This is a topic I’ve addressed many times. Let me illustrate it here with an example from some very recent measurements.
When measuring phosphate using ICP technology, you’re not actually measuring phosphate directly — you’re measuring elemental phosphorus. The assumption is then made that all phosphorus comes from free phosphate, and phosphate is calculated accordingly (phosphorus × 3). This inevitably leads to a falsely elevated phosphate value to some extent. The more organic matter you have in the water, the more exaggerated this calculated value can become.
If you want to measure true phosphate, it must be done via photometry.
When measuring phosphate at home, you’re always measuring true phosphate — either by visual interpretation or using a photometer.
In the samples shown in the attached images, I sent filtered water to an ICP lab that measures both true phosphate using an extremely advanced and expensive photometric instrument, as well as calculated phosphate via ICP-derived phosphorus. As you can see, theory and practice align perfectly — the calculated phosphate value is higher than the true phosphate measured with photometry.
In the third image (yellow text) , I’ve also included my own home measurement using my photometric device — which, as expected, aligns perfectly with the lab’s photometric result.
So keep this in mind when receiving ICP-calculated phosphate values from your ICP lab
All the best,
Jonas Roman
When measuring phosphate using ICP technology, you’re not actually measuring phosphate directly — you’re measuring elemental phosphorus. The assumption is then made that all phosphorus comes from free phosphate, and phosphate is calculated accordingly (phosphorus × 3). This inevitably leads to a falsely elevated phosphate value to some extent. The more organic matter you have in the water, the more exaggerated this calculated value can become.
If you want to measure true phosphate, it must be done via photometry.
When measuring phosphate at home, you’re always measuring true phosphate — either by visual interpretation or using a photometer.
In the samples shown in the attached images, I sent filtered water to an ICP lab that measures both true phosphate using an extremely advanced and expensive photometric instrument, as well as calculated phosphate via ICP-derived phosphorus. As you can see, theory and practice align perfectly — the calculated phosphate value is higher than the true phosphate measured with photometry.
In the third image (yellow text) , I’ve also included my own home measurement using my photometric device — which, as expected, aligns perfectly with the lab’s photometric result.
So keep this in mind when receiving ICP-calculated phosphate values from your ICP lab
All the best,
Jonas Roman
