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Detection and Estimation of Selenium





Detection of Selenium

Selenium is recognisable by the characteristic odour, resembling that of rotten radishes, which it emits when warmed, and by its combustion with a blue flame, forming the dioxide. The element and most of its compounds impart a bright blue coloration to the Bunsen flame, and a piece of cold porcelain held in the flame may receive a red deposit of the element in the reducing zone or a white deposit of the dioxide in the oxidising zone. By heating with sodium carbonate in the reducing flame, selenium compounds can be made to yield sodium selenide, from which the characteristic odour of hydrogen selenide will be developed on the addition of acid.

A wet test to identify traces of selenium has been described by Deniges. The reagent used is a solution of mercurous nitrate prepared by dissolving 10 grams of the salt in a mixture of 10 c.c. of concentrated nitric acid and 100 c.c. of water. Selenious acid and its salts yield a crystalline precipitate of mercurous selenite either immediately or on cooling. This precipitate consists of characteristic long needle-shaped crystals. Solutions containing 0.1 per cent, of the acid will give this reaction. Selenic acid and its salts when treated with an equal bulk of the reagent give a precipitate of mercurous selenate which rapidly becomes crystalline and under the microscope consists of small clusters.

Small quantities of selenium in solution may be detected by the test described by Meunier, which is similar to Marsh's test for arsenic. If there is a relatively large amount of selenium present it is readily detected by its red colour, but if the amount of the element be small the deposit closely resembles that of arsenic, and in this case the following process may be adopted to detect the selenium. A current of hydrogen sulphide is passed into the hot solution containing a little sulphurous acid. The precipitate of finely divided sulphur carries down with it any selenium present and arsenic as arsenious sulphide, and it is clotted by stirring. In the presence of selenium the clots are brown; they may be dried and the sulphur and selenium separated by careful sublimation in a closed tube.

Selenium may also be detected in solution by the addition of a little concentrated sulphuric acid and a few small crystals of hydrazine sulphate. On warming, a red coloration or precipitate indicates the presence of the element, the reaction being sensitive to 5 mgms. of selenium per litre.

To detect selenious acid in the presence of selenic acid, 3 c.c. of the solution may be treated with 5 c.c. of concentrated sulphuric acid, a little sodium sulphite added and the mixture boiled. A red coloration indicates the presence of selenious acid. After filtering off any precipitate, 3 c.c. of concentrated hydrochloric acid and more solid sodium sulphite are added and the solution again boiled; a red coloration or precipitate then indicates selenic acid. The sensitiveness of this test is the same as for the preceding test.

The presence of selenium in sulphuric acid may be detected by distilling with a few c.c. of bromine water containing potassium bromide. The distillate, collected in a few c.c. of concentrated hydrochloric acid and saturated with sulphur dioxide and cooled, gives a precipitate of the element if present. By this means 1 part of selenium in 107 parts of acid can be detected. Another method depends upon the addition of aspidospermine, which gives an intense violet colour when selenium is present. This test is less satisfactory than the preceding one, however, since the colour varies with acid concentration and time of heating. Moreover, although pure sulphuric acid gives no coloration with aspidospermine, if an oxidising agent such as potassium chlorate or lead dioxide be present, a rose-red coloration develops. A process that will detect 0.0005 per cent, of selenious acid in sulphuric acid involves the use of either morphine or codeine. As an example of the test, mix 1 c.c. of a 0.1 per cent, aqueous solution of selenious acid with 200 c.c. of pure concentrated sulphuric acid; 10 c.c. of this mixture will with 0.1 gram of codeine phosphate after one minute produce a distinct green coloration and after fifteen minutes a bluish-green colour. Smaller quantities of the selenious acid mixture will produce the same colour phenomena.

Against a white surface this colour reaction may be observed even with a 0-00001 per cent, concentration of selenious acid. The reaction is more sensitive with sulphuric acid above 75 per cent, concentration than with dilute acid. The test is not applicable if the sulphuric acid contains iron. Tellurous acid retards the reaction.

Still another method for the detection of small quantities of selenious acid consists in adding sodium hydrosulphite, Na2S2O4, to the solution, which is then neutralised with solid sodium carbonate. The presence of the least trace of selenium causes a red coloration due to the colloidal element.

A colorimetric method for the detection and estimation of small quantities of selenious acid, which is sensitive to 0.002 per cent., may be carried out as follows: Into two cylinders are put, respectively, 5 c.c. of the solution to be tested and 5 c.c. of a solution of selenious acid of approximately the same strength. 70 c.c. of water are added to each and also a drop of gum arabic; then 5 c.c. of 5 per cent, hydrochloric acid solution and the whole diluted to 99 c.c. 1 c.c. of a solution of potassium iodide is added to each tube and the contents vigorously stirred. The respective colorations (due to iodine and colloidal selenium) are compared after five minutes in a Kriiss colorimeter.


Estimation of Selenium

The general method for the estimation of selenium consists in oxidising it to selenious acid and subsequently precipitating it as the element by means of a suitable reducing agent, such as sulphurous acid or hydrazine hydrate. The red selenium produced is converted to the more granular grey variety by boiling, and can then be filtered on to a Gooch crucible, dried at 105° C. and weighed.

Hydrazine hydrochloride or sulphate as the reducing agent is also said to give good results, when the volume of nitrogen evolved is measured:

SeO2 + N2H4 = Se + 2H2O + N2.

The gas is collected over boiled water. The evolution is soon complete and stops abruptly, so that the end of the reaction is clearly seen. This method is available for small quantities of selenium, since 1 gram of the dioxide yields 201.83 c.c. of nitrogen at N.T.P.

Hypophosphorous acid in slightly alkaline solution reduces selenic acid quantitatively to the element, which can be separated, collected, dried and weighed. If the selenium is originally in a high stage of oxidation it should first be reduced by boiling with hydrochloric acid until no more chlorine is evolved.

A rapid method of dealing with the precipitated selenium is to wash it and dissolve it in a cold saturated solution of sodium sulphide. The red colloidal solution obtained can be titrated with a standard potassium cyanide solution, the end-point being marked by a change of colour from red to yellow:

Se + KCN = KSeCN.

Selenious acid readily decomposes potassium permanganate, but analytical results are untrustworthy in the presence of more than a limited quantity of sulphuric acid. The oxidation should be carried out at 50° C., a known quantity of 0.1N potassium permanganate being used and the excess .determined either by means of standard oxalic acid solution or by electrometric titration with ferrous sulphate. In the presence of tellurium, the latter is also oxidised and should be determined in a separate sample by oxidation with potassium di-chromate, which does not oxidise the selenium, and the necessary deduction can then be made.

By the interaction of a thiocyanate and selenious acid in the presence of hydrochloric acid, thiocyanoselenious acid is formed:

NH4CNS + HCl = HCNS + NH4Cl,
2HCNS + H2SeO3 = (HCNS)2.H2SeO3.

This compound is rapidly decomposed by hot water with the deposition of amorphous selenium, which may be filtered off, dried and weighed. This amorphous selenium, however, may contain some admixed sulphur, which should be estimated by treatment with aqua regia and precipitation with barium chloride, and then allowed for.

A method involving electrometric titration depends on the volumetric reduction of selenious acid to selenium by means of titanium sulphate. The method is rapid and accurate if the solution is in cold concentrated hydrochloric acid saturated with sodium chloride. The presence of the latter is important, for it ensures rapid and uniform coagulation of the selenium hydrosol and increases the sharpness in the change of voltage at the end-point. The use of the hydrochloric acid in the cold eliminates the otherwise almost inevitable loss of selenium by volatilisation. Under these conditions any tellurium which may be present is unreduced and only has the effect of modifying the nature of the end-point.

Estimation of Selenium in Organic Compounds

A trustworthy method consists in oxidising the substance with fuming nitric acid, refluxing with concentrated hydrochloric acid until the nitrous fumes disappear and then precipitating the selenium with sodium sulphite.

The Carius method can also be adapted for the estimation of selenium in organic compounds. About 0.2 gram of the substance is treated with nitric acid (sp. gr. = 1.4) and about 0.5 gram of silver nitrate. The contents of the tube are transferred to a basin, evaporated to dryness, washed with alcohol and then dissolved in 20 per cent, nitric acid.

This solution is then diluted and titrated with standard potassium thiocyanate solution, using ammonium iron alum as indicator.

Another method consists in burning the organic substance in oxygen in the presence of platinum as catalyst; the selenium is oxidised to selenious acid, which is titrated with N/100 sodium hydroxide. If sulphur is present in addition to selenium the mixed sulphuric and selenious acids obtained are titrated, the sulphuric acid being afterwards estimated separately as barium sulphate. If, however, the substance contains either nitrogen or a halogen, titration is useless. The selenious acid should, in this case, be reduced by means of sodium sulphite in the presence of a large amount of hydrochloric acid. The selenium thus formed may be weighed.

In the absence of the halogens the selenium may be oxidised to selenious acid by heating with nitric acid in a Carius tube at 240° to 300° C. The resulting liquid is nearly neutralised, excess of zinc oxide added, and the mixture titrated with silver nitrate using chromate as external indicator:

H2SeO3 + 2AgNO3 + ZnO = Ag2SeO3 + Zn(NO3)2 + H2O.

A blank titration against a suspension of zinc oxide is necessary.

Estimation of Selenium in Sulphide Minerals

In various sulphite- cellulose manufactories difficulties have occurred which have been traced to the presence of selenium in the pyrites used for burning. Part of the selenium remains in the burnt pyrites and part volatilises with the sulphur dioxide. 20 to 30 grams of pyrites are dissolved in hydrochloric acid (dens. = 1.19) and potassium chlorate. Zinc is added to reduce the iron to the ferrous condition; more hydrochloric acid is then added, the solution boiled and stannous chloride added to precipitate selenium. Since the selenium may contain arsenic, it is collected on an asbestos filter, dissolved in potassium cyanide and reprecipitated using hydrogen chloride and sulphur dioxide. The element may then be estimated by the iodometric method described below. In order to determine the relative proportion of volatile to non-volatile selenium, the pyrites may be roasted in a current of oxygen. After this treatment the contents of the tube are dissolved in warm potassium cyanide and the selenium reprecipitated and estimated in the ordinary way.

In the iodometric method for the quantitative estimation of small quantities of selenium in pyrites use is made of the reaction

SeO2 + 4HI = Se + 2H2O + 2I2.

The dioxide should be dissolved in water and the solution diluted largely. A few drops of hydrochloric acid are added and the whole heated on a water-bath. A stream of carbon dioxide is introduced to replace the air and solid potassium iodide added. The flask should be corked, its contents shaken and kept in the dark for about an hour, when the liberated iodine may be titrated with standard thiosulphate. Care must be taken that the hydrochloric acid used is free from chlorine, and that the potassium iodide contains no iodate. The method is trustworthy if the amount of iodide used is four times the theoretical quantity, and it is applicable to solutions of selenious acid in general. The degree of accuracy is improved by the addition of carbon disulphide before the liberation of iodine. The latter then completely dissolves in the organic solvent and is not partly adsorbed on the precipitated selenium; hence the subsequent titration with thiosulphate is more easily accomplished.

The detection and estimation of selenium in the mother-liquors from the preparation of sulphite-cellulose is rendered difficult by the colour of these liquors, due to the organic substances present. The presence of the element is indicated by the fact that a red deposit is formed on the sides of the combustion tube when the organic matter is being burnt off. Estimation may be carried out by evaporating the liquor until all the free sulphur dioxide has been evolved, acidifying with hydrochloric acid and boiling to remove the last traces of sulphur dioxide. After allowing to stand in a warm place for a few days the selenium settles to the bottom of the container and may be estimated as described. The method of estimation of selenium in sulphur depends upon the fact that sulphur and selenium bromides are decomposed by water according to the equations:

2S2Br2 + 3H2O = H2SO3 + 3S + 4HBr,
SeBr4 + 3H2O = H2SeO3 + 4HBr.

The reaction with the selenium compound is much more rapid than that with the sulphur compound. Some of the selenious acid becomes oxidised to selenic acid. When these selenium acids are treated with excess of hydriodic acid they become reduced to selenium:

H2SeO4 + 6HI = 4H2O + 3I2 + Se,
and
H2SeO3 + 4HI = 3H2O + 2I2 + Se.

The selenium formed is collected, dried at 100° C. and weighed.
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