FEBRUARY, 1906, Vol. XXXI., No. 359. THE ANALYST. PROCEEDINGS OF THE SOCIETY OF PUBLIC ANALYSTS. NOTE ON THE REMOVAL OF ARSENIC FROM HYDROCHLORIC ACID FOR USE IN THE MARSH-BERZELIUS METHOD. BY ARTHUR R. LIXG, F.I.C., AND THEODORE RENDLE. (Read at the Neetiq, December 6, 1905.) IT is a well-known fact that hydrochloric acid sufficiently free from arsenic for use in the Marsh-Berzelius method cannot be purchased, nor is this surprising, seeing that the acid acts on the glass bottle in which it is stored, and gla+s usually contains arsenic. Even the purest hydrochloric acid obtainable in commerce has therefore to be submitted to some preliminary purification before it can be used for any method of determining arsenic. The Joint Committee of the Society of Chemical Industry and of the Society of Public Analysts recommended a method for the purification of hydrochloric acid which depends in principle on the fact first pointed out by Rohmer (Ber., 1901, xxxiv., 31), that the presence of hydrogen bromide expedites the volatilization of arsenious oxide from a solution when it is distilled in a current of hydrogen chloride and sulphur dioxide.The Joint Committee’s method consists in diluting the hydro- chloric acid to be purified with water until it has B specific gravity of 1.10, adding about 5 C.C. per litre of bromine, and decolorizing the yellow solution with an excess of sulphurous acid. The acid, after standing for at least twelve hours, is boiled or distilled. The whole of the arsenic is contained in the first portion of the acid (one- fifth of the whole) which passes over.One of us employed this method regularly until about two years ago, when it was discarded in favour of distillation with metallic copper (the Reinsch method). The reasons for rejecting the Joint Committee’s method were threefold. In the first place, it was found very difficult to employ the method successfully with commercial hydro- chloric acid. The Committee’s instructions are that acid purchased as ‘( pure ” is to be treated. I n the second place, the yield of arsenic-free acid seldom, in practice, exceeds three-fifths of that originally taken ; and, lastly, the method is a lengthy one. The Fteinsch method was carried out under conditions devised by Dr. L. T. Thorne, who very kindly gave us complete details of his wzodus operandi. I n our experience, Dr.Thorne’s method is superior to that of the Joint Committee, as it can be used for the purification of commercial hydrochloric acid. When, however, the acid contains much arsenic the method is extremely slow.38 THE ANALYST. The method of separation of arsenic from non-volatile substances by distillation in presence of hydrochloric acid was considerably simplified and rendered much more exact by Emil Fischer’s proposal (Ber., 1880, xiii., 1778) to add ferrous chloride as a reducing agent before distillation. When, however, the residue as well as the distillate has to be examined, the addition of iron is obviously undesirable. In view of this, various other reducing agents have from time to time been proposed, and C. Friedheim and P.Michaelis (Bey., 1895, xxviii., 1414) proposed methyl alcohol for this purpose. A few months ago H. Cantoni and J. Chautems (Arch. Sc. phys. nat. Geneve (4), xix., 364) stated that when a current of air is passed through a solution of arsenious oxide in hydrochloric acid to which methyl alcohol has been added at the ordinary temperature the arsenic is volatilized, probably in the form of the ester methyl arsenite. As soon as this work became known to us we made numerous attempts to purify arsenical hydrochloric acid by making use of the principle just stated, but without success. It was found impossible to remove arsenic by this means at the ordinary temperature. We need not enter further into a description of the numerous preliminary experi- ments we carried out before finally adopting our present process.Briefly, the process consists in digesting a mixture of constant-boiling hydrochloric acid and methyl alcohol under diminished pressure in a flask furnished with a reflux condenser for some hours in presence of bright electrolytic copper free from arsenic, and then dis- tilling the acid over similar copper. Digesting with methyl alcohol alone has been found to be capable of removing all but the last traces of arsenic, but the purification is rendered complete and the process considerably expedited by the conjoint use of metallic copper. I t is our practice, before commencing the digestion, to add a trace of metallic zinc, and our experience has shown this to be an advantage. The details of the process are as follows : To 1,500 C.C.of commercial hydrochloric acid, slightly above 1.1 specific gravity, about 40 C.C. of redistilled commercial wood-spirit are added. The mixture is con- tained in a Wurtz flask of 2 litres capacity. About 5 to 10 grams of arsenic-free granulated zinc are then added. The flask is connected with the reflux condenser by an ordinary cork in which is fixed a glass rod supporting a coil of electrolytic copper foil, having a surface of about 120 square inches. The side tube of the Wurtz flask having been plugged, the, condenser is connected with an exhaust-pump, and the boiling commenced. With acid containing from 3; to 1 grain per gallon, about three hours’ gentle digestion is sufficient, the copper being withdrawn and cleaned at least once during this period. During the digestion a black, tarry, fuming liquid distils over, and the greater part of this is caught in a vessel intermediate between the condenser and the pump. I t is also advisable to interpose a second somewhat larger vessel containing water to absorb any hydrogen chloride which passes off. This water should be changed as often as necessary.