The electrolytic test for detection of arsenic alone and in the presence of other metals

Talanta, VoL 29, pp, 883 to 885, 1982 Printed in Great Britain, Ali rights reserved

0039-9140/82/100883-03$03.00/0 Copyright © 1982 Pergamon Press Ltd

THE ELECTROL YTIC TEST FOR DETECTION OF ARSENIC ALONE AND IN THE PRESENCE OF OTHER METALS ROBIN J. SPRING Gramerci, Colville Road, High Wycombe, Bucks, England

(Received 27 August 1981. Revised 28 April 1982. Accepted 11 May 1982)

Summary-A historical account is given of the development of the electrolytic generation of arsine in the Marsh test, showing how it was early realized that a step in an analytical procedure could be advantageously replaced by a more efficient one. It also shows that well-designed apparatus never really becomes outmoded.

The detection of toxins in the dead, especially in criminal cases, posed analytical problems in the 19th century. The same problems arose in the detection of toxins in adulterated food. In the preface to his book On Poisons in Relation ta Medical Jurisprudence,1

Taylor stated "There are many poisons which cannot at present be detected by chemical analysis, there are numerous circumstances which occur to prevent their detection in the food, the vomited matters, or the contents of the viscera in the dead." The Marsh test (1836), the Berzelius adaptation of this (1837) and the Reinsch test (1838) were available, but they only detected arsenic and antimony, and were quantitative only if applied with the utmost care. Their reliability often depended on the skill of the operator. Thus, according to Henry's diary for 1837,2 Faraday insisted on the necessity for much practice before ex pressing an opinion about suspected arsenic poisoning, thought the Marsh test needed a skilled operator, and that, without proper precautions, the whole of the metal might escape without being caught. Henry also noted that Daniell considered the Marsh test a good one, but that arsenic-free tin was difficult to obtain, and a reagent blank was therefore necessary. Long before these tests were developed, however, Cruikshank, by observing the ease with which copper was deposited electrolytically, was the first to suggest e1ectricity as a possible analytical tool for the detection of metals. By 1812, Fischer had detected arsenic electrolytically by the "galvoplastic" method, wherein the substance was precipitated on the cathode of the cel\. 3 Fischer also first recognized the displacement of one metal from solution by another in what is now called the e1ectrochemical series. 4 In 1840, Cozzi detected metals in animal fluids. In 1850 de C1aubeny detected poisonous metals by electrolytic deposition on platinum, followed by dissolution and analysis by the standard methods. He considered his method totally reliable, especially for the detection of copper in bread. In 1857, Otto's Lehrbuch de Chemie gave a delicate test for manganese and lead by electrolysis 883

and in the same year Despretz described the e1ectrochemical decomposition of certain salts. Copper and lead acetates gave lead dioxide at the anode and copper at the cathode. Manganese gave no deposit on the cathode, but a black dioxide film on the anode. Potassium antimonyl tartrate gave a crystalline deposit of antimony at the cathode and a yellowish-red coating, supposed to be anhydrous antimonic acid, at the anode. Bismuth nitrate gave a reddish-brown deposit on the anode. Despretz thought his antimony, lead and manganese results were new, and that the separation of copper from lead was virtually complete. 5 In 1861, in a paper on the application of e1ectrolysis to the detection of the poisonous metals in the presence of organic matter,6 Bloxam stated "Every analyst is only too weil aware of the difficulties which beset the detection of the poisonous metals in mixtures containing organic matters, such as the contents of the stomach, the solids and fluids of the body, and the articles of food." This may have been inspired by Todd's letter to "The Times" in 1859, following the conviction of Dr. Smethurst for murdering his mistress, Isabella Bankes, which stated: "1 trust this very important case will not be lost on toxicologists ... and that it will induce analytical chemists to review ca refully ail the pro cesses hitherto in use for the purpose of detecting minerai and other poisons with a view to clear up every possible source of fallacy."7 ln his revision of Bowman's Practical Handbook of Medical Chemistry (4th Ed., 1862), Bloxam described his electrolytic test for arsenic,8 listing the objections to the Marsh test, such as the presence of arsenic in commercial sulphuric acid, and of arsenic and antimony in zinc, the frothing of the reaction mixture despite the addition of alcohol, and the impossibility of further analysis of the mixture because of the large excess of zinc sulphate. He then stated: "The detection of the poisonous met ais by the decomposing action of the galvanic current is, 1 think, free from the se objections, and so minute quantities of the poisono us met ais may be detected by this method, that it

884 Open for escape of oxygen

ANNOTATIONS

~~0~:"1 baltery 1

"'

Heal 10

~

redness

-

Pt wire

r• r-

C-

Dilute sulphuric acid Water thermosla Pt plate 2 x 0.7 5 in.

Fig. 1. may safe1y be relied upon in most cases of chemicolegal investigations."6 Bloxam's first apparat us for the analysis is shown in Fig. 1. First the U-tube was charged with 1 fi. oz of sulphuric acid (1 + 4) and the emission tube was heated to redness for 15 min before introduction of the specimen, to verify that no arsenic was present in the acid. Then portions of arsenious acid solution containing 0.076, 0.0076 and 0.00076 grn of arsenic were successiveiy introduced. In all cases good mirrors were obtained. Bloxam then pulped 1 oz of le an meat, It oz of milk and t oz of white of egg in a mortar, mixed all this with 5 fi. oz of hydrochloric acid (1 + 4), digested the mixture on the water-bath for 15 min, filtered, and evaporated the filtrate to Ji fi. oz of dark brown viscid liquid. To this he added 0.1 grn of arsenious oxide and placed a quarter of the whole in the decomposition tube. He obtained an arsenic mirror for this and smaller quantities of arsenious oxide, down to 0.01 grn. 6 One problem Bloxam had to overcome was that the arsenious sulphide often present in such mixtures was insoluble in hydrochloric acid, and he wondered whether heating with potassium chlorate and hydrochloric acid would lead to a positive result. He showed that it was necessary to reduce the resuIting arsenic(V) back to arsenic(III) with sulphurous acid if a mirror was to be obtained from small quantities (0.05 grn). A mixture similar to that just mentioned gave a good arsenic mirror. He also showed that arsenic could be detected in beer (added to the mixture). He was still not satisfied, however, since not ail the arsenic was detected. Using two cells, separated with a parchment membrane, to prevent the chlorine evolved at the anode from passing to the cathode and converting the liberated arsine into arsenic(III) chI oride, he was able to show that this was the source of the discrepancy, and with this apparatus he could detect 0.0001 grn of arsenious oxide in a mixture of foods. 6 His apparatus at this stage was as shown in Fig. 2. The advantages of this method were the abiIity to detect arsenic by use of platinum, a metal not con-

taminated with it, the sa me sulphuric acid could be used throughout and tested fDr any length of time before the introduction of the specimen, the ex periment could be interrupted at any time by breaking the circuit, and both the cIearest and the foulest mixtures could be analysed equally well, leaving a residue that could,be further analysed for other metals. However, Bloxam concIudes: "On considering the detection of the other poisonous metals in this way, it is obvious that lead must be altogether excepted on account of its insoluble sulphate. Sil ver must also be omitted, where hydrochloric acid is the solvent; and baryta, of course, would not be expected to answer. The remaining important poisonous metals, antimony, copper, mercury, bismuth and zinc, were therefore tried, bismuth being incIuded on account of the medicinal use of its compounds."6 Bloxam was very interested in the determination of antimony at the cathode. To the food mixture used for the detection of arsenic he added 0.01 grn of tartar emetic (i.e., 0.036 grn of Sb) and analysed as for arsenic. The antimony deposited on the cathode was dissolved in ammonium polysulphide solution and the solution was evaporated on a watch-glass till an orange stain (indicating antimony) appeared. On the strength of his resuIts Bloxam compiled a scheme of analysis for the poisonous metals antimony, mercury, copper and bismuth but forgot to state specifically that arsenic was also detectable. If mercury interfered with the arsenic determination, the Iiquid from the decomposition cell or a fresh portion of the original hydrochloric acid solution should be distilled to separate mercury from arsenic. Later, Bloxam showed how the interference of mercury could be avoided and how the error caused by the evolution of stibine might be obviated. 9 He also improved his apparat us yet again by using broad strips of platinum foil instead of wire for the electrodes, and adding the sample to the cathode compartment through a thistle funnel. To prevent the evolution of stibine, he boiled the sam pIe with hydrochloric acid and potassium chlorate, evaporated the solution to low bulk, saturated it with hydrogen sul phi de and placed it in the decompo-

+ Caoutchouc Heat tubing

Ul=:=n!~~~=tr=~

Pt wires

Dilute sulphuric acid

+ - - Cold

water-bath

parchment L~=========~::::JL~Ve:..'!g':.eltab'e allached with Ptwire ligature

Fig. 2.

885

ANNOTATIONS

sition cell; deposits of arsenic and arsenious sulphide were formed in the heated tube. The dark precipitate collected after the experiment was identified as Sb 2 S3 • Bloxam showed that, with 1 grn of mercuric chloride and 0.01 grn of arsenious oxide (or 0.25 and 0.0025 grn respectively) mixed with white of egg, bread, milk and beer, there was no difficulty in detecting the arsenic. The addition of hydrogen sulphide to the electrolytic cell yielded a crust of arsenic, whereas without it only mercury was deposited on the cathode. Similar results were obtained with remains which had putrified for about a year. Bloxam was particularly concerned about the pu rit y of his reagents (with good reason) since he was un able to obtain hydrochloric acid pure enough for large quantities to be used without contributing a detectable blank. The sulphuric acid available was also impure, and he later published a paper on the production of pure sulphuric acid for the purpose. 10 He also feared that the potassium chlorate and hydrogen sulphide used were impure. He recommended the reaction: Sb 2 S3

+ 6HCl

=

2SbCl 3

purified, the standard arsenic mirrors were made with 4, 6, 8 and 10 J1g of arsenious acid, and the test still involved visu al comparison. 12 In inorganic analysis, however, continued interest in the electrolytic arsenic test was predictable, and Thorpe 13 and Hefti 14 used it in minerai analysis. 13 The typical electrolytic apparatus used for this purpose in the present century,13 apart from the use of a mercury cathode, is very reminiscent of Bloxam's 1862 apparat us. The early researches of Bloxam into arsenic detection are thus seen to be both an important link in electrolytical analysis and illustrative of the zeal of an analyst to apply new methods to the problems of toxicology, as weil as providing a period piece of instructive electrochemistry. Acknowledgements-My best thanks go to Professor S. F.

Mason, F.R.S., King's College, London and Professor D. Betteridge, University College, Swansea, for helpful cri ticisms of initial drafts, and to Julie Kimpton, S.CM., for steadfast encouragement.

+ 3H 2 S

for the production of the hydrogen sulphide, but did not suggest how to obtain pure potassium chlorate. Underlying the tests for the poisonous metals there clearly run organic (medical) and inorganic (mineraI) criteria of applicability. Not only the poisonous metals, however, were to come under investigation by electrolytic methods. As the science of electrochemistry developed, its range was extended to estimation of many more substances. This progress is weil outlined by Smith. 1 1 Bloxam's work in this direction was seminal. His papers of 1861 and 1862 were the efforts of an analyst attempting to achieve workable detection techniques for arsenic, originally in connection with death by arsenic poisoning. He also saw the use to which his test might be put in inorganic minerai analysis. The medical analyst, seeing (as Bloxam saw) difficulties in the application of the test to large quantities of viscera, might weil have been deterred from pursuing its application. For example, the Joint Committee of the Society of Public Analysts and the Society of Chemical Industry was still recommending the standard Marsh-Berzelius test for arsenic in 1909Y By this time, the hydrochloric and sulphuric acids were pure enough for the purpose, if subjected to special further purification. The zinc used was also specially

REFERENCES

1. A. S. Taylor, On Poisons in Relation to Medical Jurisprudence, p. vii, Churchi11, London, 1848. See A. S. Taylor, Dictionary Natl. Biog., 1898-99, XIX, 402. 2. From Joseph Henry's European Diary, in N. Reingold (ed.), Science in Nineteenth Century America, p. 78. Macmillan, London, 1966. 3. 1. R. Partington, A History of Chemistry, Vol. 4, p. 123. Macmillan, London, 1972. 4. Idem, op. cit., p. 123. 5. E. F. Smith, Electro-analysis, pp. 18-20. Blakiston, Philadelphia, 1918. 6. C. L. Bloxam, J. Chem. Soc., 1861,13, 12. 7. The Times, 1859, September 5, as quoted in H. W. Lyle, King's and some King's Men, p. 66. Oxford U.P., London, 1935. 8. 1. E. Bowman, A Practical Handbook of Medical Chemistry, 4th Ed., C L. Bloxam (ed.), p. 240/f. Churchill, London, 1862. 9. C L. Bloxam, J. Chem. Soc., 1861, 13, 338. 10. Idem, ibid., 1863, 15, 52. Il. Ref. 5, pp. 18-30. 12. A. W. and M. W. Blyth, Foods: Their Composition and Analysis, p. 437/f. Griffin, London, 1909. 13. T. E. Thorpe, Proc. Chem. Soc., 1903, 19, 183. 14. F. Hefti, Inaugural Dissertation, Zürich, 1907; see F. P. Treadwell and W. T. Hall, Analytical Chemistry, Vol. 2, Quantitative Analysis, 6th Ed., p. 204/f. Wiley, New York,I924.