Tech Tip - Phosphate

Dear Technical Section,

I'm the curriculum based education coordinator for Oregon Caves National Monument. We recently purchased three of the 30 test kits from your company. We worked with school children on a stream study project and came up with an important technical question. The phosphate test states to wait five minutes prior to testing. When we took the tests the water turned a light blue at five minutes. More time passed and the color darkened. Could you tell me if waiting more than five minutes will provide a more accurate representation of the phosphate test? Thank you for your time. It was a very fun project as well. - Joe

From Tim Parent, Technical Service Manager

Hi Joe,

It sounds like you have one of our Green Water Test Kits or possibly the Phosphorus TesTabs® module. The TesTabs® used in these kits contain ammonium molybdate which reacts with phosphorus to form a phosphomolybdate complex. This complex is reduced to a blue complex by ascorbic acid. Because this method involves a reduction, it is critical to follow the exact time frame listed in the instructions (5 minutes). It is normal for the sample to develop a darker blue color after the 5 minutes BUT this color development must be ignored. Keep a watch or timer handy when running these tests to achieve the best accuracy at 5 minutes.