An investigation of the chemical changes taking place in the mixed lime sulphur-lead arsenate spray

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Abstract
1. The reaction between lime sulphur and acid lead arsenate is shown to be small and to have little effect on the chemical properties of either material as a spray.2. It is shown that the oxidation of lime sulphur proceeds according to the empirical formula:CaS.Sx + 30 = CaS2O3 + Sx-1and that the calcium sulphides are hydrolysed in aqueous solution.3. The addition of lead arsenate has no effect on the amount of sulphur precipitated from the lime sulphur by oxidation or by the action of carbondioxide.4. Lead arsenate is only slightly decomposed by lime sulphur or by the oxidation products of lime sulphur, the main decomposition being due to the action of sulphuretted hydrogen formed by the hydrolysis of the calcium sulphides. This decomposition becomes marked in the presence of carbon dioxide which reacts on the calcium sulphide to form sulphuretted hydrogen.5. The fungicidal value of the mixed spray—as judged by the mildew killing properties of the polysulphides—is not less than that of lime sulphur alone. Additional fungicidal properties may be expected from the presence in the spray of soluble arsenates and thioarsenates.6. Judging by the chemical changes which take place in the mixed spray the insecticidal value of the lead arsenate would not appear to be greatly affected by the addition of lime sulphur.7. There is an increased amount of soluble arsenic formed by the action of carbon dioxide on the mixed spray which may prove sufficient in amount to cause spray injury.8. The A.O.A.C. method for the determination of sulphate sulphur in lime sulphur solutions is shown to be inaccurate. A method yielding more concordant results is proposed.

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