Thiocyanate Volhard titration

Volhard titration analychem Determination of the halogen content of a solution by titration with a standard thiocyanate solution. fol,hart tT tra-shan voltameter See coulometer. val tam-ad-ar ... 

Potassium thiocyanate is used in dyeing and printing textiles to make artificial mustard oil as a slimicide in paper production for controlling microbial growth in cooling water and in the preparation of organic thiocyanates. The salt also is used in analytical chemistry in Volhard titration. 

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. 

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. 

The indicator concentration is not critical in the Volhard titration. In fact, calculations similar to those shown in Feature 13-3 demonstrate that a titration error of one part in a thousand or less is possible if the iron(III) concentration is held between 0.002 M and 1.6 M. In practice, an indicator concentration greater than 0.2 M imparts sufficient color to the solution to make detection of the thiocyanate complex difficult because of the yellow color of Fe-. Therefore, lower concentrations (usually about 0.01 M) of iron(IIl) ion are employed. 

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. 

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 ... 

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. 

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. 

Gravimetric method such as AgCl or fire assay is used. Gay-Lussac method involves the titration of Ag with NaCl and/or KBr in acid solution. The endpoint is taken as the point at which no further precipitation of silver halide is observed upon the addition of more precipitant. Fajans method is based on the precipitation of Ag with chloride or bromide ions, using an adsorption indicator to detect the endpoint. Rhodamine 6G or dichlorofluorescein is used as the adsorption indicators. Volhard method is based on the titration of Ag with potassium or sodium thiocyanate to form insoluble silver thiocyanate. This titration is... 

Jacob Volhard (Darmstadt, 4 June 1834-Halle, 14 January 1910) was assistant to Liebig in Munich, Hofmann in London, and Kolbe in Marburg. He was assistant (1863), and associate professor (1869), in Munich, professor at Erlangen (1882) and Halle. He synthesised sarcosine, and creatinine by a method suggested by Strecker (1861), devised the thiocyanate silver titration, and (with H. Erdmann) synthesised thiophene by heating a mixture of sodium succinate and phosphorus trisulphide. ... 

PREPARATION AND USE OF 0.1 M AMMONIUM OR POTASSIUM THIOCYANATE TITRATIONS ACCORDING TO VOLHARD S 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. 

This method can naturally only be used when no other acids are formed. In that case the halogen is titrated with thiocyanate by Volhard s method. 

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

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 +. 

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. 

In the Volhard method, silver ions are titrated with a standard solution of thiocyanate ion ... 

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. 

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. 

According to some authors, silver iodide might be formed through an intermediary organic complex.) The silver ions in excess are titrated with ammonium thiocyanate according to the last stage of Charpentier-Volhard s procedure. 

For back-titration the method of Volhard is used and is probably the most widely employed technique for determination of halide ions. The halide is precipitated from a nitric acid solution by addition of excess silver nitrate and the excess is back-titrated with ammonium thiocyanate using ferric alum as indicator at the end-point the blood-red ferric thiocyanate is formed. Since silver thiocyanate is much less soluble than silver chloride, ammonium thiocyanate will attack the silver chloride precipitate and so cause erroneous results. For this reason the silver chloride must be removed or protected before titration of the excess silver nitrate is begun. This is best obtained by filtering off the precipitate before back-titration, but this step is time-consuming. As an alternative, some organic solvent may be added which will form a protective coating around the silver chloride and for this purpose nitrobenzene appears to be the most satisfactory. It should be remembered, however, that nitrobenzene is a powerful... 

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. 

An insecticide containing chlorine was digested in nitric acid to convert the chlorine to soluble chloride. Silver nitrate was added in excess, and the excess titrated with potassium thiocyanate by Volhard s method. It is important to know whether all the chlorine is converted to chloride. 

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