What could prevent a blue color from developing in orthophosphate method 8178?

Document ID

Document ID TE376

Published Date

Published Date 08/14/2018
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Question
What could prevent a blue color from developing in orthophosphate method 8178?
Summary
Method 8178 does not turn blue
Answer
Orthophosphate concentrations that are outside of the range of the method may result in colors other than blue. As the concentration of phosphate increases, the color changes from blue to green, then to yellow and finally to brown. The brown color may suggest a concentration as high as 100,000 mg/L PO4. If a color other than blue is formed, dilute the sample and retest.

Nitrite in the sample may bleach the blue color. Remove nitrite interference by adding 0.10 g of sulfamic acid to 50 mL sample. Swirl to mix. Use this treated sample in the test procedure.


 

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