The SU-5 test kit from Hach tests for Total Sulphites by a simple drop-count method.
The true method can be modified for the measurement of Free Sulphites – though you should test this modification first.
Following the prescribed method of the SU-5 Test Kit will produce a result for Total Sulphite. This is due to the addition of the Sulphamic Acid, which dissolves any insoluble Sulphites and brings them into the overall result.
You may be able to achieve a result for Free Sulphites only by skipping the addition of the Sulphamic Acid, leaving the insoluble Sulphites out of the result.
Since this is a deviation from the tried and tested standard method, it should be noted that the manufacturers cannot guarantee the accuracy of the results. You could trial this with known standard solutions with and without the addition of insoluble sulphites to test the performance.
For more information on this test kit, or any other kits for water testing, follow the links above or contact us;