308 QUANTITATIVE AGRICULTURAL ANALYSIS BORDEAUX MIXTURE Bordeaux mixture consists of copper sulphate and calcium hydroxide. It is one of the most reliable of the fungicides, its poisonous properties being due to the copper and hydroxyl ions. Chemical tests show that when Bordeaux mixture is applied to the leaf, a small amount of copper enters and com- bines with chlorophyl of the cells. This seems to give the leaf an increased resistance to insect injury. The spray spreads rapidly over the leaf and forms a thin colloidal membrane composed of basic copper and calcium salts. Both copper and calcium hydroxide are fungicidal and when spores fall upon a sprayed leaf, they are either killed or germinate very slowly. Moisture. — The determination of moisture in Bordeaux mixture powder is made by drying at 105° to 110° to constant weight. The determination in the paste is complicated by the fact that basic carbonates of copper (formed through interaction of copper sulphate, calcium hydroxide and carbonic acid) lose carbon dioxide during the first drying process : (CuOH)2C03->2CuO + C02 + H20. A determination of carbon dioxide must then be made and the proper correction applied. Determination of Moisture: (a) In Powder. — Dry 2 gm of sample as directed for lead arsenate powder, page 300. Calculate the loss as moisture. (b) In Paste. — Heat about 100 gm (weighed in a porcelain dish) at 90° to 100° until dry enough to powder readily. Weigh and calculate the per cent loss. Denote this by a. Powder the partly dried sample, mix and determine the per cent loss on drying about 2 gm of this as directed above for powder. Call this 6. Determine carbon dioxide (see below) in both paste and partly dried powder. Let c — per cent of carbon dioxide in the partly dried material and d the total carbon dioxide in the paste. Since b and c are based upon a partly dried sample the factor — r^r — will correct these to a basis of the original paste. Then total moisture - d. (The student should prove this formula. Note that the formula given in the Official Methods, separate volume, first edition, is incorrect.) Determination of Carbon Dioxide. — Weigh 2 gm of the powder or 10 gm of the paste, place in the reaction flask together with 20 cc of water and ,1^4 — njn(J £}ie carfoon dioxide by one of the methods discussed on pages 77 Part I. Calculate the per cent of carbon dioxide in the sample as used.