ICP vs home tests

[mook1178]

I got my first ICP back the other day. When I took the water samples I tested for TA, Ca, No3, PO4, and salinity on home kits at the same time.

Salinity:
ATI-35.72 Hanna-35.0 Within Hanna precision.

TA
ATI-8.00 Salifert 7.6 Just outside of Salifert precision of 0.3. I always struggle with this titration of when to stop at the color change, it is not a distinct change similar to the Ca and is more gradual. May be I go to far with the titration.

Ca
ATI-412.4 Salifert- 395 This is 2x outside of the precision of 10 ppm.

NO3
ATI-9.47 Hanna-9.4 Really can't get much better than that

PO4
ATI-0.13 Hanna-0.19 This is 3x outside of the precision. Concerning as one may actually bottom out faster.

Nutrients have me a bit concerned even though NO3 was spot on. If there is bio activity in transit, I can see nutrients decreasing.

Just food for though and another data point.

 

[Randy Holmes-Farley Verified Community Expert]

Thanks for posting that comparison.

Overall, I think it is quite close. :)

 

[mook1178]

Absolutely. It was my first ICP, so I thought I would compare my home tests. It came out close enough for me. Also all numbers are well within good ranges, so as long as the home tests are consistent, my parameters will be stable, which is key to a happy reef.

 

[EnterName]

I would not worry about the phosphate results. ATI doesn't use photometric methods (as far as I know) to measure phosphate and instead approximates PO₄ from the reported Phosphorus by simply multiplying by 3.066.

To cite Christoph from Oceamo:

[...]

For phosphate, however, photometric methods are still the best approach—they are highly sensitive and generally free from significant interferences. The main challenge with phosphate measurement in shipped samples is its limited stability. That’s why we chemically stabilize phosphate at the time of sampling (as done in the ICP-MS test kit).

[...]

There is also a great thread on this issue from @JonasRoman.

The expected error of the HannaChecker is ±0.02ppm ±5% of the displayed value. This gives you an expected error of ±0.0295. This means it is closer to 2 times outside the advertised accuracy.