Paragraph 3 REAGENTS

Distilled or demineralised water, free from carbon dioxide.

Buffer solution, pH 6.88 at 20°C

3.1

Dissolve 3.40 ± 0.01 grams of potassium dihydrogen orthophosphate (KH₂PO₄) in approximately 400 ml of water. Then dissolve 3.55 ± 0.01 gram of disodium hydrogen orthophosphate (Na₂HPO₄) in approximately 400 ml of water. Transfer the two solutions without loss into a 1 litre graduated flask, make up to the mark and mix. Keep the solution in an airtight vessel.

Buffer solution, pH 4.00 at 20°C

3.2

Dissolve 10.21 ± 0.01 grams of potassium hydrogen phthalate (KHC₈O₄H₄) in water, transfer without loss into a 1 litre standard flask, make up to the mark and mix.

Keep this solution in an airtight vessel.

3.3

Commercially available pH standard solutions may be used.