RECOMMENDATIONS. It is recommended (1) That further cooperative work be done on the methods for the determination of lead, copper and zinc in a compound that may contain arsenic, antimony, lead, copper, zinc, iron, calcium, magnesium, etc. (2) That further cooperative work be done on the comparison of the zinc oxid-sodium carbonate and the blood charcoal adsorption methods with the official iodin method for the determination of the total arsenic in London purple. (3) That further cooperative work be done on the comparison of the Gyory bromate and the Jamieson iodate methods for the titration of arsenic trioxid in hydrochloric acid solution, with the official iodin method for the determination of arsenic trioxid; and that special emphasis be placed on the temperature of the solution when titrating with potassium bromate. (4) That the official method for the determination of total arsenic in lead arsenate be made official for the determination of total arsenic in magnesium arsenate; and that the official method for arsenic trioxid in lead arsenate be made official for the arsenic trioxid in calcium and magnesium arsenates. (5) That a study be made of methods for the determination of calcium, magnesium and zinc, respectively, in calcium and magnesium arsenates and zinc arsenite. (6) That a study be made of methods for the determination of the soluble arsenic in calcium and magnesium arsenates and zinc arsenite; and that special attention be given that these methods be measures of the safety for the use of these compounds for spraying purposes. THE SOLUBILITY OF CALCIUM AND MAGNESIUM ARSE- By A. J. PATTEN (Agricultural Experiment Station, E. Lansing, Mich.). In 1915, Scott and Siegler1 published some results of spraying tests with calcium arsenate that seemed to recommend this material as a promising substitute for lead arsenate. Since then it has been put upon the market in considerable quantities by several manufacturers, and during 1919 magnesium arsenate was first offered for sale by one manufacturer. These new preparations contain from 5 to 10 per cent more total arsenic oxid (As2O5) than lead arsenate and can be produced at a somewhat lower cost. When first put upon the market, calcium arsenate showed rather high solubilities but of the samples analyzed in the writer's laboratory during 1919, only one sample out of six, representing five manufacturers, showed a water-soluble content in excess of 1.50 per cent of arsenic oxid. In this one case the excess was only 0.37 per cent. The one sample of magnesium arsenate analyzed gave 1.25 per cent of water-soluble arsenic oxid. In spite of these facts, however, many reports of more or less severe foliage injury have come from fruit growers who have used these arsenicals. The greatest injury has been obtained on peaches, but apples and other tree fruits were also damaged. In a spraying experiment conducted by the Horticultural Department of the Michigan Agricultural Experiment Station during the summer of 1919, foliage injury was noted on apples and peaches from the use of calcium arsenate (II) and severe injury from the use of magnesium arsenate (I). From the following table it will be seen that calcium arsenate (II) and magnesium arsenate (I) contained only 0.34 per cent and 1.25 per cent of water-soluble arsenic oxid, respectively. It is evident, therefore, that some other explanation must be found for the cause of the injury than the solubility in water. In considering this problem, it was thought that carbon dioxid might be a factor, especially since the trees give off large quantities of carbon dioxid during the night, when the greatest deposition of moisture occurs in the form of dew. Therefore, it would be reasonable to suppose that the dew would contain a rather high concentration of carbon dioxid. Acting upon this theory, the solubility of a number of samples of lead, calcium and magnesium arsenates in carbonated water was determined. 1U. S. Dept. Agr. Bull. 278: (1915). The official method for the determination of water-soluble arsenic was followed except that water saturated with carbon dioxid was used instead of pure water. The results are shown in the following table: In studying these results, it will be noted that the solubility of lead arsenate was lower in carbon dioxid-water than in pure water in all but one sample. The differences were small, however, in all cases. With calcium arsenate there is considerable variation in the amount of arsenic oxid dissolved in carbon dioxid-water. This amount, however, bears no relation to the total or water-soluble arsenic and to the amount of free lime in the samples. The solubility of magnesium arsenate (I) was similar to that of the calcium arsenates. It was further demonstrated that by suspending 1 gram of magnesium arsenate (I) in 1 liter of water and passing carbon dioxid through it for 2 hours, with frequent shaking, 96 per cent of the arsenic oxid was dissolved. After this work was completed, the sample of magnesium arsenate (II) was received from the manufacturer, and the results are shown in the table for the sake of comparison. It was claimed that this sample would show a low solubility, both in pure water and carbonated water. It will be seen, from the table, that it does exhibit characteristics quite different from those of magnesium arsenate (I). Upon passing carbon dioxid through a suspension of the sample in water for 24 hours only 4.29 per cent of arsenic oxid was dissolved, or 11.28 per cent of the total. Its solubility in dilute mineral acids was also much less. Further investigation showed this sample to be magnesium pyroarsenate instead of the ordinary ortho form. From the results presented herewith, together with the many reports of injury from varied sources, the evidence seems quite conclusive that the determination of water-soluble arsenic oxid is not a satisfactory measure of the safety in using these arsenicals. Whether calcium or magnesium arsenate under any circumstances can be used safely on tender foliage and, if so, whether its solubility in carbon dioxid can be used as a measure of the safety, are problems for the future. Furthermore, the insecticidal efficiency and the action of magnesium pyroarsenate on foliage must be investigated. All of these problems must be solved by field experiments, but the members of this association can assist greatly by exercising a careful chemical control over such experi ments. THE DETERMINATION OF WATER-SOLUBLE ARSENIC OXID IN CALCIUM ARSENATE. AN INVESTIGATION TO DETERMINE THE CORRECTNESS OF THE OFFICIAL METHOD FOR WATER-SOLUBLE ARSENIC IN LEAD ARSENATE, WHEN APPLIED TO CALCIUM ARSENATE. By J. J. T. GRAHAM (Bureau of Chemistry, Washington, D. C.), Associate Referee on Insecticides and Fungicides. In the absence of an official method for the determination of watersoluble arsenic in calcium arsenate, the official method for water-soluble arsenic in lead arsenate1 is being used for calcium arsenate by most of the laboratories charged with the examination of this product. In order to clear up several points in connection with the accuracy of this method, as applied to calcium arsenate, certain investigations were made which may be divided into two parts: (1) The determination of whether or not shaking during the first part of the 24-hour digestion period gave any difference in results from shaking during the latter part of the period; (2), a comparison of the iodin titration method for the arsenic in the water extract, with the distillation and the gravimetric methods. For the first part of the investigation three commercial calcium arsenates, A, B and C were used. Two-gram charges were transferred to Florence flasks with 1 liter of carbon dioxid-free water and placed in the constant temperature bath at 32°C. One set of determinations was started at 9 A. M. and shaken at 1-hour intervals during the first 8-hour period of the digestion, and the second set was started at 4 P. M. and 1 Assoc. Official Agr. Chemists, Methods, 1916, 68. shaken at 1-hour intervals during the last 8-hour period of the digestion. At the expiration of the 24-hour digestion period for each set, they were removed from the bath, filtered, and the arsenic determined by the official method for water-soluble arsenic in lead arsenate1, with the following results: These results are practically identical and show that it is immaterial whether the flasks are given the eight shakings during the first or the last part of the digestion period. For the second part of the investigation, solutions were prepared as in the preceding case using four samples of commercial calcium arsenate, A, B, F and D, the shakings being made during the first part of the 24hour digestion period. Extractions were made in quadruplicate and, after filtering, the solutions for each sample were combined in order to have sufficient material to make all determinations on the same solution. The arsenic was determined by the following methods: (a) Official method for water-soluble arsenic in lead arsenate1. (b) Distillation method.—An aliquot of 250 cc. of the solution representing 0.5 gram of the sample, was placed in an Erlenmeyer flask, 4 cc. of sulphuric acid were added and the solution concentrated by boiling to about 50 cc. This was transferred to a distillation flask and the arsenic determined by the official distillation method2, titrating the entire distillate. (C) Gravimetric method.-In this determination aliquots representing 1 gram were used on solutions A, B and F, and aliquots of 0.5 gram were used on solution D. The aliquots were placed in Erlenmeyer flasks and boiled with 5 cc. of nitric acid. In the case of solutions A, B and F the boiling was continued until the volume was reduced to about 100 cc. The solutions were transferred to beakers, cooled, and made alkaline with sodium hydroxid after adding 1-2 drops of phenolphthalein. They were then made very faintly acid with acetic acid and the arsenic was precipitated by adding a neutral solution of 1 Assoc. Official Agr. Chemists, Methods, 1916, 68. 2 Ibid., 63. |