Detection and Estimation of Bismuth

Dry Tests

Bismuth compounds, when heated on a charcoal block, are reduced; a brittle globule of metallic bismuth is obtained, with a yellow incrustation of bismuth trioxide. If the original compound is mixed with powdered charcoal, potassium iodide and sulphur, and the mixture heated on a charcoal block, a scarlet incrustation is obtained.

When a mixture of metals containing bismuth is heated in a hard glass tube under a hard vacuum, a characteristic ring is formed on the tube indicating the presence of bismuth. With a vacuum of 0.001 mm. the presence of 0.01 per cent, of bismuth can be detected by this method.

Wet Tests

Bismuth is usually identified in solution by precipitation as brown trisulphide. The precipitation is effected by passing a current of hydrogen sulphide through a warm solution acidified with hydrochloric acid. The precipitate is insoluble in yellow ammonium sulphide, but is soluble in hot, dilute nitric acid and in sulphuric acid. The presence of bismuth may be confirmed by dissolving the precipitate of trisulphide in dilute nitric acid, reprecipitating the bismuth as hydroxide with ammonium hydroxide, redissolving the hydroxide in dilute hydrochloric acid, and allowing the clear solution of bismuth trichloride to drop slowly into excess of water. A copious white precipitate of bismuth oxychloride is obtained by hydrolysis.

Bismuth compounds are readily reduced by reducing agents such as formaldehyde in alkaline solution, hypophosphorous acid, or sodium or potassium stannite. In each case a black stain or residue is obtained, by means of which the presence of bismuth may be determined.

Potassium iodide reacts with solutions of bismuth salts to form black bismuth triiodide, which readily dissolves in excess of the reagent to form a yellow or orange solution which probably contains potassium iodobismuthate. This reaction may be employed as a spot test for the detection of traces of bismuth in the presence of copper, lead and cadmium.

An electrolytic method has been suggested for the detection of bismuth in slags. A piece of the slag is connected to the positive pole of a battery and an aluminium plate to the negative pole. If a filter paper, saturated with a solution of potassium iodide, is pressed between the slag and the aluminium plate, the presence of bismuth is indicated by the formation of a reddish-yellow coloration on the filter paper.

Bismuth may be detected in gold alloys by employing the streak test. The streak is dissolved in aqua regia, the metals are precipitated as sulphides by hydrogen sulphide, the sulphide precipitates are digested with yellow ammonium sulphide and finally dissolved in nitric acid. The presence of bismuth is detected by adding to a minute drop of this solution a drop of solution of potassium sulphate, when the double sulphate of bismuth and potassium is precipitated.

Many organic reagents have been employed for the detection of bismuth. A 2 per cent, solution of 8-hydroxyquinoline containing nitric acid or sulphuric acid is mixed with a 4 per cent, solution of potassium iodide immediately before use. This reagent produces a flocculent orange precipitate when added to a solution containing bismuth if the concentration of the bismuth exceeds 1 part in 100,000. The precipitate is soluble in a mixture of acetone and ammonium acetate, and in cyclohexane, and these solutions may be employed in the colori- metric determination of bismuth.

Other reagents that have been employed are methylephredine methiodide, viscose and sodium alizarinsulphonate.

Gravimetric Methods

As bismuth trioxide. Bismuth is precipitated from solution as basic bismuth carbonate, the reagent being ammonium carbonate. The precipitate is then ignited and weighed as bismuth trioxide. This method is not suitable for solutions which contain hydrochloric acid or sulphuric acid.
As bismuth oxychloride. In this method the solution containing bismuth, which is faintly acid, is just neutralised by ammonium hydroxide, care being taken to avoid precipitation. The solution after neutralisation may be opalescent. A small quantity of dilute hydrochloric acid is now added, and the precipitated oxychloride allowed to settle for a considerable time. The precipitate is finally dried at 100° C. and weighed as oxychloride.
As bismuth trisulphide. Hydrogen sulphide is passed through a warm, acid solution containing the bismuth, the trisulphide being precipitated. The precipitate is washed successively with a solution of hydrogen sulphide, alcohol, and freshly distilled carbon disulphide. Final washings are made with alcohol and ether, after which the precipitate is dried at 100° C. and weighed as trisulphide.
As metal,
1. The method of Rose. Bismuth is precipitated as basic bismuth carbonate and the dried precipitate is reduced by fusion with potassium cyanide. The salts formed are dissolved out with water and the metal collected. (Precautions must be taken as the crucible is liable to be attacked during fusion.)
2. The method of Vanino and Treubert. The bismuth compound in solution is reduced by the action of formaldehyde in the presence of excess of sodium hydroxide. The bismuth may either be estimated as metal, or preferably it may be redissolved in nitric acid and estimated as oxide (see method (1)). This method can be employed in the presence of hydrochloric and sulphuric acids. The results, however, are liable to be high unless special precautions are taken.
Bismuth may be precipitated from its solutions by means of selenious acid. The precipitation is effected in a nitric acid solution. It is preferable to convert the bismuth selenite to bismuth trioxide before weighing.
Miscellaneous methods. Bismuth may be precipitated as a brick-red, granular complex substance, bismuth chromium thiocyanate, BiCr(CNS)₆, by the addition of potassium chromium thiocyanate to a solution of bismuth containing nitric acid. The precipitate may be dried at 120° to 130° C. and weighed. This method may be employed in the presence of iron, chromium and sulphuric acid, and it is claimed that, in certain circumstances, it is to be preferred to the selenious acid method.

Triethylenediamine cobaltic chloride has also been used as a reagent for the estimation of bismuth. The solution of the bismuth compound is made in dilute hydrochloric acid, and is treated with potassium iodide to form potassium iodobismuthate. To this solution is added a concentrated solution of triethylenediamine cobaltic chloride, "the bismuth being precipitated as a reddish-yellow, crystalline precipitate.

" Cupferron" may be employed for the estimation of bismuth in either hydrochloric or nitric acid solution. The bismuth is ultimately determined as trioxide. This method may be employed for the separation of bismuth from many other metals.

Volumetric Methods

Various methods of estimating bismuth volumetrically have been described. A solution is made in nitric acid, and from this, basic bismuth oxalate is precipitated with ammonium oxalate. This precipitate is then dissolved in hydrochloric acid, the solution neutralised with ammonium hydroxide, and any precipitated hydroxide redissolved in sulphuric acid. This final solution is then heated to 70° C. and titrated with a standard solution of potassium permanganate.

The following indirect method may also be noted. To a fairly acid solution containing bismuth, a solution of potassium bromide is added until the precipitate of bismuth oxybromide which forms at first is redissolved. The solution is now neutralised with sodium hydroxide and a freshly prepared, saturated solution of [Cr(NH₃)₆](NO₃)₃ is added in excess. The precipitate is then distilled with a solution of sodium hydroxide and the expelled ammonia collected in a known volume of standard acid solution, the excess of which is ultimately titrated with a standard solution of sodium hydroxide. In this reaction six molecules of ammonia are equivalent to one atom of bismuth.

Other volumetric methods are mentioned in the section on the microchemical estimation of bismuth.

Colorimetric Methods

The yellow-orange or red colour produced by the solution of bismuth triiodide in excess of potassium iodide is frequently employed for the colorimetric estimation of bismuth. The sample is dissolved in nitric acid, glycerine is added, followed by a solution of potassium iodide. Comparison is made by a similarly treated standard solution containing bismuth.

Thiourea may also be employed in the colorimetrie estimation of bismuth. Solid thiourea is added to a bismuth solution containing a slight excess of acid. A yellow coloration is obtained, due to the formation of various complex compounds, and may be compared with the colour obtained with a standard solution containing bismuth. If ferric iron is present, the solution should be boiled with hydrazine sulphate.

If a solution of bismuth nitrate is added to a solution of cinchonine potassium iodide a crimson or orange coloration is produced, the intensity of the colour depending upon the amount of bismuth present. Lead, arsenic, antimony and tin must first be removed, and the bismuth solution must be added to the reagent. Comparison is made with the colour obtained with a standard bismuth solution. This method is suitable for the estimation of small amounts of bismuth of the order of 0.00003 to 0.00015 gram.

Microchemical Methods

A volumetric method has been devised for the micro-estimation of bismuth. The bismuth is precipitated as oxyiodide and the precipitate is decomposed by treatment with a solution of potassium hydroxide. The iodide present is then oxidised to iodate by the action of chlorine, potassium iodide is added and the liberated iodine titrated with a standard solution of sodium thiosulphate.

A concentrated solution of trans-dithiocyanato-diethylenediamino-cobaltic thiocyanate reacts with a faintly acid solution of bismuth nitrate (to which excess of potassium iodide has been added) to form the orange- yellow compound dithiocyanato-diethylenediamino-cobaltic iodobis-muthate, [Co en₂(SCN)₂]BiI₄. This reaction may be employed both for macro- and micro-estimation of bismuth.

Other reagents which have been suggested for use in the micro-estimation of bismuth are quinoline, a solution of piperazine in acetone, viscose, and hexamethylene-tetramine.

Electrolytic Methods

Bismuth may be estimated electrolytically in acid solution. A nitric acid solution is frequently employed containing not more than 2 per cent, of free acid. Hydrazine hydrate may be employed as a reducing agent, the electrolysis being carried out at 80° to 85° C. using a 0.01N HNO₃ | quinone auxiliary electrode, a current of 1.3 amperes, and a cathode potential (referred to the quinone electrode) of -0.45 to -0.6 volt. This method is suitable for the separation of bismuth from lead. A solution of bismuth trichloride may also be used; it is suggested that the solution should also contain sodium chloride, calcium chloride or magnesium chloride. Additions of pyrogallol and resorcinol to the electrolyte improve the deposit, but additions of hydroquinone and benzoic acid are not so effective.

Good deposits of bismuth are obtained from an electrolyte of perchloric acid containing bismuth, with a little oil of cloves as an addition agent. The electrolysis is carried out at 40° C. with a current density of 1.2 to 3.6 amperes per square foot. If lead is present, however, both lead and bismuth are deposited.

The dropping mercury electrode may also be employed for the electrolytic estimation of bismuth. The electrolyte is composed of a nitric acid solution, neutralised by sodium hydroxide, to which Rochelle salt has been added.

Spectrographic Methods

In recent years spectrographic methods have been adopted for the identification and the approximate estimation of bismuth. These methods have been employed mainly in connection with alloys, but they have also been adopted in the examination of ash from organic remains.