Ph tester shows phosphate?

[kilnakorr]

Background story.
A friend got his water tested at the LFS.
Phosphate showed 3ppm instantly.
As he tested low at home, he bought a new kit, and tested at home. New test showed low, so he borrowed my hanna tester which also showed low (0.04 ppm).
Back to the LFS, as everyone was curious about the results.
Retested at th LFS, and all tests low.
Untill, my friend remembered they tested the water first with a handheld ph tester.
Tried that again, and retested for phosphate and bam, dark blue showing high PO4.

Can anyone explain how a ph tester can increase the phosphate, or interfere with the result?
Might be a question for reef chemist @Randy Holmes-Farley :)

 

[Randy Holmes-Farley Verified Community Expert]

Phosphate is the typical ingredient in pH 7 buffers. A typical concentration is about 5,000 ppm phosphate.

If the probe moved from the buffer to the sample without careful rinsing, phosphate will transfer.

 

[kilnakorr]

Phosphate is the typical ingredient in pH 7 buffers. A typical concentration is about 5,000 ppm phosphate.

If the probe moved from the buffer to the sample without careful rinsing, phosphate will transfer.

Thank you, but the ph device wasn't calibrated prior as far as I know.
However, it could have been earlier without a good rinse afterwards.
Bottom line, it is most likely cross contamination?

 

[taricha]

yep. Phosphate buffers are common for stable pH solutions and have tons of PO4 by comparison to tank water, so a little bit of transfer could definitely cause a high contamination.

 

[Randy Holmes-Farley Verified Community Expert]

Thank you, but the ph device wasn't calibrated prior as far as I know.
However, it could have been earlier without a good rinse afterwards.
Bottom line, it is most likely cross contamination?

I cannot think of any other way. They might even store it in a buffer between uses.

 

[kilnakorr]

I cannot think of any other way. They might even store it in a buffer between uses.

Thank you.
It is very plausable. A tiny left over at 5000 ppm, could easily contaminate 50-100 ml water.