Yes. Liquid samples can be tested for phytic acid using Phytic Acid (Total Phosphorus) Assay Kit .
At Step 1 of the procedure (Sample Extraction), dilute a given volume of the liquid sample with an appropriate volume of hydrochloric acid (0.66 M), e.g. 1 mL liquid sample plus 19 mL hydrochloric acid (0.66 M) and proceed with the extraction incubation as for solid samples. The level of sample dilution can be adjust accordingly depending on the phytic acid content of the sample. This will need to be determined experimentally.
In the calculation for liquid samples, the sample weight (g) is replaced with sample volume (mL), e.g. if 1 mL of sample was added to 19 mL of hydrochloric acid (0.66 M) in Step 1 of the procedure (Sample Extraction) then in the calculation the sample volume is 1 and extraction volume is 20. The results of phosphorus and phytic acid content for liquid samples are given as g/100 mL instead of g/100 g.
The K-PHYT MegaCalc spreadsheet contains a separate worksheet for the calculation of results using liquid samples.