Sodium Phosphate Dihydrate
(Disodium Phosphate Dihydrate, Ph. Eur. monograph 0602)
Na2HPO4,2H2O 178.0 10028-24-7
Action and use
Excipient.
Preparations
Phosphates Enema
Phosphate Oral Solution
DEFINITION
Content
98.0 per cent to 101.0 per cent (dried substance).
CHARACTERS
Appearance
White or almost white powder or colourless crystals.
Solubility
Soluble in water, practically insoluble in ethanol (96 per cent).
IDENTIFICATION
A. Solution S (see Tests) is slightly alkaline (2.2.4).
B. Loss on drying (see Tests).
C. Solution S gives reaction (b) of phosphates (2.3.1).
D. Solution S gives reaction (a) of sodium (2.3.1).
TESTS
Solution S
Dissolve 5.0 g in distilled water R and dilute to 100 mL with the same solvent.
Appearance of solution
Solution S is clear (2.2.1) and colourless (2.2.2, Method II).
Reducing substances
To 5 mL of solution S add 5 mL of dilute sulfuric acid R and 0.25 mL of a 3.2 g/L solution of potassium permanganate R and heat on a water-bath for 5 min. The colour of the permanganate is not completely discharged.
Monosodium phosphate
Maximum 2.5 per cent.
From the volumes of 1 M sodium hydroxide (n1 mL, n2 mL and n3 mL) used in the assay, calculate the following ratio:
(n2 – n3)/ (n3 – n1)
This ratio is not greater than 0.025.
Chlorides (2.4.4)
Maximum 400 ppm.
To 2.5 mL of solution S add 10 mL of dilute nitric acid R and dilute to 15 mL with water R.
Sulfates (2.4.13)
Maximum 0.1 per cent.
To 3 mL of solution S add 2 mL of dilute hydrochloric acid R and dilute to 15 mL with distilled water R.
Iron (2.4.9)
Maximum 40 ppm.
Dilute 5 mL of solution S to 10 mL with water R.
Loss on drying (2.2.32)
19.5 per cent to 21.0 per cent, determined on 1.000 g by drying in an oven at 130 °C.
ASSAY
Dissolve 2.000 g (m) in 50 mL of water R and add 25.0 mL of 1 M hydrochloric acid. Carry out a potentiometric titration (2.2.20) using 1 M sodium hydroxide. Read the volume added at the 1 inflexion point (n1 mL). Continue the titration to the 2 inflexion point (total volume of 1 M sodium hydroxide required, n2 mL). Carry out a blank titration (n3 mL).
Calculate the percentage content of Na2HPO4 from the following expression:
1420(n3 – n1)/ m(100 – d)
d = percentage loss on drying.
