concentration of 0 2m with the 0 5m dtpa the amount of barium sulfate dissolved after 7 hours at 100 u00b0 c was 50 6 g 1 still significantly more than the 47 2 g l obtained with the use of 0 5m dtpa alone as shown in comparative example a the results of comparative examples a and b and examples 1 and 2 are graphically depicted in the drawing in which the points on the curves indicate the amounts of barium sulfate dissolved in the various solutions of the examples after 1 3 and 7 hours of reflux at 100 u00b0 c determined by atomic absorption the curves show that for all periods of reflux the amounts of barium sulfate dissolved in the solutions are in the following order equ1 the curves also show that with the three solutions comprising 0 5m dtpa which were the only examples in which a significant amount of barium sulfate was dissolved over 90 of the dissolution obtained after 7 hours of reflux was achieved after the first hour these examples illustrate that unlike the case with pbtc various well known polycarboxylic threshold scale inhibitors reduce rather than enhance the barium sulfate dissolving capacity of an apca such as dtpa the procedure of examples 1 and 2 was followed except that instead of pbtc the following commercial polycarboxylic threshold scale inhibitors in 0 1n concentration were utilized with the 0 5m dtpa phosphino polyacrylic acid having a molecular.