Determination of total organic phosphate

The sample is ashed to produce chiefly phosphorus pentoxide this is hydrolysed to orthophosphoric acid, which is titrated with alkali. 

Add 100 ml dry propan-2-ol, cover with a watch-glass and boil on a steam bath for 30 min, filter, wash and evaporate to dryness in a nickel crucible. Do not use a platinum crucible. 

Dissolve the ash in a few ml of 0.5 M sulphuric acid, transfer to a round-bottomed flask using a total of not more than 20 ml of the acid, and boil under reflux for 30 min to hydrolyse condensed phosphates. 

Cool and transfer quantitatively to a 100 ml beaker. Insert pH electrodes and add 1 M sodium hydroxide until the pH is about 3.5. Titrate potentiometrically with 0.1 M sodium hydroxide through two end-points to a pH of about 10. Determine the volume used between the end-points, which are around pH 4.5 and pH 9.2. Alternatively, transfer the acid solution to a conical flask, make just alkaline to phenolphthalein with c. 1 M sodium hydroxide and neutralise carefully with 0.1 M hydrochloric acid. Titrate to bromocresol green with the same acid. 

The titration at step 5 involves the loss or gain of one hydrogen ion per 

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