Volhard’s Method

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

Discussion. This method is based upon the precipitation of lead chlorofluoride, in which the chlorine is determined by Volhard s method, and from this result the fluorine content can be calculated. The advantages of the method are, the precipitate is granular, settles readily, and is easily filtered the factor for conversion to fluorine is low the procedure is carried out at pH 3.6-5.6, so that substances which might be co-predpitated, such as phosphates, sulphates, chromates, and carbonates, do not interfere. Aluminium must be entirely absent, since even very small quantities cause low results a similar effect is produced by boron ( >0.05 g), ammonium (>0.5 g), and sodium or potassium ( > 10g) in the presence of about 0.1 g of fluoride. Iron must be removed, but zinc is without effect. Silica does not vitiate the method, but causes difficulties in filtration. 

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... 

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). 

Volhard s method is based on two major aspects, namely ... 

Explanation Chlorine combined originally to chlorobutol is being converted by hydrolysis in the presence of sodium hydroxide to ionic chloride that may be estimated quantitatively by Volhard s method in the presence of nitrobenzene. 

A good number of pharmaceutical substances can be assayed by Volhard s method and are mentioned in Table 8.2. 

Hydrolysis with ethanolic KOH helps in the conversion of organically combined chlorine to KC1 which after due acidification with HN03 is assayed by 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. 

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... 

Volhard s Method.—39 gms. potassium cyanide and 10 gms. caustic potash are dissolved in 100 c.cs. of water in a large flask. 63 gms. potassium permanganate dissolved in 1 litre of water are then added, drop by drop, from a funnel, the flask being placed in a freezing mixture. The temperature should not rise above 8°. This is filtered, and a solution of 80 gms. ammonium sulphate is added, and the whole evaporated to dryness, The residue is powdered and extracted with 80 c.cs. absolute alcohol under a reflux at boiling point for hour. It is then filtered and the residue washed with boiling alcohol. The alcohol is removed by distillation until the volume is about 50 c.cs. It is then placed in a glass dish and allowed to stand. The crystals which separate are filtered, washed with alcohol, and dried. A second crop of crystals can be obtained from the mother liquor. 

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. 

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 ... 

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... 

Another method34 involves fusion of the organosilicon with magnesium to produce magnesium silicide. This is then decomposed to produce gaseous silicon hydrides by addition of dil. sulphuric acid. The hydrides are absorbed in bromine water and thus hydrolysed to silicic acid. The latter is converted into molybdosilicic acid and determined colorimetrically as molybdenum blue. The error is 0.73 to 0.54% over the range 10.53 to 37.8% of silicon. After removal of silicon hydrides, the elementary carbon which separates is deactivated with ferric or aluminium salts and filtered off, when halogens can be determined in the filtrate by Volhard s method. 

Chlorides may also be estimated by Volhard s method, the aqueous extract being rendered slightly acid with nitric acid, a measured volume of N/10 silver nitrate solution added, and the... 

In addition, several analogues of the pulvinic acids, required for biological evaluation have been prepared by way of Volhard s method (96—J06). 

These theoretical considerations correspond to the last stage of Charpentier-Volhard s method with X = SCN . 

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