624 . REPORT— 1894. 



5. On the Tartrar seniles. 

 By G. G. Henderson, D.Sc, M.A., and A. R. Ewing, Ph.D. 



Arsenious oxide dissolves readily in a boiling solution of sodium hydro- 

 gen tartrate, and on concentration and cooling a compound of the formula 

 C4H40gA.80Na. 2A Aq. crystallises out in aggregates of needles or prisms. The salt 

 is quite stable when dry, and may even be heated for several hours at 185° without 

 undergoing further change than loss of water of crystallisation. It has a sweetish, 

 not unpleasant taste, and is easily soluble in water, but it appears to be decomposed 

 slightly by a large excess of water. It crystallises from dilute alcohol in colourless 

 plates. 



The corresponding ammonium salt, C^HjOeAsONlI^. | Aq., which is prepared 

 in a similar way, crystallises in small glistening needles which are easily soluble 

 in water. The crystals effloresce slowly, and appear to undergo partial decompo- 

 sition on standing for some time. 



The potassium salt is not so easily prepared, owing to its instability in aqueous 

 solution. It is obtained by addiug arsenious oxide to a boiling concentrated solu- 

 tion of potassium hydrogen tartrate so long as it dissolves, filtering, cooling the 

 filtrate, and then adding two volumes of alcohol to it. The white, finely crystalline 

 precipitate which is formed is washed with alcohol and dried on a porous plate. 

 Analysis of this compound gave results agreeing fairly well with the formula 

 C^H^OgAsOK. Aq. When the salt is treated with water, even in the cold, it 

 decomposes into arsenious oxide and potassium hydrogen tartrate, but it may be 

 recrystallised from dilute alcohol, from which it separates in long needles. Slight 

 decomposition occurs in this case also. 



When a dilute solution of barium chloride is mixed with a dilute solution of 

 the sodium salt, delicate glistening needles of the barium salt are gradually formed. 

 It has the formula (C^H40gAsO)2Ba. Aq.,and is only slightly soluble in hot water. 

 It is decomposed to some extent by boiling with much water. The corresponding 

 strontium and calcium salts are obtained in a similar way, but the mixed solutions 

 are boiled for a short time. They crystallise in small shining cubes, apparently 

 isomorphous, and are more soluble than the barium salt. Other compounds of this 

 series have not yet been prepared. 



All of the compounds described above are decomposed by excess of mineral 

 acids, with liberation of arsenious oxide. If, however, the salt is kept in excess, a 

 substance is obtained in solution which has the properties of an acid, but which 

 has not been isolated owing to its instability. When the barium salt, suspended 

 in water, is decomposed by sulphuric acid (taking care to keep the salt in excess), 

 barium sulphate is precipitated and a clear solution is obtained, which remains 

 unaltered even after standing for several weeks. The solution has a strong acid 

 reaction, but contains no free sulphuric acid, and with hydrogen sulphide it gives a 

 copious precipitate of arsenious acid. If heated, or allowed to evaporate spon- 

 taneously over sulphuric acid, or mixed with alcohol, it is decomposed into 

 arsenious oxide, which precipitates, and tartaric acid, which remains in solution. 

 It undergoes the some change at once if a drop or two of a mineral acid is added. 

 It decomposes the carbonates of the alkalies and of the alkaline earths, carbon 

 dioxide being evolved, and the salts described above being formed. A quantitative 

 examination of the solution showed that it contained exactly the quantity of 

 arsenic required on the assumption that a substance of the formula AsO^HjO,^ was. 

 present. It may be concluded, therefore, that there is a definite compound of thin 

 composition in the solution, and that it is stable at ordinary temperatures if tho 

 solution be not too concentrated. Taking its properties into account, this substance 



may be regarded as a tartrarsenious acid ^s(^r\\r * ^, i-e-, as a derivative of the 



hypothetical rrthoarseniousacid As(0H)3, and the compounds described above may 

 be considered to be the salts of tartrarsenious acid, or the tartrarsenites. 



Arsenic acid and various acid oxides likewise appear to forni definite compounds 

 when treated with the acid alkaline salts not only of tartaric acid but also of other 

 organic oxyacids, but the investigation of this subject has not proceeded, far enough 

 to justify the publication of results. 



