Mohr’s method

Discussion. The method is applicable to the determination of a mixture of two salts having the same anion (e.g. sodium chloride and potassium chloride) or the same cation (e.g. potassium chloride and potassium bromide). For example, to determine the amount of sodium and potassium chlorides in a mixture of the two salts, a known weight (Wj g) of the solid mixture is taken, and the total chloride is determined with standard 0.1 M silver nitrate, using Mohr s method or an adsorption indicator. Let w2 g of silver nitrate be required for the complete precipitation of Wj g of the mixture, which contains xg of NaCl and yg of KC1. Then ... 

Now suppose that the determination of potassium chloride and potassium bromide in a mixture is desired. The total halide is determined by Mohr s method or with an adsorption indicator. Let the weight of the mixture be w3 g and w4 g, be the weight of silver nitrate required for complete precipitation,... 

International Standard Organization. 1989. Water quality. Determination of chloride. Silver nitrate titration with chromate indicator (Mohr s method). ISO 9297. International Organization for Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland. 

In volumetric analysis, hydrophilic colloids alter the end point, e.g., in a titration of HC1 and NaOH, the amount of deviation in the end point is increased with increasing amounts of colloids. In the volumetric estimation of silver by Mohr s method, the phenomenon of adsorption comes into being. 

In a back-titration of silver by chloride ions, potassium chromate can be used to indicate the endpoint (Mohr s method) but with potassium thiocyanate as tifrant, anunonium iron(III) sulfate ( ferric alum ) is preferred. In the direct titration (Gay-Lussac s method) the location of the turbidimetric endpoint has been improved in detail. ... 

Dr Clark, in Journ. Hoc. Chem. Ind., 1896, recommends for antimony ores and alloys a modification of Mohr s method t of titration with solution of iodine —... 

Atgentometry in Neutral or Weakly Alkaline Medium Mohr s Method... 

Mohr s method is a direct titration. It consists of making a silver nitrate solution to react with the solution containing the halide to be determined in neutral or weakly... 

According to the principle behind Mohr s method, at the equivalence point, two precipitation phenomena coexist. As a result, we can write... 

Mohr s method is essentially used for the titration of chloride and bromide ions. It is not satisfactory for iodide and thiocyanate ions because they exhibit a strong tendency to be adsorbed on the silver iodide and silver thiocyanate precipitates. Finally, some ions that give poorly soluble compounds with silver (see Sect. 37.1.2) may disturb the titration. 

Like Mohr s method, Fajans method is a direct titration method. It is based on the use of adsorption indicators. A small volume of an acetic acid solution and an adsorption indicator are added to the solution containing the halide to be determined. Then a silver nitrate solution is added. At the equivalence point, the precipitate suddenly turns colored and separates completely from the solution. 

In a first stage, it is hydrolyzed by an excess of concentrated sodium hydroxide. Then the solution is neutralized by a standard sulfuric acid solution in the presence of phenolphthalein. The liberated chloride ions are titrated according to Mohr s method. The reactions occurring during the hydrolysis are... 

The first comment that must be made is to assert that it is a prototropic back fixation with sulfuric acid, and not only an argentometric fittafion, although, in the procedure, a quantitative determination of chloride ions is mentioned. The fixation of chlorine ions with silver ions is used to be sure that an excessive consumption of hydroxide ions does not exist. In fact, it is just this excessive consumption of hydroxide ions that liberates the chloride ions from chloroform. In other words, the consumed hydroxide ions that correspond exactly to the concenXafion of chloral hydrate are those given in the first chemical equation. The second comment is that Mohr s method is particularly well adapted to determine the liberated chloride ions, due to the basic pH of the solution imposed by the use of phenolphthalein as indicator ... 

In alkaline medium, as with Mohr s method, the following reaction might occur ... 

For direct titration a number of alternative methods of end-point detection are possible. The oldest procedure, and still in use to a considerable extent for fairly pure samples, is Mohr s method. In this method potassium chromate is added to the chloride solution which is then titrated with silver nitrate. Because it is so much more insoluble than silver chromate, silver chloride first precipitates in a colloidal form which tends to coagulate as the end-point is reached immediately all the chloride is titrated silver chromate is formed as a brownish-red precipitate which gives a good indication of the end-point. It is important that the quantity of chromate added should not be too excessive or the red precipitate may form before all chloride is precipitated and so give a premature end-point. In very dilute or hot solution the end-point indication is poor, due to solubility of silver chromate. Mohr s method may be used for chlorides or bromides, but is unsatisfactory for iodides because (i) a mixed precipitate of iodide and chromate may be obtained and (ii) the colour of silver iodide masks that of the precipitating chromate. The following general procedure may be used ... 

Determjination of chloride content of a water sample by Mohr s Method (Argentometric)... 

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