A. Mohr’s method:-In this method soln of cl- is titrated against AgNO3 soln with potassium chromate as the indicator. Which is precipipated as Ag2CrO4(red ppt) Agcl being less soluble is preferentially precipitated and when all cl- are precipitated and futher addition of AgNo3gives red ppt of AgCrO4 indicating the end point

The titration should be performed in almost neutral medium (pH=6.5-9).

In alkaline medium AgoH is precipitated. In acidic medium CrO4– are changed into HCrO4(which is weak acid) or Cr2O7– lowering conc. Of CrO4--. Such that the precipitation of Ag2CrO4 requires more AGNo3 to be added. i.e. (increasing titration error)

So,this titration is best performed using mixture of chromate and dichromate which by buffring action maintain the pH at above 7 through the titration.

B. Volhard method:-this is indirect method in which cl- is precipitated as Agcl by adding excess of AgNo3 is titrated back with thiocynate soln using Fe+++ ions as the indicator. Which gives red coloured of ferric thiocynate at the end point.

I. Titration is carried out in slight acidic medium to avoid the formation of AgoH.

II. The adsorption of Ag+ on Agcl can be avoided.

III. By removing the Agcl as soon as it is formed before back titration with scN -. The removal is done by boiling for about 3 min. followed by filtration.

IV. By addition nitrobenzene which forms layer around the precipitate(Agcl) there by preventing the adsorption of Ag+.

Role of quantative / pharmaceutical analysis in QC3. In many manufacturing industries,the chemical composition of raw materials ,intermediates and finished products needs to be monitored to ensure satisfactory quality and consistency and to meet the stringent requirements laid down by the government and the relevant department.4. To check whether manufacture standards are in accordance with official standards (IP,BP,USP etc.)5. To analyze the stability of the drugs during storage at normal condition (Real time stability) and accelerated condition (accelerated stability.)

Systemic errori. Produce blas- an overall deviation

of a result from the true value even when random errors are very small.

ii. Cause all results to be affected in one sense only –all to high or all to low.

iii. Can not be detected simply by using replicate measurements.

iv. Can be corrected , e.g. by using standard methods and meterials.

Random errori. Affect precision repeatability or

reproducibility.

ii. Cause replicate results to fall on either side of a mean value.

iii. Can be estimated using replicate measurements.

iv. Can be minimized by good techenique but not eliminated.

Hydrolysis of saltSalt Hydrolysis: In water, salts will dissociate to form ions (either completely or incompletely depending on

the respective solubility constant, Ksp. For example: NH4Br(s)→NH4+(aq)+Br−(aq)

Here, the salt NH4Br is put into water and dissociates into NH+4 and Br−.

There are four possible ways of forming salts:

1, If the salt is formed from a strong base and strong acid, then the salt solution is neutral, indicating that the bonds in the salt solution will not break apart (indicating no hydrolysis occurred) and is basic.

2. If the salt is formed from a strong acid and weak base, the bonds in the salt solution will break apart and becomes acidic.

3. If the salt is formed from a strong base and weak acid, the salt solution is basic and hydrolyzes.

4. If the salt is formed from a weak base and weak acid, will hydrolyze, but the acidity or basicity depends on the equilibrium constants of Ka and Kb. If the Ka value is greater than the Kb value, the resulting solution will be acidic and vice versa.

Acid Hydrolysis:- Water can act as an acid or a base based on the Brønsted-Lowry acid theory. If it acts as a Bronsted-Lowry acid, the water molecule would donate a proton (H+), also written as a hydroniumion (H3O+). If it acts as a Bronsted-Lowry base, it would accept a proton (H+). An acid hydrolysis reaction is very much the same as an acid dissociation reaction. CH3COOH+H2O⇌H3O++CH3COO−. In the above reaction, the proton H+ from CH3COOH (acetic acid) is donated to water, producing H3O+ and a CH3COO-. The bonds between H+ and CH3COO- are broken by the addition of water molecules. A reaction with CH3COOH, a weak acid, is similar to an acid-dissociation reaction, and water forms a conjugate base and a hydronium ion. When a weak acid is hydrolyzed,a hydronium ion is produced.

Basic Hydrolysis:-A base hydrolysis reaction will resemble the reaction for base dissociation. A common weak base that dissociates in water is ammonia:NH3+H2O⇌NH4+ + OH-.In the hydrolysis of ammonia, the ammonia molecule accepts a proton from the water (because water acts as a Bronsted-Lowry acid), producing a hydroxide anion (OH-). Similar to a basic dissociation reaction, ammonia forms ammonium and a hydroxide from the addition a water molecule.

Redox TitrationsIn a redox titration one of the solutions is a reducing agent and the other an oxidizing agent.

The equivalence point is reached after sufficient oxidizing agent has been added to react with all of the reducing agent.

A redox indicator, which changes color between reduced and oxidized states, can be used to detect the end point

Redox IndicatorsA redox indicator (also called an oxidation-reduction indicator) is an indicator which undergoes a definite color change at a specific electrode potential.

Redox visual indicators are of two types. The first type is called specific while the other type is called nonspecific. An example of a specific indicator is starch where it forms a blue complex with iodine but not with iodide. When iodine is consumed, the blue complex disappears and the solution turns almost white indicating the end point. Another specific indicator is the permanganate ion where it is also called a self indicator. The purple color of the permanganate ion is converted to the colorless Mn2+. Nonspecific redox indicators are very weak oxidants or reductants which have different colors of the oxidized and reduced forms. When all the analyte is consumed in a redox reaction, the first drops of excess titrant will react with the indicator, thus changing its color. The redoxindicator equilibrium can be represented by the equation below:

Gravimetric analysisis the process of producing and weighing a compound or element in as pure form as possible after some form of chemical treatment has been carried out on the substances to examined.Gravimetric analysis is one of the most accurate and precise method of macro quantitative analysis.

Advantages 1. It is accurate and precise when using

modern analytical balance.2. Possible sources of error are readily

checked since filtrates can be tested for completeness of precipitation and precipitates may be examined for the presence of impurities.

3. It is an absolute method; it involves direct measurement without any form of calibration being required.

4. Determination can be carried out with relatively inexpensive apparatus; the most expensive items are a muffle furnace and sometimes platinum crucibles.

5. Gravimetric analysis was used to determine the atomic masses of many elements to six figure accuracy.

Disadvantages1. The chief disadvantage is that it

requires meticulous time consuming.2. The chemist often prefers modern

instrumental methods when they can be used.

3. Gravimetric analysis usually only provides for the analysis of a single element, or a limited group of elements, at a time.

4. Methods are often convoluted and a slight mis-step in a procedure can often mean disaster for the analysis (colloid formation in precipitation gravimetry, for example).

5. Gravimetric analysis is based on the measurement of mass.

Principle of precipitation titration:To determine if a precipitate will form when two solutions are mixed then:1. The Ksp expression for insoluble combination of the ions in a mixture must be

written and its value is determined.2. The concentration of the ions present in the mixture of solution needs to be

calculated.3. The product of concentration of ions(Q) in solution is compared to the Ksp. If it

exceeds the Ksp a precipitate will form. If the Q exceeds the Ksp then a precipitates forms. If Q is larger than the values of Ksp

then the ions are above the saturation line and the ions are in a super-saturated situation. The ions will precipitate until their individual concentrations multiplied together equal the value of Ksp. i.e. Q≥ Ksp = PPt formed.

If the Q doesn’t exceed the value of Ksp then the ions are in an unsaturated situation. Which means there aren’t enough ions dissolved to form a precipitate. i.e. Q ≤ Ksp then unsaturated situation.

If the Q equals the value of Ksp then the solution is saturated. The solution can hold no more dissolved ions without a precipitate forming i.e. Q = Ksp then saturated condition.

Standardization of silver nitrate solution:Standardization is analytical process where actual concentration of reagent is know by

titrating it with standard sod. chloride solution. Nacl has a relative molecular mass of 58.44 A. About 2.9 gm of the standard Nacl is accurately weighted out, dissolved in 500ml of water and then titrated with silver nitrate to know the concentration of silver nitrated prepared.

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Activity is the result of the effects of interactions between ion or molecule and its surroundings. „ Since it is hard to define, it is usually measured with reference to an ideal state.

„ The activity coefficient measures how much an actual system deviates from a reference system .„ Mathematically, the activity coefficient is defined as the limit of the equation derived for activity and has no units. „ According to the IUPAC, the activity coefficient can be calculated in terms of mole fractions for liquids and solid mixtures, or in terms of molalities for dilute solutions.

Polyprotic Acid:A polyprotic acid is an acid which contains more than one ionizable hydrogen (H+) per acid molecule. The acid ionizes one step at a time in an aqueous solution, with a separate ionization constant for each step. The initial dissociation is the primary source of H+, so it is the main factor in determining the pH of the solution.

Phosphoric acid (H3PO4) is an example of a triprotic acid. Phosphoric acid ionizes in three steps:

1. H3PO4(aq) ⇔ H+(aq) + H2PO4-(aq)Ka1 = [H+][H2PO4

-]/[H3PO4] = 7.5 x 10-3

2. H2PO4-(aq) ⇔ H+(aq) + HPO4

2-(aq)Ka2 = [H+][HPO42-]/[H2PO4

-] = 6.2 x 10-8

3. HPO42-(aq) ⇔ H+(aq) + PO4

3-(aq)Ka3 = [H+][PO43-]/[HPO4

2-] = 4.8 x 10-1

C. fajan’s method:- This method involves the titration of cl- with AgNo3 using Flouriscence as the indicator which gives pink color at the end point. The titration is carried out in slightly basic medium since the anionic form of the indicator adsorbs Ag+. Some flourescence derivaties(like esoin) can also be used as the absorption indicator performing the titration is acidic medium. Titration involving Hg(No3)2 and ammonia for potassium thiocynate. Similar method as AgN o3.

Solubility product:Solubility product is defined as the product of ionic concentration when dissolved ions and undissolved ions are in equilibrium.In other words,When a saturated solution of sparingly or slightly soluble salt is in contact with undissolved salt, an equilibrium is established between the dissolved ions and the ions in the solid phase of the undissolved salt. Ionic product at this stage is called solubility product.Symbol:It is denoted by KspDetermination of solubility product:Consider a slightly soluble salt such as silver chloride (AgCl).

AgCl(aq) ↔ Ag+(aq) + Cl-(aq)

Applying equilibrium law:Kc=[Ag+][Cl-]/[AgCl]Kc[AgCl] = [Ag+][Cl-]Since there is no change in the concentration of salt (AgCl) at equilibrium. Therefore,[AgCl] = constant (K') or , Kc = [Ag+][Cl-]/ K' or, Kc x K'= [Ag+][Cl-] Let Kc x K' = solubility product or Ksp , Therefore,Ksp = [Ag+][Cl-].

InOX + n e = InRED Eo = x VE = EIn

o – (0.0592/nIn) log [InRED]/[InOX]

The color of the oxidized form can be clearly distinguished when 10 [InRED] = [InOX] and the color of the reduced form can be clearly distinguished when [InRED] = 10 [InOX]. Substituting into the electrode potential equation we get:

DEColor Change = EIno + (0.0592/nIn)

Therefore, the color change of the indicator occurs around Eo

In and, in fact, very close to it. For a real titration, Eo

In should be as close as possible to the electrode potential at the equivalence point, Eep, and should thus be at the potential range of the sharp break of the titration curve. It is also clear that it is not possible to use whatever indicator for whatever titration.

Standard electrode potentialThis is the 'potential' of a redox system to lose or gain electrons when compared to the standard hydrogen electrode - assigned a value of 0 volts.In any reduction-oxidation half equation the electrons are gained by the species on the left hand side:Cu2+ + 2e-- ---------- CuThis is an equilibrium and so if a more powerful reducing agent is allowed enters into electrical contact with the above system it can force the copper ions to accept electrons and push the equilibrium to the right hand side.

Galvanic cell1. The device in which chemical energy is

converted into electric energy is called galvanic cell.

2. In this cell current is produced as a result of chemical reactions.

3. Spontaneous oxidation-reduction reactions take place.

4. In this cell electric conductin takes place.

5.Daniel’s cell and fuel cell are examples.

Electrolytic cell1. The device in which electric energy is

converted into chemical energy is called electrolytic cell.

2. In this cell current is used to drive a chemical reaction.

3. Non-spontaneous oxidation reduction reactions take place.

4. In this cell, electrolysis takes place.5. Down’s cell and Nelson’s cell are

examples.

Mohr’s method:Chloride and Bromide:-The determination of chloride in neutral solution by

titration with standard 0.1M silver nitrated. If the solution is acid, neutralization may be effected with chloride free calcium carbonated, sodium tetraborate. Mineral acid may also be removed by neutralizing most of the acid with ammonia solution and then adding an excess of ammonium ethanoated.

Volhard’s method:

Determination of chloride:-The chloride solution is treated with excess of standard silver nitrate solution, and the residual silver nitrate determined by titration with standard thiocyanate solution. Now silver chloride is more soluble than thiocyanate, and would react with thiocyanate. It is avoided by the addition of a little nitrobenzene.

Determination of bromide:-Bromide can be determined by volhardmethod, but as silver bromide is less soluble than silver thiocyanate, it is not necessary to filter of the silver bromide. The bromide solution is acidified with dilute nitric acid, an excess of standard 0.1M silver nitrate added.

Determination of iodides:-Iodides can be determined by volhard method, and in this case too there is no need to filter off the silver halide, since silver iodide is very much less soluble than silver thiocynate. In this determination the iodide solution must be very dilute in order to reduce adsorption effects.

Characteristics of Good Gravimetric Techniques

Ideally Precipitates in a gravimetric analysis should be:

I. Insoluble, Ksp should be way below the amount expected in solution. Limiting factor for detection is usually the balance, not Ksp. (For larger samples, this may not be true.)

II. Easily filterable, make large crystals which separate from solution easily

III. Pure with known composition, that is, they do not make a whole bunch of insoluble complexes.