The volhard method is a procedure for titrating silver(I) with standard potassium thiocyanate (KSCN). The titration is carried out in acidic solution. When the silver(I) has been precipitated as white silver thiocyanate, the first excess of titrant and the iron(III) indicator react and form a soluble red complex. The method is widely used for silver and chloride, because the titration can be done in an acidic solution. The titration of silver with NH4SCN with ferric alum as an indicator is an example of the class of titration involving the formation of a coloured substance in the solution. During titration AgSCN is formed while at the end point excess NH4SCN reacts with Fe(III) to form deep red [FeSCN]2+. The amount of thiocyanate which will give a visible colour is very small. Thus, the end point error is very small but the solution should be shaken vigorously at the end point as silver ions are absorbed on the precipitate and are then desorbed. In Volhards method, one can easily determine chloride ions in acidic solution, otherwise in basic media Fe3+ will hydrolyse. An excess of AgNO3 is added to chloride solution and part of it is unreacted. Ag solution is back titrated with ferric alum as indicator, but the method has a source of error. AgSCN is less soluble than AgCl, So AgCl + SCN– ⇾ AgSCN + Cl– This will consume more NH4SCN and chloride content will appear lower. This error can be eliminated by filtering off AgCl precipitate before back titration or if a little nitrobenzene is added, it will adhere to AgCl and protect it from reaction with thiocyanate however, nitrobenzene slows down the reaction. This can be avoided if Fe(NO3)3 and a small measured amount of NH4SCN are added to the chloride solution at the start with HNO3 and the mixture titrated with AgNO3 until the red colour disappears. The Volhard method is designed for the direct titration of silver in dilute nitric acid with standard thiocyanate solution. Added ferric ion forms the clear, red colored ion [FeSCN]2+, at the precipitation endpoint. Here, chloride can be determined indirectly by adding a known excess of standard silver solution, then back titrating with the Volhard method. This is a less desirable approach. However, since the ethacrynate back titrant tends to react slightly with the AgCl to form the even more stable precipitate AgSCN. As with the Mohr method, a blank or a titer is advised for both the direct and indirect approaches. In the Volhard method a standard solution of thiocyanate ion (SCN) is titrated against Ag+ using Fe3+ as the indicator. In the Volhard method silver ions are titrated with a standard solution of thiocyanate ions. The volhard method is an indirect or back titration method in which an excess of a standard solution of silver nitrate is added to a chloride containing sample solution. The excess silver is then back titrated using a standardized solution of potassium or ammonium thiocyanate with ferric ion as an indicator. The amount of silver that is precipitated with chloride in the sample solution is calculated by subtracting the excess silver from the original silver content. Ag+ + Cl– ⇾ AgCl Ag+ + SCN– ⇾ AgSCN SCN– + Fe3+ ⇾ [FeSCN]2+
The Volhard method is a back-titration developed to tackle this problem. For this process, using excess uniform silver nitrate, chlorides are precipitated to the solution. The silver nitrate that is used is created by the nitric acid reaction of silver foil.Volhard Method Procedure
Volhard Method in Precipitation Titration
Volhard Method Calculation
The chloride ion concentration is calculated by subtracting the titration findings of the silver ion moles which reacted with thiocyanate from the total moles of silver nitrate added to the solution. This approach is used when the solution’s pH is acidic, after the sample is prepared.
Volhard process for assessing chlorine, bromine, and iodine in the form of halides by precipitating them with excess silver nitrate and using a thiocyanate solution to titrate excess.
The expression ‘excess’ is used because the moles of added silver nitrate are considered to surpass the moles of sodium chloride present in the sample so that all the present chloride ions react. The indicator Fe3+ (ferric ion) is then added, with the potassium thiocyanate solution titrated to the water.
Argentometry is a form of titration which involves the silver(I) ion in analytical chemistry. This is usually used to determine the chloride content present in a sample. A solution of silver nitrate of known concentration is titrated against the sample solution.
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Pharmaceutical Analysis I Practical: Limit test for (1) Chloride (2) Sulphate (3) Iron (4) Arsenic Preparation and standardisation of (1) Sodium hydroxide (2) Sulphuric acid (3) Sodium thiosulfate (4) Potassium permanganate (5) Ceric ammonium sulphate Assay (1) Ammonium chloride (2) Ferrous sulphate(3) Copper sulphate (4) Calcium gluconate (5) Hydrogen peroxide (6) Sodium benzoate (7) Sodium Chloride Determination of Normality (1) Conductometric titration of strong acid against a strong base (2) Conductometric titration of strong acid and weak acid against a strong base (3) Potentiometric titration
Aim
To determine the percent purity of sodium chloride in given sample
Principle
Sodium chloride is determined by Volhard’s method, it is a back type of precipitation titration which is characterized by formation of soluble colored compound at the end point. Sodium chloride is first treated with a known excess of silver nitrate to form a precipitate of silver chloride and the remaining silver ions are titrated with thiocyanate in acid solution using ferric ion as an indicator. Initially excess Ag+ ions react with SCN– and silver thiocyanate is precipitated. After equivalence point, when no Ag+ is present, excess of thiocyanate added reacts with Fe3+ to give reddish brown ferric thiocyanate. This method is precise and most commonly used for determination of halides. Nitrobenzene is used to coagulate the precipitate of silver chloride.
Titration is carried out in an acidic medium to prevent hydrolysis of Fe3+ ions added as an indicator. As AgSCN has a tendency to adsorb Ag+, the endpoint will be reached earlier than the equivalence point. Hence to avoid this, the solution is stirred continuously till the indicator color change is permanent.
The solubility product of AgCl is lower than AgSCN.Hence AgCl is more soluble. Hence excess of SCN– reacts with AgCl when no Ag+ remains.
AgCl + NH4SCN AgSCN + NH4Cl
The above reaction takes place before reaction between Fe3+ and SCN– and thus endpoint will be observed after equivalence point which introduces titration error. To minimize this, reaction between AgCl and SCN– should be prevented in the following manner:
- AgCl ppt is filtered off before back titration. The ppt should be boiled for few minutes to coagulate it so as to remove adsorbed Ag+ ions from ppt and send it back to solution. The cold titrate is used for titration.
- KNO3 can be added as coagulant after addition of excess of AgNO3.
- Nitrobenzene is added to form coating over AgCl which prevents its reaction with thiocyanate.
Standard
Sodium chloride (Mol. wt.: 58.4) contains not less than 99.0 percent and not more than 100.5 percent of sodium chloride calculated on a dried basis.
Procedure
Preparation of 0.1M silver nitrate
Dissolve g. in sufficient water to produce 1000ml
- Standardization of 0.1M silver nitrate
Weigh accurately about 0.1g of sodium chloride previously dried at 1100C for 2 hours and dissolve in 5ml of water. Add 5ml of acetic acid, 50 ml methanol and 0.15ml of eosin solution. Stir and titrate with silver nitrate solution
Preparation of 0.1M Ammonium thiocyanate
Dissolve 7.612g of Ammonium thiocyanate in sufficient water to produce 1000ml
- standardization of 0.1M Ammonium thiocyanate
pipette 30 ml of 0.1M silver nitrate into a glass stoppered flask ,dilute with 50ml of water, add 2ml of nitric acid and 2ml of ferric ammonium sulphate solution and titrate with ammonium thiocyanate soluition to first appearance of red-brown colour.
Dissolve 0.1 g in 50 ml of water in a glass stoppered flask. Add 50 ml of 0.1M silver nitrate, 5 ml of 2M nitric acid and 2 ml of dibutyl phthalate, shake well and titrate with 0.1M ammonium thiocynate using 2 ml of Ferric ammonium sulphate solution as indicator, until the colour becomes reddish yellow.
Reference
Indian Pharmacopoeia 2014; Vol. III; Pg. No. 2740-2741
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