6. precipitation titrations

6. precipitation titrations

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  PRECIPITATION TITRATIONS G. Nikitha,M.Pharmacy Assistant Professor Department of Pharmaceutical Chemistry Sree Dattha Institute of Pharmacy Hyderabad Subject: Pharmaceutical Inorganic Chemistry Year: Pharm-D 1st Year
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  CONTENTS  Learning Objectives Introduction  Determination of end point  Mohr’s Method  Volhard’s Method  Fajan’s Method  Application  Conclusion  Reference
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  LEARNING OBJECTIVES In thesetopic we will discuss about  Introduction and definition of Precipitation Titration  Determination of end point  Mohr’s Method Introduction  Volhard’s Method Introduction  Fajan’s Method  Applications
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  INTRODUCTION Titration involving precipitationare known as precipitation titrations Most common types involve the reactions of metallic halides with silver nitrate. Such precipitation titration also known as argentimetric processes. Nacl + AgNO3 AgCl +NaNO3 precipitation is the process of conversion of a solution into solid by converting the substance into insoluble form
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  DETERMINATION OF ENDPOINT Three different methods are used by which end point in argentimetric titration can be determined. In these three methods different visual indicators have been used. 1. Mohr’s Method 2.Volhard’s Method 3.Fajan’s Method
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  MOHR’S METHOD  Thismethod was developed by Mohr in 1856 using Potassium Chromate as indicator.  This method involves the titration of silver nitrate against halides in neutral solution using Potassium Chromate as indicator. The end point is marked by the appearance of brick red Precipitate due to the formation of silver chromate with an Excess of silver nitrate. Ag+ + Cl- AgCl 2Ag+ + CrO4 2- Ag2CrO4
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   This methodis based on the fact that silver halide is more insoluble than silver chromate. Hence so long as there is any chloride or bromide left in the solution no silver chromate is formed. In fact silver chromate even if formed will immediately change to silver chloride or bromide by interaction with chloride or bromide Example: Ag2CrO4 + 2Cl- 2AgCl + CrO4 2- When the whole of chloride has been used up sliver chromate will be formed and brick red coloration or precipitate will appear.
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  Principle: This method determinesthe chloride ion concentration of a solution by titration with silver nitrate. As the silver nitrate solution is slowly added, a precipitate of silver chloride forms. The end point of the titration occurs when all the chloride ions are precipitated.
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  VOLHARD’S METHOD  Themethod was designed by volhard (1874) for the Estimation of silver in dilute nitric acid solution by titration Against a standard thiocynate solution in the presence of ferric salt as indicator.  But latter on it has been extended to the estimation of Chlorides, bromides and several other indirect analysis.  When a standard solution of ammonium or potassium thiocynate is added from a burette to a solution of silver salt in presence of nitric acid and ferric indicator in the titration flask a precipitate of silver thiocyanate continues to be formed till the silver gets completely precipitated. Ag+ +CNS- AgCNS
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  The addition ofa further drop of thiocyanate reacts with the ferric ion to form a red coloured ferric thiocyanate complex Fe +3 +3 CNS- Fe(CNS) 3 The appearance of a reddish brown coloration makes the end point. For a halide solution having acidic medium potassium chromate cannot be used as indicator. Volhard’s method is used for acidified halide solutions.
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  Preparation and standardizationof 0.1N AgNO3 solution:  It is prepared by dissolving 17.5g silver nitrate in 1000ml of distilled water. Then it is standardized by titration with sodium chloride (0.1 N) solution.  Similarly compounds of silver and mercury may be measured by direct titration with standard ammonium thiocyanate solution. Ammonium thiocyanate precipitates mercury and silver salt as thiocynates.  At the end point ammonium thiocyanate reacts with ferric alum indicator to produce red coloured ferric thiocyanate as under: Fe(NH 4)(SO4)2 +3NH 4SCN Fe(SCN)3 + 2(NH4)2 SO4. The solution of silver and mercury are acidified with nitric acid avoiding the hydrolysis of ferric salt in neutral solution before titration with ammonium thiocyanate
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  Preparation and standardizationof 0.1 N Ammonium Thiocyanate solution: It is obtained by dissolving 8 g of ammonium thiocyanate in 1000ml of distilled water. For standardization 20ml of 0.1N AgNO3 solution is acidified with nitric acid. Then it is titration with an approximately prepared 0.1N ammonium thiocyanate solution by using ferric alum as indicator. The normality of NH4SCN is calculated.
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  FAJAN’S METHOD  Thismethod employs adsorption indicators for the Detection of end point.  This method was introduced by Fajan in 1923-1924 as A results of his vigorous studies on the nature of adsorption. The action of these indicators is based upon the simple fact That at end point the indicator gets adsorbed by the precipitate and during the process of adsorption a change in colour of the indicator will take place which may result in a substance of different colour. They are therefore termed as adsorption indicators.
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  Commonly used indicatorfor titrations of chloride against silver nitrate are: 1. Fluorescein 2. Dichlorofluorescein 3. Eosin 4. Di-iodo diethylfluorescein
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  CONCLUSION In these topicwe have discussed  Introduction and definition of Precipitation Titration  Determination of end point  Mohr’s Method Introduction  Volhard’s Method Introduction  Fajan’s Method  Applications
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  REFERENCE  Pharmaceutical Chemistry-Inorganic Volume-1 by G. R. Chatwal.  Essentials of Inorganic Chemistry by Katja A. Strohfeldt.  Indian Pharmacopoeia.  M.L Schroff, Inorganic Pharmaceutical Chemistry.  P. GunduRao, Inorganic Pharmaceutical Chemistry, 3rd Edition  A.I. Vogel, Text Book of Quantitative Inorganic analysis.  Bentley and Driver's Textbook of Pharmaceutical Chemistry.
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