This section is from the book "Alcohol, Its Production, Properties, Chemistry, And Industrial Applications", by Charles Simmonds. Also available from Amazon: Alcohol: Its Production, Properties, Chemistry, And Industrial Applications.
About 1 gram of oxidising mixture (1 part KNO3 and 3 parts Na2CO3) is added to 50 c.c. of the wine in a platinum capsule, the mixture evaporated to dryness, and the residue incinerated. In the resulting ash, taken up with dilute nitric acid and heated to boiling for a short time to convert any pyrophosphates into orthophosphates, the phosphoric acid is precipitated with ammonium molybdate solution in the usual manner, and either weighed as the ammonium-phosphomolybdate, or converted into the magnesium ammonium salt and weighed as magnesium pyrophosphate, Phosphorus exists in wine both in organic combination, chiefly as lecithin, and as inorganic compounds. "Marc " wines, made by mixing a solution of sugar with the "marc " or skins of the grapes, and fermenting the mixture, are deficient in phosphates. On the other hand, sweet wines that have been made from concentrated "must" have a high phosphate content.