of bowing.

Intensity readings were taken for each position

Intensity

of the film surface, after which the film position was again checked. No change in film position was observed. readings taken of the two cells with stopcocks were alternated Since the film position

with those of the cell with a plug.

of the latter remained constant, it served as a reference cell to correct for instrumental drift that might occur.

Figures 4 and 5 show the effect of changing the position

of the film surface on the intensity of the Ka radiations of zinc and copper respectively. The film is distended by hydrostatic pressure approximately 0.4 mm and bowing the film an additional 0.10 mm from this position causes the zinc Ka intensity to increase by 0.19%. For a solution having a zinc concentration of 30.30% by weight based on metal content, a value of 30.36% would be obtained. For copper, a 0.10 mm change in film position produces a 0.069% change in intensity and a copper value of 79.35%, by weight based on metal content, instead of 79.30%.

To determine the effect on intensity of a difference in acid concentration between solutions of unknown and standard, intensities were measured for solutions having different acid concentrations. Three nitric acid solutions of NBS C1120 were prepared, each containing the same concentration of brass, 10 mg per ml. The excess nitric acid was removed by evaporating the solution to dryness on a steam bath, adding

INTENSITY OF Zn Ka (DIGITAL VOLTMETER RESPONSE, v)

water, re-evaporating the solution to dryness and repeating this procedure. Nitric acid was pipetted to each solution to obtain concentrations of 0.64, 1.3, and 3.2 Molar.

The

Figure 5. Effect of changes in film position on
copper Ka intensity.

intensities of the fluorescent radiations were measured and graphs made relating intensities to concentrations of acid. Figures 6 and 7 show the effect of a change in the nitric

1.85

1.80

INTENSITY OF Zn Ka (DIGITAL VOLTMETER RESPONSE, v)

acid concentration on the Ka radiations of zinc and copper

respectively.

Adding 1.6 millimoles of nitric acid to 10 ml

of the brass solution, which was 1 Molar with respect to nitric acid, resulted in a 0.38% decrease in zinc intensity. This was

Figure 7.

Effect of changes in nitric acid concen-
tration on Copper Ka intensity.

quivalent to adding 0.1 ml of 70% nitric acid to the cell olume which would have resulted in a zinc value of 18.05% by eight instead of the actual concentration of 18.12%. For

opper, the same increase in nitric acid concentration resulted n a 0.48% decrease in intensity and therefore a copper value