dimethyl dithiophosphoric acid by the alkali treatment and, therefore, does not interfere.Cuprous ion interferes by forming with the dithiophosphate a colorless complex that is more stable than the cupric complex. Readily oxidizable materialse.g., mercaptans(thiols)-if not removed prior to addition of the copper reagent, will reduce the cupric ion and thus give low results. In the procedure described, mercaptans and other acidic impurities are removed by carbon tetrachloride extractions made on acidified aqueous solutionjust before addition of copper reagent.Small amounts of the following metallic ions do not interfere: iron(II), iron (III), zinc, nickel(II), cadmium, aluminum, tin (II), and lead. Wettable sulfur, fermate, parzate. cuprocide, aram-ite, toxaphene, parathion, ovotran, DDT, and methoxychlor do not interfere. Plant materials which have been analyzed by the method described include alfalfa, apples, barley, beets, broccoli, cauliflower, cottonseed, cranberries, cucumbers, eggplant, grapes, green beans (foliage and pods), kale, lettuce, lima beans (fresh and canned), mustard (fresh and canned greens), onions, peaches (canned), peas (canned), peppers, potatoes, spinach, strawberries, string beans (fresh and canned), Swiss chard, tobacco, tomatoes, and tomato juice (canned).Ginsburg, Filmer, and Reed (7) have applied the method to the analysis of corn, lima beans, lettuce, and onions; Kolbezen (2) to analysis of dates, walnut meats, pears, oranges, lemons, milk, avocados, and cantaloupe; and Westlake and Butler (3) to analysis of apples. pears, peaches, spinach, snap beans, cucumbers, broccoli, potatoes, strawberries, and peas.
