In standardising the chromate solution, the standard lead nitrate solution is used. A quantity containing about as much lead as the assay is supposed to contain is measured off, rendered alkaline with dilute ammonia, and then neutralised with acetic acid, using a small piece of litmus paper dropped into the solution as indicator. Then dilute, boil, and titrate. When the lead in the assay has been separated as sulphate and dissolved in sodic acetate, less chromate is apparently required, and in this case it will be necessary to precipitate the lead in the standard with an equivalent of sodic sulphate and redissolve in sodic acetate just as in the assay. In these solutions (although there is considerable chromate in excess) a further addition of 5 or 6 c.c. of the chromate solution will cause a further precipitate. The following experiments show the effect of variation in the conditions of the assay:-- ~Effect of Varying Temperature.~--Twenty c.c. of lead nitrate solution and 10 grams of sodium acetate were used; diluted to 200 c.c., heated to the desired temperature, and titrated. The results were:-- Temperature 15° 30° 50° 100° "Chromate" required 19.8 c.c. 19.5 c.c. 19.3 c.c. 19.2 c.c. The first two of these filtered badly, the precipitate coming through the filter; the last was very satisfactory in the working. ~Effect of Varying Bulk.~--Using 20 c.c. of lead nitrate, and 10 grams of sodium acetate as before, diluting to the required bulk, heating to boiling, and titrating, the results were:-- Bulk 100.0 c.c. 200.0 c.c. 500.0 c.c. 1000.0 c.c. "Chromate" required 19.6 " 19.3 " 19.4 " 19.4 " ~Effect of Varying Acetic Acid.~--Since the experiments are carried out in the presence of sodic acetate, acetic acid is the only acid whose effect need be considered. Working as before, but with 200 c.c. bulk and varying amounts of the acid, the results were:-- Acid present -- 10.0 c.c. 20.0 c.c. 40.0 c.c. "Chromate" required 19.7 c.c. 19.1 " 18.5 " 17.3 " These experiments show that only slight quantities of acid are admissible. ~Effect of Varying Sodium Acetate.~--With the same conditions as before, but with varying weights of sodium acetate, the results were:-- Sodium acetate present -- 5 grams 10 grams 25 grams 50 grams "Chromate" required 19.7 c.c. 19.6 c.c. 19.6 c.c. 18.8 c.c. 17.8 c.c. These experiments show that excessive quantities of sodium acetate must be avoided. Ammonium acetate interferes to a greater extent, and if both acetic acid and this salt are present, each exerts its disturbing influence. With 10 grams of ammonium acetate, 19.4 c.c. of the chromate solution were required instead of 19.7 c.c. in the absence of this salt; with 10 grams of the acetate and 10 c.c. of acetic acid, only 18.6 c.c. were required.