Category: Chemistry Notes PPT

The valency of an element is described as a measure of its combining capacity, and it can be defined as “the number of electrons that should be gained or lost by an atom to obtain the stable electron configuration.”

What does Oxidation State Mean?

The oxidation state of an atom is defined as the number of electrons gained or lost by it.

Valency and Oxidation State are the most fundamental properties of the elements and are studied with the electron configurations’ help. The valency of the element is a measure of its ability to combine with other elements and is defined as the number of electrons that an object must lose or receive in order to achieve a stable electron configuration.The number of electrons lost or acquired by an atom determines its oxidation state.

Oxidation is one of the most fundamental aspects of elements is state and valency, which may be explored using electron configurations.

Valency and Oxidation State

Generally, electrons that are found in the outermost shell are referred to as valence electrons. At the same time, the number of valence electrons defines the valency or valence of an atom.

• In general, the elements’ valencies belonging to both the s-block and the p-block of the periodic table are calculated as eight minus the number of valence electrons or the number of valence electrons.

• For the d and f-block elements, valency can be determined not only based on the valence electrons but also on both d and f orbital electrons. However, these d and f block elements’ general valencies are given as 2 and 3.

The general oxidation state of the elements present in the periodic table can be illustrated in the chart provided below.

Valence electrons are electrons that are present in the valence shell of a molecule, the outermost shell, and the quantity of valence electrons determines an atom’s valency (or valence).

• The number of valence electrons or eight subtracted from the number of valence electrons is used to compute the valencies of elements in the s-block and p-block of the periodic table.

• Valency is determined for the d-block and f-block elements not only by valence electrons but also by d and f orbital electrons. The typical valencies of these d and f block elements, on the other hand, are 2 and 3.

In the graphic below, the general oxidation state of the elements of the periodic table is depicted.

Valency of First 20 Elements

The periodic table’s valency of the first 30 elements can be tabulated as follows:

Periodic Trends in the Oxidation States of Elements

1. Changes in Oxidation Levels Over Time

The number of valence electrons of elements increases and ranges between 1 and 8 while travelling left to right over a period. However, when elements are mixed with H or O first, their valency increases from 1 to 4, then decreases to zero. Consider the following two oxygen-containing compounds: Na₂O and F₂O. F has a higher electronegativity than oxygen in F₂O.

2. Oxidation State Variation Within a Group

The number of valence electrons does not change as we travel down in a group. As a result, each group’s elements have the same valency.

While moving from left to right across a period, the elements’ number of valence electrons increases and changes between 1 to 8. Whereas the valency of the elements, when first combined with H or O, increases from 1 to 4, and after that, it reduces to zer
o. Let us consider two compounds with oxygen Na₂O and F₂O. In the F₂O compound, the electronegativity of F is more than that of oxygen.

Thus, each of the F atoms will attract one electron from the oxygen compound. It means F will exhibit a -1 oxidation state, and O will exhibit a +2 oxidation state. On the other side, in the case of Na2O, oxygen is highly electronegative compared to a sodium atom. Therefore, oxygen will attract 2 electrons from each of the sodium atoms exhibiting a -2 oxidation state, and the Na compound will hold the oxidation state of +1.

The element’s oxidation state represents the charge possessed by an atom because of the gain or loss of electrons (because of the electronegativity difference that exists between the combining atoms) in the molecule.

While we move down in a group, there occurs no change in the number of valence electrons. Thus, all the elements of one group hold the same valency.

Guidelines for assigning the Oxidation States

• Oxidation states of the elements such as S8, O₂, H₂, Fe, P₄, and more are zero.

• Oxygen contains a -2 oxidation state. Whereas, in its peroxides such as H₂O₂ and Na₂O₂, it contains an oxidation state of -1.

• In the same way, hydrogen contains +1. But coming to the Metal Hydrides, like LiH, NaH, and more related, it has -1

• Also, a few elements contain similar oxidation states as in their compounds like

1. Halogens contain -1 except the time they produce a compound with Oxygen or one another.

2. Alkali Metals like K, Na, Rb, Cs, -Li; have +1

3. And, the Alkali Earth Metals holds +2 such as Ca, Mg, Ba, Sr, -Be, and more related.

Finding Valency of the Elements

As we probably already know, the element’s valency measures its ability to combine with the other elements. The number of electrons counted within the outer shell of the element determines its valency. There are several methods to calculate the element’s valency (otherwise molecule, for that matter).

Let us look at one of the methods of finding the valency of the elements.

The first and the easiest method is simply to consult the periodic table: the elements are sorted into the groups, and the elements present in the groups (1–8 respectively) contain similar valency the same as others in their group. For suppose, all the elements in group 8 contain 8 electrons (with high stability).

A System of Rules that governs the Assignment of Oxidation States

• The oxidation states of elements such as O₂, S₈, H₂, P₄, Fe, and others are all zero.

• The oxidation state of oxygen is -2. However, in peroxides such as Na₂O₂ and H₂O₂, it has an oxidation state of -1.

• Hydrogen, too, has a +1. However, it has a negative value in metal hydrides such as NaH, LiH, and others.

• Some elements, such as carbon, have the same oxidation states as their compounds.

• Except when they form a compound with one another or with Oxygen, halogens have a -1.

• Alkali metals, such as Na, K, Rb, Li, and Cs, have a positive charge.

• Mg, Ca, Ba, Be, Sr, and other alkali earth metals have a +2 rating.

Alcohols are chemical compounds in which a hydroxyl group replaces the hydrogen atom of an aliphatic carbon. As a result, an alcohol molecule is made up of two components. The first has an alkyl group, while the second has a hydroxyl group.

They feature a unique set of physical and chemical properties and have a sweet odor. The existence of a hydroxyl group is the most important aspect in defining an alcohol’s characteristics.

Alcohol are derivatives of hydrocarbons whose functional group is -OH as -OH has replaced a hydrogen atom. Depending on the presence of hydroxyl groups in the compound  there are different types of alcohol i.e primary alcohol, secondary alcohol, tertiary alcohol. Functional group of alcohol is known as -OH( Hydroxyl group). Nature of alcohol is mainly covalent in nature as the -OH group is attached to the carbon by covalent bond. Ethyl alcohol is considered as primary alcohol or one of the main alcohol and ethyl alcohol is also considered as ethanol. General formula of alcohol is CnH2n + 1OH.

Physical Properties of Alcohol

1. The Boiling Point of Alcohols

Alcohols have higher boiling points than other hydrocarbons with equivalent molecular weights. This is responsible for the formation of intermolecular hydrogen bonds between the hydroxyl groups of alcohol molecules. Furthermore, as the number of carbon atoms in the aliphatic carbon chain grows, the boiling point of alcohol rises.

 

Boiling points of alcohol are generally higher when compared to other hydrocarbons having the same molecular mass. The concept behind the higher boiling point of alcohols is the presence of intermolecular hydrogen bonding between hydroxyl groups of alcohol molecules. With the increase in a carbon chain of aliphatic alcohol boiling point increases whereas with an increase in the branching of alcohol boiling point decreases.

2. The Solubility of Alcohols

The hydroxyl group determines the solubility of alcohol in water. Intermolecular hydrogen bonds are formed by the hydroxyl group of alcohol. Alcohol is water-soluble due to hydrogen interaction between water and alcohol molecules. Because of the hydrophobic nature of the alkyl group, the solubility of alcohol reduces as the size of the alkyl group increases.

 

The solubility property of alcohol is determined by the presence of hydroxyl groups. Hydroxyl groups help in the formation of intermolecular hydrogen bonding as the hydrogen bond formed between water and alcohol molecules make them soluble in water, where the alkyl group is hydrophobic in nature. Thus solubility is directly related to the size of the alkyl group.

3. The Acidity of Alcohols

Alkoxides are formed when alcohols react with active metals such as sodium, potassium, and other elements. These reactions demonstrate the acidic nature of alcohol. The polarity of the –OH bond determines how acidic alcohol is. The acidity of alcohol decreases when an electron-donating group is added to the hydroxyl group. This is because it increases the electron density of the oxygen atom.

Alcohol reacts with metals to form the corresponding alkoxide. Example ethanol reacts with sodium metal to form Sodium ethoxide. This reaction shows the acidic property of alcohol as the -OH bond shows the polarity of alcohol. Their acidic property decreases when the electron-donating group is attached to the hydroxyl group.

Chemical Properties of Alcohol

1.  Oxidation of Alcohol

Aldehydes and ketones are generated when alcohols are oxidized in the presence of an oxidizing agent, and these can then be further oxidized to form carboxylic acids.

2. Dehydration of Alcohol

Alcohol dehydrates (loses a molecule of water) when exposed to protic acids, resulting in alkenes.

3. Catalytic Reduction of Butanal

Butanol is generated when butanal is dissolved. This is caused by a hydrogenation process. The hydrogens are added to the carbon-oxygen double bond, which is then changed to a carbon-oxygen single bond, resulting in the carboxyl oxygen group being transformed into a hydroxyl group.

 

A reduction process, also known as catalytic hydrogenation, is the addition of hydrogen to a carbon-carbon double bond to generate an alkane. The hydrogenation of a double bond is advantageous thermodynamically because it yields a more stable (lower energy) product.

 

Preparation of Alcohol

There are several methods for the preparation of alcohol, some of these methods are given below:

Hydrolysis of Halides: When alkyl halide is boiled with an aqueous solution of an alkali hydroxide, they form alcohol due to the nucleophilic substitution mechanism. Under this reaction, primary and secondary alcohols are formed.

Reaction:

R-X + KOH → R-OH + KX

Hydration of Alkanes: Under this reaction there occurs direct hydration of alkanes in the presence of a catalyst. 

 

Hydroboration of Alkenes: Under this reaction, an alkene is treated with diborane to form alkyl boranes, further alkyl boranes on oxidation with alkaline hydrogen peroxide to give alcohol as a final product. 

 

 Phenols

Phenols are chemical compounds that contain a benzene ring as well as a hydroxyl group. Carbolic acids are another name for them. They have special physical and chemical features due to the presence of a hydroxyl group.

 

It is an aromatic compound. It consists of phenyl groups attached to each other. Phenol is crystalline in nature having white color.

 

Physical and Chemical Properties of Phenol

1. Boiling Point of Phenols

Phenols have higher boiling points than other hydrocarbons with identical molecular weights. The primary explanation for this is the presence of intermolecular hydrogen bonding between the hydroxyl groups of phenol molecules. In general, as the number of carbon atoms increases, the boiling point of phenols rises.

2. Solubility of Phenols

The hydroxyl group determines phenol’s water solubility. The development of intermolecular hydrogen bonds in phenol is due to the hydroxyl group. As a result, hydrogen bonds develop between water and phenol molecules, making phenol water-soluble.

3. Acidity of Phenols

When phenols combine with active metals like salt or potassium, they produce phenoxide. The acidic character of phenols is indicated by these reactions. The electron-withdrawing group in phenol is the sp2 hybridized carbon of the benzene ring linked directly to the hydroxyl group.

 

As a result, the electron density of oxygen is reduced. Phenoxide ions are more stable than alkoxide ions due to the delocalization of the negative charge in the benzene ring. As a result, we might conclude
that phenols are acidic in comparison to alcohol.

4. Chirality of Phenols

Catechin, for example, is a phenol containing chirality inside its molecules. The lack of planar and axial symmetry in the phenol molecule accounts for this chirality.

 

Preparation of Phenol

1. From Haloarenes

Haloarenes include chemicals like chlorobenzene. The monosubstitution of a benzene ring yields chlorobenzene. We get sodium phenoxide when chlorobenzene reacts with sodium hydroxide at 623K and 320 atm. Finally, phenols are produced when sodium phenoxide is acidified.

2. From Benzene Sulphonic Acid

By reacting benzene with oleum, we can make benzene sulphonic. The resulting benzene sulphonic acid is treated with molten sodium hydroxide at a high temperature. Sodium phenoxide is formed as a result of this mechanism. Finally, phenols are produced when sodium phenoxide is acidified.

This reaction is the very first commercial step of phenol synthesis. In this process, sodium benzene sulphonate is fused with sodium hydroxide to form sodium phenoxide, which further undergoes acidification to yield phenol.

3. From Diazonium Salts

We may easily generate diazonium salts by treating an aromatic primary amine with nitrous (NaNO2 + HCl) acid at 273–278 K. In nature, these diazonium salts are quite reactive. When heated with water, these diazonium salts hydrolyze to phenols. We can make phenols by treating diazonium ions with dilute acids.

When a diazonium salts solution is treated with steam distilled or is added to boiling dil.H2SO4, it forms phenol as a final product.

4. From Cumene

Cumene is an organic chemical made by alkylating benzene with propylene in the Friedel-Crafts reaction. Cumene hydroperoxide is formed when cumene (isopropylbenzene) is oxidized in the presence of air.

Ethers

They belong to organic compounds that have an oxygen atom attached to two same or different alkyl or aryl groups. The general formula of ether is R-O-R, R-O-Ar or Ar-O-Ar.

 

Physical Properties of Ethers

1. Ethers have a comparable boiling point as alkanes. When compared to alcohols of comparable molecular mass, however, it is significantly lower. This is true despite the polarity of the C-O bond.

2. Ethers are water-miscible in the same way as alcohols are. Ether molecules are miscible in water. This is because, like alcohols, the oxygen atom of ether may form hydrogen bonds with a water molecule.

 

Chemical Properties of Ethers

1. Cleavage of C-O bond: Ethers are normally non-reactive. Cleavage of the C-O bond occurs when an excess of hydrogen halide is added to the ether. Alkyl halides are formed as a result of this reaction.

The following is the order of reactivity:

HI > HBr > HCl

R-O-R  +  HX  →  RX  +  R-OH

2. Electrophilic substitution: For electrophilic substitution, the alkoxy group in ether activates the aromatic ring at ortho and para locations. Halogenation, Friedel Crafts reaction, and other electrophilic substitution reactions are common.

3. Ether halogenation: Aromatic ethers undergo halogenation, such as bromination, when a halogen is added in the presence or absence of a catalyst.

4. Aromatic ethers undergo Friedel Crafts reaction, which involves the addition of an alkyl or acyl group when they are introduced to an alkyl or acyl halide in the presence of a Lewis acid as a catalyst.

 

Preparation of Ethers

There are several methods for the preparation of ether, some of these methods are given below:

 

Preparation of Ether by Dehydration of Alcohol: This reaction takes place in the presence of protic acid i.e sulphuric acid alcohol undergoes dehydration to produce alkenes and ether as their minor and major products. This reaction occurs at approx 443K. This is the ideal method of preparation of ether.

 

Williamson’s Synthesis: Under this reaction, alkyl halide reacts with sodium alkoxide and ether is formed as the main product. This reaction generally follows the SN2 mechanism for the formation of primary alcohol.

Photochemical reactions are of immense importance as these are the basis of many such processes which are the basis of sustainable life on earth. For example, photosynthesis, the formation of vitamin D with sunlight, etc. are photochemical reactions. Photochemical reactions are studied or come under the branch of chemistry called photochemistry.  It is a branch of chemistry that deals with the chemical effects of light.

 

Trommsdorff described the first photochemical reaction in 1834. He observed the reaction on crystals of santonin. These crystals when exposed to sunlight turned yellow and burst. 

 

Those reactions which take place by absorption of light energy are called photochemical reactions. Generally, it takes place by the absorption of ultraviolet light, visible light, or infrared radiation. Wavelength of all these radiations are given below in the table –

 

 

Photochemical reactions proceed differently than temperature-driven reactions or thermal reactions. In paths of photochemical reactions, high energy intermediates are formed which cannot be formed thermally. In these reactions, large activation barriers are crossed in a short time. Some photochemical reactions are destructive such as photodegradation of plastics.

 

Examples of Photochemical Reactions 

Most common example of a photochemical reaction is photosynthesis. In photosynthesis, plants use sunlight and water to convert carbon dioxide into glucose (Carbohydrates) and oxygen. Reaction is given below –                 

[ 6CO_{2} + 6H_{2}O overset{sunlight}{rightarrow} C_{6}H_{12}O_{6} + 6O_{2}]

Some other examples of photochemical reactions are given below –

• Bioluminescence reactions which occur mainly in marine animals. 

• Silver chloride absorbs light and decomposes it into silver and chlorine. Reaction is given below –

[ 2AgCl overset{sunlight}{rightarrow} 2Ag + Cl_{2}]

[ 2AgBr overset{sunlight}{rightarrow} 2Ag +Br_{2}]

• Many polymerization reactions use light energy. These are also photochemical reactions. In many polymerization reactions, free radicals are formed by photoinitiation. It is known as photolysis. Reaction is given below –

• Photodegradation of many substances takes place by photochemical reactions. For example, photodegradation of polyvinyl chloride. 

• Photodynamic therapy is based on photochemical reactions. Photochemical reaction takes place when doctors use light to destroy tumors. 

• Vision is initiated by the photochemical reaction of rhodopsin. 

• Photochemical reactions take place or are used for the production of anti-malarial drugs. 

• Photoalkylation is also an example of a photochemical reaction. In these reactions, alkyl groups are attached in the molecules by using light energy. 

• Electrocyclic reactions, radical reactions, photoisomerization, and Norrish reaction 1 and 2 are examples of photochemical organic reactions.  

• Industrial production of benzyl chloride is also a photochemical reaction. Reactions are given below –

[Cl_{2} + hvcdot  rightarrow  2Cl]

[C_{6}H_{5}CH_{3} + 2Clcdot rightarrow  C_{6}H_{5}CH_{2}cdot + HCl]

[C_{6}H_{5}CH_{2}cdot + Clcdot rightarrow  C_{6}H_{5}CH_{2}Cl]

• Free radical halogenation reactions are also examples of photochemical reactions.

• Coordination complexes and organometallic compounds are also photoreactive and show photochemical reactions. 

 

Laws of Photochemistry 

As we know, the arrival of a reactant to an excited state is the 1st step of photochemical processes. Which is called photoexcitation. With this photochemical reactions or processes follow the laws of photochemistry as well. There are two laws of photochemistry which are Grotthuss-Draper law and Stark–Einstein law. 

• Grotthuss – Draper law – This law states that light must be absorbed by a chemical substance in order to take place in a chemical reaction. This law was given by chemists Theodor Grotthuss and John W. Draper.  

• Stark – Einstein law – This law states that for each photon of light absorbed by a chemical system, no more than one molecule is activated for a photochemical reaction, as defined by the quantum yield. This law was given by physicists Johannes Stark and Albert Einstein. 

 

Tips to Study Photochemical Reactions

The above material gave you a thorough insight into the Photochemical Reactions with appropriate examples along with the laws of Photochemistry. 

Let’s understand the best way to prepare for a particular topic thoroughly. 

The utmost important thing to do for students is to focus on understanding each and every concept that is a part of the course exam. No matter what level of examination you’re appearing for or how difficult it is, the only way to crack it is to be clear about every topic. 

The art of making notes might seem a time-consuming task to do but it could help in unimaginable ways. Whether you think of the examination days or the preparation days, they come in handy and can be of the best help at all times. Hence, students are advised to prepare good revision notes for themselves to make their study swift and concise. 

To keep a track of whatever the students have understood, they need to revise the topics repeatedly. This never lets them lose their confidence. An important thing that a student shall remember at all times is that every bit of the syllabus is equally important and hence, leaving any of it can cost you more than you could ever imagine. 

Another important part of your preparation strategy is to attempt some mock tests. It gives you an idea of what the questions would look like and what is the exam pattern that is followed. Once you understand it, it becomes easier for you to attempt the final paper. Practicing sample papers also helps you to boost confidence and charge you for the final days. 

An important skill that the students must have
is to be able to measure their progress. Be your competitor, keep your previous results handy and compare them with the latest ones. This would give you an insight into how your preparation is going and what areas need more practice or attention. 

Once you’re done with observing your study patterns and habits, you’re required to make an effective, time-based, and realistic study plan which helps you upraise your preparation strategy. Students shall also understand that it is not only important to make a schedule but it is more important for them to stick to it. As much as you stay true to your goals and focus on achieving the targets, even the smallest ones, you get closer to your final destination. 

Self-doubt might stop you time and again but once you decide to work past them with a positive attitude, you’re already halfway there! You might face a lot of challenges across the way but adopt a ‘Just Do It’ attitude and your dreams would turn into reality in no time!

What are Alcohols?

The compounds obtained by replacing one hydrogen atom from aliphatic hydrocarbons by a hydroxyl group are alcohols whereas those obtained by replacing hydrogen atoms of aromatic hydrocarbons are phenols. Alcohols occur widely in nature and have various industrial and medicinal applications. In this article, we will talk about the classification of alcohols, alcohol physical and chemical properties and their uses.

Chemical and Physical Properties of Alcohol

What are The Physical Properties of Alcohol?

The important physical properties of alcohols are:

• Physical State- The lower members are colourless liquids having a characteristic smell and burning taste. The higher members (having more than 12-13 carbon atoms) are colourless, odourless, wax-like solids. 

• Associated Nature- Alcohols exit as bonded molecules having intermolecular hydrogen bonds as shown below:

This hydrogen bonding is due to the large difference in electronegativities of oxygen and hydrogen atoms. As a result, the OH bond is strongly polar and forms hydrogen bonds. 

• Boiling Points- The lower members have low boiling points but with the increase in molecular mass, the boiling points keep on increasing gradually. This is because of an increase in van der Waals forces. Isomers of alcohol have the same number of carbon atoms, the boiling points are in the order:

Primary > secondary > tertiary.

This is because, with branching, the surface area increases and therefore, van der Waals forces decrease. Consequently, the boiling point also decreases.

• Solubility- The members with the low carbon of alcohols are highly soluble in water but the solubility in water decreases with the increase in molecular weight. The solubility of alcohols with less carbon in water is due to the formation of hydrogen bonds between alcohols and water molecules.

However, as the number of carbon in the alcohol molecule increases, the alkyl group becomes larger and prevents the formation of hydrogen bonds with water molecules and hence the solubility goes on decreasing with increase in the length of the carbon chain.

Chemical Properties of Alcohol

Alcohols can behave both as nucleophiles (electron-donating group) as well as electrophiles (electron-withdrawing group).

1. They behave as nucleophiles in the reaction where the bond between O-H is broken as shown below:

2. They behave as electrophiles in which the bond between C-O is broken. These reactions are carried out in the presence of acids to form protonated alcohols. 

Protonated alcohols react as electrophiles. 

Chemical Reactions of Alcohols

1. Reaction with active metals- Alcohols are weakly acidic in nature and react with active metals such as sodium, potassium, magnesium, aluminium, etc. to liberate hydrogen gas and form metal alkoxide. For example,

2CH3CH2OH + 2Na → 2CH3CH2ONa + H2

The above reaction shows that alcohols (R-OH) are acidic in nature (pH less than 7).

2. Reaction with metal hydrides- Alcohols react with metal hydrides and form sodium alkoxides and evolve hydrogen gas as a byproduct.

CH₃OH + NaH →CH3O–Na+ + H2

3. Reaction with carboxylic acids- Alcohols react with the carboxylic acid, in the presence of concentrated sulphuric acid or dry hydrochloric gas as a catalyst, to form esters. The reaction is known as esterification. The function of concentrated sulphuric acid is to act as a protonating agent as well as a dehydrated agent. 

CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O

4. Reaction with grignard reagent

Alcohols react with Grignard reagents to form hydrocarbons. For example,

CH3OH + C2H5MgBr → C2H6 + CH3OMgBr

5. Reaction with acyl chloride or acid anhydride- When alcohols are treated with an acid chloride or acid anhydride in the presence of bases like pyridine, the hydrogen atom of -OH group is replaced by an acyl group.

6. Reaction with hydrogen halide- Alcohols react with hydrogen halide and form alkane halide.

ROH + HX → RX + H2O

7. Reaction with phosphorus halide- Phosphorus halide when reacts with alcohols it forms haloalkanes.

ROH + PCl5 → R-Cl + POCl3 + HCl

8. Reaction with thionyl chloride- On treatment with thionyl chloride in the presence of pyridine, alcohols form chloroalkanes.

ROH + SOCl2 → R-Cl + SO2 ↑ + HCl↑

Did you know?

• Ethanol and methanol are alcohols that act as fuel.

• Alcohol can lower blood sugar level.

• Alcohol can turn blue litmus to red.

• Ethanol is used as an intoxicating agent.

Polarography, also known as Electrochemical Polarography, is an Electroanalytical technique that measures the reduction potential of Electroactive species. Polarographic sensors are the mainstays of field microelectrodes, but field microelectrodes can be made Polarographic too. Polarographic techniques are a subset of potentiometric techniques, which includes coulometry, anodic stripping voltammetry, and atomic emission spectrometry. The principle is based on measuring the Current or voltage drop that occurs in a polarizable electrolyte that is in contact with a polarized electrode. The ability of the electrode to become polarized is a Direct function of the activity of the substance in the electrolyte. The polarization of the electrode is expressed in mV as a difference between anodic and cathodic overpotentials.  Polarographic electrodes are widely used in electroanalytical chemistry and biochemistry for the determination of oxidation-reduction potentials, pK, the detection of electroactive contaminants in wastewaters, and the detection of chemical and biological species such as pesticides, heavy metals and microbes.

Instrumentation 

A Polarographic instrument consists of three main parts: a cell or electrochemical cell, an electrode and a potentiostat.  In the case of the instrument’s probe, the potentiostat is also the electrode.  A cell is an electrochemical cell that is used to hold a solution with electrodes.  Each electrode in a cell has one or more potentials on it that are applied to the cell, and a potential is applied to each electrode relative to a second electrode in the cell.  The probe is the instrument’s electrode.  An electrode can be either the working electrode or the counter electrode. 

The working electrode is the electrode that the Current is measured by or the electrode that is charged with the potential.  The counter electrode is the electrode that the Current is NOT measured by.  The reason for this is that if the working electrode was charged with potential, the counter electrode would be the electrode that measured the Current.  Since the counter electrode doesn’t measure the Current, there is no Current flow on the counter electrode.  

When the Current is measured, it is the working electrode that is charged, and the potential is that which the counter electrode is held at.  The potentiostat is the electrode and instrument that applies the potential to the electrode.  It has two or more probes to measure the Current between the potential of the working electrode and the potential of the counter electrode, as well as the Current between the potential of the working electrode and the ground.  

There is one potentiostat for each electrode in a cell, with one probe connected to the working electrode and another probe connected to the ground.  A potentiostat can also have several probes connected to different potentials.  In this way, the potentiostat can be used to control how much Current goes into the cell (by setting the potential of the working electrode), as well as how much Current leaves the cell (by changing the potential of the counter electrode).

Examples 

Potentiostats are used to test the performance of cells in many applications, including photovoltaic cells, fuel cells, batteries, corrosion cells, and various chemical Analysis applications. A potentiostat may also be used to control electrodeposited materials in the form of electroplating baths and electrochemical baths for chemical deposition.

In addition to the galvanostatic charging application described above, potentiostats can be used to measure specific electrochemical reactions. The potential difference across an electrochemical cell is controlled using the potentiostat to control the Current flow and the cell is held at a constant potential. This is referred to as the galvanostatic mode of operation.

Some potentiostats may have more than one set of wires and one set may be used to control the Current while the other set is used to control the potential, to create a bridge circuit.

Advantages

Compared to other methods, the advantages of Polarographic measurements are as follows:

• Electrochemical cells used for polygraphic measurements are simpler to construct and operate than electrolytic cells because they do not require the use of a mercury or silver amalgam. 

• The concentration of a TM species is measured electrochemically; there is no need to separate the solution. 

• The Polarographic technique is sensitive, reproducible and provides a quantitative measurement. 

• The technique allows the determination of valencies in one order of magnitude.  

• It is an accurate technique and the precision may be defined by the type of equipment employed. 

• The standard deviation is typically 0.01–0.05 mV for a single Analysis. The method gives information on the number of electrons (or ions) carried by the species studied. 

• The voltammetric studies of electroactive compounds are carried out easily using Polarographic equipment. 

• The technique is well suited for the fast determination of valencies and the investigation of the reduction potential of compounds that undergo thermally activated processes. 

• In aqueous solutions, most of the organic compounds give stable voltammograms with a high signal to noise ratio. 

• The technique may be used for a wide variety of stable metal cations, weakly or moderately electrophilic, including lanthanides, actinides, uranium, vanadium, molybdenum, tungsten, etc. 

• The method may also be used for electrochemically active inorganic compounds. 

• The electrode material has to be chosen carefully. Most metal oxide electrodes are not suitable because of the irreversible oxidation of the electrogenerated ions.

Polarographic Analysis

Polarography is considered to be an electroanalytical technique that is used for measuring the Current flowing between two electrodes present in a solution. This technique is possible only in the presence of applied voltage which seems to increase gradually. The purpose of this technique is to determine the concentration of a particular solute as well as the nature of the solute, respectively. Polarography is also known as Polarographic Analysis in analytical chemistry. This technique is considered to be an electrochemical method that is responsible for analyzing solutions of reducible or oxidizable substances.

 

Overview

In analytical chemistry, Polarography is also known as voltammetry, and Polarography is known to be a type of voltammetry where the working electrode is considered to be the same as a dropping mercury electrode or static memory drop electrode; these electrodes are believed to be very useful as they possess a wide cathodic range and renewable face.

Voltammetry is considered to be an electroanalytica
l method in which varied information is obtained about the analyte when the Current gets measured as the potential. The analytic data which is meant for a voltammetric experiment gets depicted in the form of a voltammogram. Voltammogram is considered to be a polarogram in the case of Polarography. Voltammogram is responsible for plotting the Current which is produced by the electrolyte.

The simple principle of Polarography is known to be the study of solutions or different electrode processes using electrolysis in the presence of two electrodes, among which one is polarizable, and one is non-polarizable. Non-polarizable electrodes are formed when mercury regularly drops from a capillary tube.

 

Types of Polarography

After having a brief about the principle of Polarography, let’s discuss its types in a brief and detailed way to get a clearer understanding of Polarography:-

 

Direct Current Polarography (DCP)

In dc Polarography, it is witnessed that a constant potential seems to be applied during the entire drop-life time. It constructs a Current-voltage curve by applying a series of potential steps; these steps are synchronized with the drop fall. In most of the instruments, however, linearly changing potential gets applied at such a slow rate that the change of potential throughout the drop lifetime is found to be in a few millivolts.

 

Square Wave Polarography (SWP)

In the case of Square Wave Polarography, the Current present in the working electrode gets measured when the potential between the working electrode and a reference electrode is swept linearly with time. One can view the potential Waveform in the form of a superposition of a regular Square Wave onto an underlying staircase. The Current which is measured gets sampled two times – once at the end of the forward potential and again at the end of the reverse potential Pulse. Due to this Current sampling technique, the contribution that the Current signal receives from the capacitive Current is minimal.

 

Normal Pulse Polarography (NPP)

In the case of Normal Pulse Polarography, the potential doesn’t get altered due to a potential ramp that seems to increase continuously but gets altered by the Square Wave potential Pulses whose height is increasing and is overlaid on a constant initial potential. The mercury drop electrode is held at a constant potential for most of its duration. During this time, no electrochemical reaction seems to take place under a given experimental condition. The limiting Current that is there in NPP is considered to be diffusion controlled.

 

Differential Pulse Polarography (DPP)

Differential Pulse Polarography is considered to be the most efficient Pulse method among all. In digital instruments, an increasing Direct potential which is in the shape of a staircase is present in the excitation signal. In periodic succession, small Square Wave Pulses having a constant potential get applied to this increasing Direct potential. The superimposition gets synchronized with the drop time and seems to take place when the surface of the electrode experiences no changes. The Current gets measured two times at each mercury drop, before each Pulse, and at the end of the Pulse time. The difference between the measurements seems to be plotted against the Direct potential and has the potential to produce peak-shaped polarograms.

 

Polarographic Cell

The Polarographic cell is considered to be used for continuous Analysis of flowing situations. When the Polarographic cell gets used for the Analysis, it should get designed in such a way that it can have a low holdup volume in respect to the flow rates that are being used so that the response to changes in the composition can be faster than before.

Just like disaccharides and monosaccharides, Polysaccharides also play important roles in our lives. To know more about them in detail, we have come up with this article. In this article, you will learn about the different types of Polysaccharides and their structure. 

You must have already heard about simple and complex carbohydrates. Starch is the most common type of complex carbohydrate which is present in the food you consume. So, these complex carbohydrates consist of severely branched molecular structures. They are nothing but the Polysaccharide itself. They are complex carbohydrates formed with a chain of monosaccharides. The bonds that keep the chain together are glycosidic. Some common Polysaccharides examples are starch, glycogen, and cellulose. Homopolysaccharides and Heteropolysaccharides are the two fundamental types of Polysaccharides. In this article, you can learn about Polysaccharides in detail, their structure, types, and examples and so on.  

What is a Polysaccharide?

Polysaccharides are nothing but complex carbohydrates, formed with a chain of monosaccharides. Glycosidic linkage is what keeps the chain of monosaccharides bonded together. You can say that a molecule of a Polysaccharide has a certain number of sugar molecules that come together to form a larger molecule. You can also call them Glycans. Polysaccharides can also be defined as the long polymers of carbohydrates that are made up of repeating mono- or di-saccharide units joined by glycosidic connections (e.g., glucose, fructose, galactose). They might be linear or very branching in structure. Storage Polysaccharides like starch and glycogen, as well as structural Polysaccharides like cellulose and chitin, are just a few examples. More than 10 monosaccharide units make up a Polysaccharide. Personal preferences differ on how large a carbohydrate must be to be classified as Polysaccharides or oligosaccharides. Polysaccharides are classified into two parts, which are Homopolysaccharides and Heteropolysaccharides. Below you can learn more about the same. 

Types of Polysaccharides

Homopolysaccharide: In this type, molecules get formed with a single type of monosaccharides. You can determine a general theme for such molecules by studying their monosaccharide units. When Homopolysaccharide gets formed using nothing except glucose molecules, then you can call them The roots of Polysaccharide that lucans. If it uses galactose molecules while forming, then you may call it Galactus. In the topics below, you can find plenty of explanations about Glucans. 

  

Heteropolysaccharide: These are Polysaccharide molecules consisting of more than a single kind of monosaccharides. Hyaluronic acid, heparin, and chondroitin sulfate are some common examples of Heteropolysaccharides.   

Structure of Polysaccharides:

Following is how a typical Polysaccharide structure looks –

Every Polysaccharide gets formed with the same process. In which the monosaccharides form a chain using glycosidic bonds. These bonds have oxygen molecules bridging the two carbon rings. The bond gets created when the carbon of one molecule loses the hydroxyl group, and the hydroxyl group of a different monosaccharide loses the hydrogen. An oxygen molecule connects two carbon rings in these glycosidic linkages. Since two molecules of hydrogen and one molecule of oxygen get expelled, it becomes a dehydration reaction. In other words, the bond is produced when a hydroxyl group is removed from one molecule’s carbon, and the hydrogen is removed from another monosaccharide’s hydroxyl group. The obtained structure of molecules tells you about the various properties and structures of the final Polysaccharide. A Polysaccharide utilized for energy storage will allow simple access to the constituent monosaccharides, but a Polysaccharide used for support will typically be a lengthy chain of monosaccharides forming fibrous structures.

 

Polysaccharide Classification 

Depending on the monosaccharide components, Polysaccharides can be Homopolysaccharides or Heteropolysaccharides. A Homopolysaccharide (also known as homoglycan) has only one form of monosaccharide, whereas a Heteropolysaccharide (also known as heteroglycan) contains multiple types of monosaccharides.

Polysaccharides are classed as storage or structural Polysaccharides based on their function. Polysaccharides that are employed for storage are known as storage Polysaccharides. Plants, for example, store glucose in the form of starch. Glycogen is the storage form for simple sugars in animals. Carbohydrates with a structural purpose are known as structural Polysaccharides. celluloses, which are polymers of repeating glucose units connected by beta-linkages, are found in plants. Chitin is produced by certain animals and is used as a structural component of exoskeletons, for example.

Polysaccharide Examples

Now that you have learned what a Polysaccharide and its structure is, it’s time to learn about Polysaccharide examples. Naturally, there are three main Polysaccharides. You come across them every day in your life, and they are starch, glycogen, and cellulose. Below you can read about them in detail. 

You can find starch in all photosynthetic plants. The roots and seeds of the plant have more starch in general. When plants synthesize glucose, excessive glucose gets stored in the form of starch. It consists of a linked glucose molecule, and that’s why it’s a glucan. 

The molecular formula for starch is (C6H10O5)n. Here, the ‘n’ indicates the number of linked molecules. Below is a typical structure of starch.

 

You can find starch in the seeds of plants as granules. Upon heating those granules in water, you get a colloidal suspension. Further, you get two components from the process, Amylose and Amylopectin. 

Glycogen is a glucan, which exclusively contains D-glucose units. It acts as a reserved source of carbohydrates for plants as well as animals. Below you can see the structure and function of glycogen. 

()

The structure of glycogen is quite similar to amylopectin. However, the branching takes place more frequently in a glycogen molecule. As you can see, it is indeed a larger molecule. Glucose molecules are small in size, and they can diffuse out of a cell membrane. Since glycogen is bigger, it doesn’t diffuse out of the cell membrane. It serves the essential function of storing glucose within cells. 

Cellulose is a crucial structural component of the cell walls of plants that are photosynthetic. It is fibrous in nature and highly insoluble in water too. Keep in mind that cellulose is a glucan. The D-glucose units have connections in the (1 – 4) fusion. It is a beta linkage, and it’s indeed different from the glycogen. 

()

The structure of the cellulose –OH group points outwards from the chain structure. When two chains come in contact with each other, they get stacked on each other because of the hydrogen bonding between hydroxyl groups. In the end, you obtain an insoluble fibrous structure that is perfect for the functions of cellulose in the cell walls.   

Differences and Similarities between Chitin and Cellulose 

Both cellulose and chitin are long-chain polymers of t
housands of glucose monomers strung together in long strands. The side-chains connected to the carbon rings of the monosaccharides are the only difference between the two Polysaccharides. The glucose monosaccharides in chitin have been changed to include a group with additional carbon, nitrogen, and oxygen. A dipole is created by the side chain, which promotes hydrogen bonding. Cellulose is known to form hard materials but chitin can form even harder materials. 

Long, linear chains form from both Polysaccharides. Long fibers are formed by these chains and are deposited outside of the cell membrane. The fibers weave into a complicated structure that is maintained in place by hydrogen bonds between side chains thanks to the help of certain proteins and other factors. As a result, simple glucose molecules that were previously used for energy storage can be transformed into molecules with more structural stiffness. The monosaccharides used change relatively slightly between structural and storage Polysaccharides. Instead of a structural Polysaccharide, glucose molecules can branch and store many more bonds in a smaller space by modifying the shape of the molecule. The configuration of the glucose used is the only difference between cellulose and starch.