Anhydrous Disodium Hydrogen Phosphate
(Disodium Phosphate, Ph. Eur. monograph 1509)
Na2HPO4 142.0 7558-79-4
Action and use
Excipient.
Preparation
Phosphates Enema
DEFINITION
Content
98.0 per cent to 101.0 per cent (dried substance).
CHARACTERS
Appearance
White or almost white powder, hygroscopic.
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 100.0 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 10 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 200 ppm.
Dilute 5 mL of solution S to 15 mL with dilute nitric acid R.
Sulfates (2.4.13)
Maximum 500 ppm.
To 6 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 20 ppm, determined on solution S.
Loss on drying (2.2.32)
Maximum 1.0 per cent, determined on 1.000 g by drying in an oven at 105 °C for 4 h.
ASSAY
Dissolve 1.600 g (m) in 25.0 mL of carbon dioxide-free 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.
STORAGE
In an airtight container.
