45-51
SODIUM CHROMATE, continued
SAMPLING AND STORAGE
Collect samples in clean glass or plastic bottles.
ACCURACY CHECK
Standard Additions Method
a) Snap the neck off a Sodium Chromate Voluette Ampule Standard
Solution, 25,000 mg/L Na
2
CrO
4
.
b) Use a TenSette Pipet to add 0.1, 0.2 and 0.3 mL of standard,
respectively, to three 25-mL samples. Mix well.
c) Analyze the sample according to the above procedure. The sodium
chromate concentration should increase by 100 mg/L for each 0.1 mL
addition of standard.
d) If these increases do not occur, see Standard Additions (Section I) for
more information.
Standard Solution Method
Use a 1000 mg/L Sodium Chromate Standard Solution listed under
Optional Reagents to check accuracy.
INTERFERENCES
Large amounts of turbidity will result in high readings.
STATISTICAL EVALUATION
A single operator repetitively tested samples of two laboratory prepared
solutions, using one DR/700, matched sample cells and two
representative lots of testing reagents. Testing 934 mg/L Na
2
CrO
4
concentration samples, the standard deviation was ±5.4 mg/L Na
2
CrO
4
.
Testing zero concentration samples, the limit of detection was 8.1 mg/L
Na
2
CrO
4
. The limit of detection was calculated as three times the
standard deviation when testing zero concentration samples (Adapted
from Analytical Chemistry, 1980, 52, 2242-2249).
SUMMARY OF METHOD
The test directly measures the intensity of the alkaline yellow color of
the sodium chromate solution. In acid media, the solution is orange and
must be treated. A neutralizing agent is added to raise the pH, giving the
yellow color necessary for the determination.
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