I extracted phosphate from my sample using a sodium bicarbonate solution. When I acidify the sample to a very low pH, I see that the color development (using ammonium molybdate and ascorbic acid) takes longer than when I directly add the reagents to the extract without acidification. However, I understand that when I add the reagents without acidifying, carbon dioxide is produced due to the presence of bicarbonate.
In this case, would it be more ideal to adjust the sodium bicarbonate solution to around pH 6-8? I read somewhere that this is the ideal sample pH.
Kendric - thank you for this interesting question. Although I'm an inorganic chemist, I'm certainly not a specialist in this particular field. However, it might be helpful reading the article entitled "Stabilization of Molybdate Reactive Phosphorus in 0.5 M Sodium Bicarbonate Extracts of Soils" which is available as public full text on RG:
Article Stabilization of Molybdate Reactive Phosphorus in 0.5 M Sodi...
According to this article, there is a significant difference in the pH values during the extraction and the phosphate analysis. For the phosphate extraction, the pH of the sodium bicarbonate (NaHCO3) solution must be adjusted to 8.5. The subsequent colorimetric analysis must be done under acidic conditions. For this purpose, the exctract is acidified with sulfuric acid (H2SO4). Under these conditions the bicarbonate decomposes under evolution of CO2. This is OK because at this stage the bicarbonate is not needed any more.
Thank you for your response Prof. Edelmann. Specifically, I am using 5% sodium bicarbonate solution to extract phosphate from my sample. I measured the pH of the solution before extraction and got a value close to 8.0, but I guess the pH value will change when some bicarbonate ions are exchanged for phosphate.
I also found that the pH tends to affect the color development: When I add the reagent after extraction (without any pH adjustment), there is hardly any color development, but when I add some acid, the color development becomes more pronounced up to a certain point.
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