Лако-красочные материалы — производство

To the acid mixture from (a) is added, in small portions, 125 grams of 80 per cent Weldon mud.* The additions are made over a period of 2 to 3 hours at a temperature of about 25°C. When the addi­tions are completed, the mixture is stirred for 3 hours at 30°, and then is heated slowly to 120°. At this temperature, the mixture usually be­comes so thick that it can no longer be stirred. The dark color of the manganese dioxide gradually gives way to a light gray, but it is rarely possible, in the laboratory, to carry the reaction to the point where the dioxide completely disappears. After 12 hours, the mass is diluted with 2 liters water and treated with enough slaked lime to neutralize the mineral acid. The solution should not be made strongly alkaline to litmus, however, because excess alkali destroys the benzaldehydedisul — fonic acid. The calcium sulfate paste is treated with concentrated soda solution in an amount such that a filtered test portion gives no precipitate on the addition of more soda. The solids are now filtered off and washed well, and, if possible, resuspended in water and refiltered. The weakly alkaline, clear solution is evaporated to 250 cc. under reduced pres­sure with the introduction of C02, and filtered again, if necessary, to remove any residual calcium sulfate and manganese oxides. The yield can be determined by treating a measured test sample, in the presence of sodium acetate, with an acetic acid solution of phenylhydrazine of known strength. The endpoint is reached when no more yellow colora­tion is formed on adding more of the reagent to a salted-out portion of the test sample. The determination is not very accurate.