Percentage Formula for addition of salts, etc., to completed media

Formula for preparing any desired percentage of a given salt, etc., in tubed media; e. g., to make 4 per cent. solution of KNO3 in a series of tubes of broth each containing 10 c.c. of medium, when there is already available a 25 per cent. stock aqueous solution of potassium nitrate.

(N + X) Y A (X)

100 100

N = number of cubic centimetres contained in each tube.

X = amount of stock solution to be added to each tube.

Y = percentage required in the medium.

A = percentage of stock solution.

Then

(10 + X) 4 25X

100 100

Therefore, 40 + 4X = 25X.

Therefore, 21X = 40.

X = 1.9 c.c.

This allows for solution added to the original bulk of medium.

Therefore, 10 c.c. broth + 1.9 c.c. of a 25 per cent. aqueous solution KNO3 makes 11.9 c.c. medium containing 4 per cent. KNO3.