How can I prepare a pure dry sample of potassium phosphate?

Question

Materials:
phosphoric acid (H3PO4)
5.0g solid potassium nitrate
5.0g solid potassium carbonate
potassium chloride
potassium hydroxide
phenolphthalein

apparatus:
evaporating dish
glass rod
watch glass
250ml beaker
electronic balance
filter paper and funnel
bunsen burner
usual volumetric measurement apparatus

Answer 1

Some additional details to ag's answer:

If you want to make 10 grams of K3PO4:

Measure 4.6g of H3PO4 (if it's solid). If it's the 85% "syrup", use 5.4 g (3.2 mL if you want to use a grad cylinder). Add that to about 50 mL of water in a beaker and dissolve.

Make up a solution of 9.0 g KOH in 50 mL water.

Phenolphthalein is likely in solution form already. If not, make up a dilute solution, a pinch (on spatula tip) into 100 mL water and dissolve. Test it out by mixing a drop of this solution with 1 mL of the KOH solution. It should be a very perceptible pink. If it's too light, make a more concentration solution by adding solid phenolphthalein to its solution.

Now put about 5 drops of the phenolphthalein soln into the phosphoric acid solution. It should be colorless. Titrate (while stirring) with the KOH solution till you see it turn pink. Continue titrating but slower till you see the pink get dark and remain dark. You should use up about 45 mL of the KOH solution.

Filter out any particulates and then evaporate the solution in the evap dish with the bunsen burner under it till its dry.

Or just buy some from Sigma-Aldrich for $10.

Answer 2

Add KOH to H3PO4, titrate with phenolphthalein.