Silver Volhard titration

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 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. 

Ammonium thiocyanate-silver nitrate titrations (Volhard s Method). 

AMMONIUM THIOCYANATE-SILVER NITRATE TITRATIONS (VOLHARD S METHOD)... 

The reactions of potassium thiocyanate in aqueous solution are essentially those of the thiocyanate anion. Its reaction with ferric ammonium sulfate, apphed in Volhard titration, results in the formation of ferric thiocyanate, Fe(SCN)3. Similarly, in titration against shver nitrate, it forms insoluble silver thiocyanate, AgSCN. 

Elemental composition K 40.23%, S 33.00%, C 12.36%, N 14.41%. Potassium can be measured in an aqueous solution by flame photometry, AA, or ICP/AES (see Potassium). Thiocyanate anion can be measured by Volhard titration against a standard solution of silver nitrate in the presence of ferric ammonium sulfate. The color of the solution turns red at the end point. 

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. 

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. 

Volhard titration, which involves the formation of a soluble coloured compound. This approach is exemplified by the quantitative analysis of chlorides, bromides and iodides by back titration. In this case, the halide is titrated with silver ... 

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 ... 

Chemical analysis methods maybe used for assay of silver alloys containing no interfering base metals. Nitric acid dissolution of the silver and precipitation as AgCl, or the Gay-Lussac-Volhard titration methods are used interchangeably for the higher concentrations of silver. These procedures have been described (4). 

This method of silver recovery can be applied to any silver precipitates from quantitative analysis, such as silver chloride, bromide, or thiocyanate. First wash the residues well with water by decantation and on the Buchner funnel, then spread out on paper to dry. A little nitrobenzene absorbed on the precipitate from the Volhard titration will not do any harm the greater part of the nitrobenzene will be removed by washing, in any case. 

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. 

The official methods of determining pharmaceutical salts of halogen acids are by the Volhard silver nitrate titration for chlorides and bromides and the Lang iodate method for iodides. The equivalents are given in Table 19 ... 

Indicators for silver-halide precipitation titrations are of two types. The first react specifically when an excess of titrant becomes present immediately after the end point - for example, if a small amount of potassium chromate is added, it will react with excess silver ions to produce deep red silver chromate in neutral solutions (Mohi s method). In acid solutions, the silver is titrated with potassium thiocyanate (KCNS) solution (Volhard s method). Iron (III) ammonium sulfate solution is added and reacts with an excesss of thiocyanate to produce a deep red iron thiocyanate species. 

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

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, D. of as silver bromide, (g) 491 by EDTA, (ti) 339 by mercury(I), (cm) 542 by oxygen flask, 113 by silver ion, (cm) 546 by silver nitrate, (ti) 351 by Volhard s method, (ti) 356 with iodide, (ti) 352 4-Bromomandelic acid 473 Bromophenol blue 265, 267 Bromopyrogallol red 182, 319 Bronsted-Lowry bases titration with strong acids, 277... 

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. 

Yolumetrically, thiocyanate is estimated by Volhard s method, which involves titration with standard silver nitrate solution containing nitric acid, ferric alum being used as indicator. Cuprous thiocyanate dissolved in ammonium hydroxide solution and acidified with dilute sulphuric acid may be titrated with permanganate.6 An iodometrie method has also been described.7... 

Silver sulfadiazine is dissolved in 65% nitric acid and the solution is diluted with water to a tenfold volume. The silver is assayed by the Volhard procedure titration with thiocyanate, indicator Fe +. 

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. 

Perchlorates. These may be present in appreciable quantity in conversion-saltpetre. The following method gives very accurate results 20 g. of the sample is heated to 545 in a covered nickel crucible and maintained for an hour at this temperature in a Gilbert furnace or electric furnace, to reduce Ihe perchlorate to chloride, which is then titrated with silver nitrate by Mohr s or Volhard s method. The chloride present in an unheated sample is deducted. Any chlorate or iodate present must also be allowed for, Lenze recommended heating for half an hour in a porcelain crucible at 580 to 600 . The first quarter of an hour is occupied in raising the temperature to the required point. At higher temperatures losses of chloride occur. Methods involving the addition of iron... 

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