I have some frozen stream water samples of which some are from hard water streams. For soluble reactive phosphorous analysis it is generally recommended not to freeze samples especially when they contain a lot of Ca. Is it possible to re-dissolve Phosphate precipitates by acid addition in order to measure SRP?
This is article that may have some insight.
Wong, G. T. F., Li-Tzu Hou, L. and Li, K.-Y. (2017), Preservation of seawater samples for soluble reactive phosphate, nitrite, and nitrate plus nitrite analyses by the addition of sodium hydroxide. Limnol. Oceanogr. Methods, 15: 320–327. doi:10.1002/lom3.10160
The answer is dependent on whether the sample was filtered before freezing. I would always recommend filtering a subsample before freezing. if the sample is filtered before freezing, freezing quickly can prevent precipitation and coprecipitation. Slow freezing permits crystals of pure water to exclude solutes so that they become more concentrated in the unfrozen interstices. It is that degree of concentration that can cause Ca, carbonate, and phosphate to increase in concentration to the point that they might exceed the solubility product for calcium carbonate or calcium phosphate. I suggest always filtering a subsample first, cooling in ice/water to 0 degrees C, and then putting on dry ice. This freezes the samples so quickly that crystalization of precipitates is unlikley. if the sample was filtered and frozen, it is easy to see if there is any precipitate after thawing if it is put in clear glass or a spectrophotmeter cell. If there is a precipitate, then adding HCl to about pH 3 or 4 is unlikely to interfere because the pH of the Murphy-Riley reagent is below pH 1 anyway.
On the other hand if the sample was not filtered before freezing then many interactions can occur (1) sorption to clays caused by concentration of the solutes during slow freezing, (2) inability to tell if there was any additional precipitation beyond the particles present before freezing. If acid is added after thawing, then the anion that accompanies the acid (chloride, sulfate, nitrate) may cause desorption of previously adsorbed phosphate. In fact weak acids are frequently used to measure weakly adsorbed phosphate. In theory weak HCl would be the least likely to cause desorption, but only at low concentrations.
Robert G. Qualls
University of Nevada
Dear Robert and James, thank you very much for your answers! This is exactly what I needed! I did filter my samples (0.2 um), but I did not 'shock' freeze them. I will definitely do that next time. All the best, Sabine
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