122 Estimation of Total Sulphate in Blood Serum
separated with 1.8 c.c. acid, the solution in the tube was quite clear; the
figure obtained, viz., 0.61 mg. S, was due to the soluble benzidine salts adhering
to the sides of the tube made insoluble by the pure acetone used for washing.
(b) Effect of acidity of the liquid on the benzidine sulphate precipitate.—
The trichloracetic acid present in the serum filtrate was also removed
along with the hydrochloric acid used for hydrolysis when the hydrolysed
sample was dried under vacuum. The dried substance left in the tube
was practically neutral. This was dissolved in water and several attempts
were made to precipitate sulphate in this neutral solution but they were not
successful. The liquid should be acidic. The following table shows the opti-
mum acidity required for the complete precipitation of benzidine sulphate :—
TABLE IV
|
Amount of acid added to the |
Mg. S per cent as total sulphate |
||||||
|
Sample |
Sample |
Sample |
Sample C |
||||
|
2 c.c. water ; solution neutral . |
No pre- |
No precipi- |
No precipi- |
No precipi- |
|||
|
2 c.c. 0.5 per cent trichloracetic |
.. |
10.00 |
9.62 |
.. |
.. |
.. |
.. |
|
2 c.c. 1 per cent trichloracetic acid |
.. |
.. |
.. |
2.78 |
.. |
7.81 |
6.25 |
|
2 c.c. 2 per cent trichloracetic acid |
3.33 |
5.26 |
5.26 |
2.79 |
2.78 |
2.94 |
2.84 |
|
2 c.c. 3 per cent trichloracetic acid |
.. |
.. |
.. |
.. |
.. |
2.84 |
2.84 |
|
2 c.c. 4 per cent trichloracetic acid |
3.26 |
5.10 |
5.10 |
2.29 |
.. |
2.75 |
2.63 |
|
By macro method . . |
3.50 |
5.15 |
2.91 |
2.91 |
|||
Five c.c. of 0.5 per cent benzidine in acetone was added for precipitation.
There was no precipitate when the solutions were neutral. But sulphates
separated as soon as the solution was made acid with one drop of 20 per cent
trichloracetic acid. The above figures show that best results were obtained
when 2 c.c. of 2 per cent acid was used. Four per cent acid gave decidedly
lower results.
(c) Interference caused by the presence of phosphates.—The results
obtained with less acid in table IV are of interest. With sample B
much higher results (100 per cent) were obtained when the liquids were
just acid. Similarly in the case of sample C, one per cent acid gave