7.2. PROCEDURE
Preparation of the calibration solutions
See Method 25d (7.2).
For an optimum interval of 0 to 5 μg/ml of zinc, place 0, 0.5, 1, 2, 3, 4 and 5 ml, respectively, of the working solution (4.4.2) in a series of 100 ml volumetric flasks. Where necessary, adjust the concentration of hydrochloric acid to bring it as close as possible to that of the test solution. Add 10 ml of the lanthanum salt solution used in (6.2) to each volumetric flask. Make up to 100 ml with the 0.5 M hydrochloric acid solution (4.2) and mix thoroughly. These solutions contain, respectively: 0, 0.5, 1, 2, 3, 4, and 5 μg/ml of zinc.