Dear all,
we are trying for the first time to perform the quantification of P with the Olsen method. We had followed the protocol proposed by Guide to laboratory establishment for plant nutrient analysis by FAO FERTILIZER AND PLANT NUTRITION BULLETIN attached. The calibration curve is always very bad and I observed the absence of potassium antimonyl tartrate compound in molybdate solution. Can it be linked with that the absence of linearity of the curve? Please give me an advice about this method.
I suggest you abandon the HCl/SnCl2 system and follow Murphy and Riley with ascorbic acid and additional Sb
Apologies for the piecewise response!
The reproducability problem is likely due to a neutralisation problem during manual colorimetry. My suggestion is to make the solution slightly acidic using H2SO4 before colorimetry.
The previous effort had the wrong year.
Murphy, J., Riley, J.P., 1962. A modified single solution method for the determination of phosphate in natural waters. Analytica Chimica Acta 27, 31–36.
Please let me know how this works out.
To be more explicit the pH must be acidic before the molybdate and ascorbic acid are added to get reproducible results. It's also important to mix vigorously after reagent addition.
Thanks so much for the advise! I just ordered the citric acid and Sb. In the meantime I increased the amount the molybdate to 2.5% and I just obtained pretty good results.
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