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Determination of orthophosphate ions using spectrophotometry


Determination of a sample:

Insert 2 ml of sample (in some cases it may be necessary to dilute) into a 25 ml test tube.
Add 2.5 mL of sulfomolybdic reagent. Stir vigorously.
Then add 15.5 mL of ascorbic acid. Stir again.
Cover the test tube with a piece of parafilm.
Place the test tube (in a rack) in a water bath (set at 85°C) for 10 mins.
Then cool the test tube under cold running water or in a cold water bath.
Take an aliquot, place it in a disposable spectrometer cuvette, and then proceed to the spectrometric measurement at 830 nm.

Refer to the spectrometer data sheet

Blank test:

Perform a blank test under the same conditions as the determination of a sample, replacing the sample with distilled water.


In a series of 25 mL test tubes, proceed as for the sample, initially introducing 2 mL of a standard solution of orthophosphates. Perform a calibration with, for example, 6 solutions: 0; 2; 4; 6; 8; 10 mg.L-1 of P.

Refer to the calibration sheet

Information pictogram
Practical tip: the samples, blank test and calibration solutions are prepared at the same time, in order to avoid losing time at the water bath stage.
Illustration of a chronometer ≈ 20 minutes / sample
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