I want to extract the phosphorus fractions in fish pond sediment by "SMT method" and measure them by the "Murphy and Riley method". Although I am sure the sediment contains a lot of organic and inorganic phosphorus, but unfortunately the sample does not turn blue, after adding the Murphy and Riley colorimetric solution, and the OD becomes zero or even negative.
What could be the reason for this problem?
Can anyone guide me in this?
Mina - Thanks for this interesting question. Although I'm not a specialist in this particular field, I found a very interesting articles which could help you solving your problem. The paper is entitled "Potential errors in the use of the Murphy and Riley method for determination of phosphorus in soil extracts" and it is available as public full text on RG:
Article Potential errors in the use of the Murphy and Riley method f...
Thanks, Professor for your guidance and introduced article, but still my problem is not solved.
@ Mina :What is the pH of your solution? The pH of the samples must be between 2 and 7.
Best regards,
malgorzata
The pH of acidic solutions was between 1-2 and the pH of alkaline solutions was between 13-14.
I was told that Murphy and Riley's method is for near-neutral environments (pH about 7), which has not been mentioned in the articles I have read so far.
I adjusted the pH of the solutions to about 7 with ammonia and sulfuric acid and read the samples and compared them with the original samples.
The blue color and OD were higher in the samples whose pH was adjusted than in the original samples (The original solutions were completely colorless, and their OD was zero or negative).
When adjusting the pH with ammonia, a brown compound formed which gradually precipitated. This compound may remove some of the phosphorus from the solution and cause the experiment to wrong.
to avoid precipitates formation adjust the pH with NaOH and/or HCl
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