Volhard method

Volhard method Titration of Ag with NCS" in the presence of Fe. A deep red colour is formed at the end-point. 

A second end point is the Volhard method in which Ag+ is titrated with SCN in the presence of Fe +. The end point for the titration reaction... 

For Volhard methods identified by an asterisk ( ) the precipitated silver salt must be removed before carrying out the back titration. 

The analysis for I using the Volhard method requires a back titration. A typical calculation is shown in the following example. 

A 0.1093-g sample of impure Na2C03 was analyzed by the Volhard method. After adding 50.00 ml of 0.06911 M AgN03, the sample was back titrated with 0.05781 M KSCN, requiring 27.36 ml to reach the end point. Report the purity of the Na2C03 sample. 

The exact amount of silver oxide required may be determined by titrating a sample of the solution with silver nitrate by the Volhard method. 

Bromides can also be determined by the Volhard method, but as silver bromide is less soluble than silver thiocyanate it is not necessary to filter off the silver bromide (compare chloride). The bromide solution is acidified with dilute nitric acid, an excess of standard 0.1M silver nitrate added, the mixture thoroughly shaken, and the residual silver nitrate determined with standard 0.1 M ammonium or potassium thiocyanate, using ammonium iron(III) sulphate as indicator. 

The end points of precipitation titrations can be variously detected. An indicator exhibiting a pronounced colour change with the first excess of the titrant may be used. The Mohr method, involving the formation of red silver chromate with the appearance of an excess of silver ions, is an important example of this procedure, whilst the Volhard method, which uses the ferric thiocyanate colour as an indication of the presence of excess thiocyanate ions, is another. A series of indicators known as adsorption indicators have also been utilized. These consist of organic dyes such as fluorescein which are used in silver nitrate titrations. When the equivalence point is passed the excess silver ions are adsorbed on the precipitate to give a positively charged surface which attracts and adsorbs fluoresceinate ions. This adsorption is accompanied by the appearance of a red colour on the precipitate surface. Finally, the electroanalytical methods described in Chapter 6 may be used to scan the solution for metal ions. Table 5.12 includes some examples of substances determined by silver titrations and Table 5.13 some miscellaneous precipitation methods. Other examples have already been mentioned under complexometric titrations. 

In each class the problem may be resolved into two essential parts (i) the breakdown of the organic compound under appropriate conditions to give a quantitative yield of fluoride ions in aqueous solution, and (ii) the determination of the concentration of these fluoride ions. Methods of breaking down the organic compounds were examined and the procedure adopted for the phosphorofluoridate was different from that used for the fluoroacetate series. From both, however, sodium fluoride was obtained as the breakdown product containing all the fluorine present. After numerous preliminary experiments we came to the conclusion that on the macro-scale a very convenient method of determining the quantity of fluoride ions in the products was by precipitation as lead chlorofluoride,2 PbCIF, which was then dissolved in dilute nitric acid and the chloride was determined by the Volhard method and calculated to the equivalent amount of fluorine. We determined carefully the conditions for the quantitative precipitation of lead chlorofluoride. 

Applications of precipitation titrations are listed in Table 7-1. Whereas the Volhard method is an argentometric titration, the Fajans method has wider applications. Because the Volhard titration is carried out in acidic solution (typically 0.2 M HN03), it avoids certain interferences that affect other titrations. Silver salts of CO -, C20 -, and AsO " are soluble in acidic solution, so these anions do not interfere. 

Determine chlorine in a 25-ml. portion also. Add about 2 ml. of 6N nitric acid, and oxidize the excess sulfite exactly by the addition of a 1 per cent solution of potassium permanganate. Then determine chlorine by the Volhard method or any other that may be convenient. Several methods have been described for the analysis of nitrogen trichloride, which may also be used for monochloroamine.1... 

The analysis of chloride in water will be performed by a modified Volhard method. The principle of this method is the insolubility of AgCl in water. Thus when silver nitrate and sodium chloride are mixed, the net ionic reaction can be written as ... 

Halide Analysis by the Modified Volhard Method General Procedure - ... 

Br-, 1", AsOi Volhard method precipitate removal is unnecessary... 

For detailed explanations of the chemistry of specific prreipitation titrations the reader is referred to other sources. These include methods such as the Mohr method for halides using the silver chromate end point, the Fajans adsorption indicator method, the Volhard method for FeSCN end point, the titration of halide mixtures with the attendant solid-solution and adsorption effects, the titration of fluoride with thorium(IV), and the titration of sulfate with barium ion. 

Determination of Benzyl Bromide. The determination of this substance may be carried out by the method already described for benzyl chloride. However, according to Van der Laan, it is sometimes more convenient to decompose the substance directly with a measured volume of standardised alcoholic silver nitrate solution and to titrate the excess of the latter with ammonium thiocyanate solution by the Volhard method. 

Alternatively, when the precipitated silver(I) carbonate from the first stage of the preparation has been washed by decantation, a solution of 10 g (0.1 mole) of sulfamic acid in 50 mL of hot, distilled water is carefully added. The concentrated solution of silver(I) sulfamate is filtered rapidly with suction and, on cooling, deposits a large crop of tiny, transparent crystals. Purification is achieved by recrystallization from warm water. Anal. Calcd. for NHaSOaO Ag, 52.89. Found Ag, 52.73 (Volhard method). 

The Volhard method of endpoint detection involves using Fe " " ions as the indicator. This procedure requires a suitably acidic solution to prevent precipitation of iron(III) as the hydrated oxide. It has the disadvantage that it is useful only for the reaction ... 

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