Silver Volhard determination

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 most important application of the Volhard method is the indirect determination of halide ions. A measured excess of standard silver nitrate solution is added to the sample, and the excess silver is determined by back-titration with a standard thiocyanate solution. The strong acidic environment required for the Volhard procedure represents a distinct advantage over other titrimetric methods of halide analysis because such ions as carbonate, oxalate, and arsenate (which form slightly soluble silver salts in neutral media but not in acidic media) do not interfere. 

Butylchloral hydrate does not yield chloroform on reacting with alkali, the products of decomposition being dichloropropylene, hydrochloric acid and formic acid. In mixtures, estimation follows the lines of that for chloral hydrate. Back titration with acid after heating with N aqueous potassium hydroxide for half an hour gives unreliable figures, but a Volhard determination of the chloride formed by this treatment is satisfactory only one chlorine atom is replaced. 1 ml OTN silver nitrate = 0 01935 g. 

The %w/w K in a 0.6712-g sample was determined by a Volhard titration. After adding 50.00 mb of 0.05619 M AgNOa and allowing the precipitate to form, the remaining silver was back titrated with 0.05322 M KSCN, requiring 35.14 mb to reach the end point. Report the %w/w K in the 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. 

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. 

The purity of the crystallized product, determined volu-metrically by Volhard s method, exceeds 98%. In this procedure, 10 ml. of a 1% solution of methylisourea hydrochloride is acidified with a few drops of nitric acid and treated with 20 ml. of 0.1 N silver nitrate. After removal of the silver chloride by filtration, the excess of the silver nitrate is estimated with 0.1 TV thiocyanate solution, using ferric alum as indicator. Alternatively, 10-ml. portions of 0.1 N silver nitrate, acidified with nitric acid, may be titrated directly with the 1% methylisourea hydrochloride solution in the presence of tartrazine. 

Chlorides, bromides, and iodides can be quantitatively determined by treatment with silver nitrate, and, with suitable precautions, the precipitated halide is washed, dried, and weighed. Chlorides in neutral soln. can be determined by F. Mohr s volumetric process 27 by titration with a standard soln. of silver nitrate with a little potassium chromate or sodium phosphate as indicator. When all the chloride has reacted with the silver nitrate, any further addition of this salt gives a yellow coloration with the phosphate, and a red coloration with the chromate. In J. Volhard s volumetric process, the chloride is treated with an excess of an acidified soln. of silver nitrate of known concentration. The excess of silver nitrate is filtered from the precipitated chloride, and titrated with a standard soln. of ammonium thiocyanate, NH4CN8—a little ferric alum is used as indicator. When the silver nitrate is all converted into thiocyanate AgN03-fNH4CNS=AgCNS +NH4NOS, the blood-red coloration of ferric thiocyanate appears. 

Silver thiocyanate, formed as a white, flocculent precipitate by double decomposition, is insoluble in dilute mineral acids. It is upon the formation of this salt that Volhard s volumetric method for the determination of silver or of thiocyanate (see p. 283) depends. 

Chlorine.—1 gram of the substance is fused with nitre and sodium carbonate, the mass being dissolved in water, the solution acidified with nitric acid, and the chlorine determined either gravimetrically as silver chloride or volumetrically by Volhard s method. 

Often, greater accuracy may be obtained, as in Volhard type titration, by performing a back titration of the excess silver ions. In such a case, a measured amount of standard silver nitrate solution is added in excess to a measured amount of sample. The excess Ag+ that remains after it reacts with the analyte is then measured by back titration with standard potassium thiocyanate (KSCN). If the silver salt of the analyte ion is more soluble than silver thiocyanate (AgSCN), the former should be filtered off from the solution. Otherwise, a low value error can occur due to overconsumption of thiocyanate ion. Thus, for the determination of ions (such as cyanide, carbonate, chromate, chloride, oxalate, phosphate, and sulfide, the silver salts of which are all more soluble than AgSCN), remove the silver salts before the back titration of excess Ag.+ On the other hand, such removal of silver salt is not necesary in the Volhard titration for ions such as bromide, iodide, cyanate, thiocyanate, and arsenate, because the silver salts of these ions are less soluble than AgSCN, and will not cause ary error. In the determination of chloride by Volhard titration, the solution should be made strongly acidic to prevent interference from carbonate, oxalate, and arsenate, while for bromide and iodide analysis titration is carried out in neutral media. 

Chlorine is determined by a modified Volhard procedure.3 At the beginning of the analysis, the aliquot is made strongly acid with about 10 ml. of freshly boiled concentrated nitric acid. This excess nitric acid prevents the phosphite from reducing the silver ion. 

In a second series of six experiments the chlorine in the neutral chloride was determined volumetrically by Volhard s method after removing the cobalt, with potassium carbonate. The ratio of cobalt to silver was thus established and checked by two experiments in which cobalt was allowed to displace silver from silver sulphate solution ... 

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. 

Silver chloride is more soluble than silver thiocyanate. As a consequence, in chloride determinations by the Volhard method, the reaction... 

Table 13-3 lists some typical applications of precipitation titrations in which silver nitrate is the standard solution. In most of these methods, the analyte is precipitated with a measured excess of silver nitrate, and the excess is determined by a Volhard titration with standard potassium thiocyanate. 

Challenge Problem. The Volhard titration for Ag is being evaluated for use in determining silver in a thiosulfate photographic fixing bath. An independent analysis of the bath solution by atomic absorp-... 

A second example of this type of indicator is illustrated in the Volhard titration. This is an indirect titration procedure for determining anions that precipitate with silver (Cl , Br , SCN"), and it is performed in acid (HNO3) solution. In this procedure, we add a measured excess of AgNOa to precipitate the anion and then determine the excess Ag" " by back-titration with standard potassium thiocyanate solution ... 

EXPERIMENT 10 DETERMINATION OF SILVER IN AN ALLOY VOLHARD S METHOD... 

The test for the determination of chlorine is performed by burning the coal mixed with Eschka mixture in an oxygen bomb followed by potentiometric titration with silver nitrate solution or by a modified Volhard colorimetric titration. The repeatability interval for either procedure is the same. 

Chloride may be determined by the Volhard method. The sample is acidified with concentrated nitric acid and silver nitrate is added, which reacts with the chloride, and then the mixture is boiled. After cooling, the pale yellow solution is diluted and the excess silver nitrate titrated with potassium thiocyanate solution using ammonium ferric sulfate as indicator. The ash may also be used for this determination. 

Butter Butter should be analyzed for wateg salt, and butterfat content. Water can be determined by the usual drying procedure and salt by Volhard titration. Alternatively, salt may be measured by diluting it in hot water and titrating with silver nitrate using potassium chromate indicator. 

Sodium chloride levels are routinely determined in canned vegetables and legumes. Titrimetric methods are most commonly used with the sodium chloride first isolated by either ashing at 500-550°C, followed by aqueous dissolution of the ash and titration with silver nitrate solution (Mohr method), or by boiling the food in dilute nitric acid, adding excess silver nitrate and back-titration with potassium thiocyanate (Volhard method). Of these, the latter method is generally more accurate than the former method however, it is also more time-consuming. 

Because the Volhard method is a titration of Ag", it can be adapted for the determination of any anion that forms an insoluble silver salt. 

Saturated solutions were prepared at a temperature about 5 K higher than that of the Isotherm to be studied. The samples were equilibrated for 4 hours with constant stirring. After being quiescent for 1 hour the phases were separated from each other and samples were taken for analysis. Silver content was determined by the Volhard method HP0 ions were precipitated with excess BiCNO,), and the Bl " " ions were back titrated with Komplexon III. 

Iodide ions can be determined quite well with argentimetric titrations. The Volhard method, electrometric endpoint indications, and adsorption indicators work well. The Mohr endpoint indication, however, does not give good results because of the adsorption of the chromate on the silver iodide precipitate. The presence of chloride and bromide ions disturbs the argentimeric iodide determination. 

Following Charpentier-Volhard s method (1874), a nitric acid solution and a known quantity in excess of silver nitrate solution are added to the medium under study. Silver ions in excess are determined by titration with a standard solution of an alkaline... 

Fig. 37.1 Error made in the determination of chloride ions by Charpentier-Volhard s method when the silver chloride precipitate is not removed before the addition of thiocyanate ions...

To about 0-4 g add 25 ml of 95 per cent ethanol and warm on a water-bath until dissolved. Cool, add 10 ml of N alcoholic potassium hydroxide made with 95 per cent ethanol, swirl gently and allow to stand for ten minutes. Dilute to 150 ml with water, neutralise with dilute nitric acid and add 10 ml in excess, followed by 50 ml of 0 1 N silver nitrate. Complete by Volhard s method (see under Halogen Acids, p. 290). Carry out a blank determination on about the same amount of sample as above (10 mg), adding the same volume of all reagents in the order, dilute nitric acid, ethanol, potassium hydroxide solution and silver nitrate. 1 ml 0-1N 0 009695 g of CgHgCle. Alternatively the hydrolysed chlorine may be determined electrometrically. 

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