Standard reagents (H2SO4, antimony, molybdate then ascorbic acid). Normal samples (water washed biochar, which would remove some salts plus a few nutrients, thus the PO4 test). Instant reaction turning the mixture white. I have done P analysis on thousands of aqueous samples of all kinds of sources and never seen this. Any suggestions are appreciated.
There are a few possibilities. It could be precipitation of the Mo oxide, but it could also be precipitation of a molybdate salt, e.g., the Cs salt is not very soluble.
Second thought: make sure to acidify the extract as per Murphy and Riley before adding the molybdate
Hi Paul, thanks very much for the thinking on this. The vexing thing is that we prepared 8 samples, all basically the same way (10g biochar with 20ml DI water, shaken overnight), the only difference being the source of biomass and the firing temp of the char. Only two samples had this milky white response to the reagent. I took the pH of all the samples, and the anomalous one was actually the lowest pH (range was 10 to 12), coming in at 10. It sure looks like a precipitate, but the confusing part is that none of the other samples behave like this, and I have done thousands of samples in my career and never seen this. I suppose I could acidify the sample, catch and dry the precipitate and run it on XRD.