PH ? - Liquid test kit vs ph meter - The Planted Tank Forum
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post #1 of 6 (permalink) Old 10-15-2015, 05:58 PM Thread Starter
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PH ? - Liquid test kit vs ph meter

Here's the stats:

RO/DI water, KH by potassium carbonate/snail shells, GH by Seachem Replenish. (switching to GLA GH booster in the near future.)

TDS out of the RO/DI is 0ppm, checked with two TDS meters.

KH is 3
GH is 4-5
No CO2 injection, yet.
Two india almond leaves in tank.

Now onto my question.

When I check the pH with my calibrated meter I get a pH of ~6-6.2, doesn't mater what time of day.

When I check the pH with the API kit I get ~7.0-7.2.

I check both samples together.

What I do:
I keep the meter in the standard solution which is pH 7.0, I kick on the meter before pulling it out of solution and it always reads 7.0.
I then rinse the meter and API glass tube in RO/DI water, then rinse in aquarium water. Dump all that out.

I then extract water from the tank, put it in the glass container rinsed by RO/DI-aquarium water, stick the pH meter in, swirl water with it, and then take the 5ml of water out for the API test.

So I'm watching my meter as I put drops into the tube and shake.

The test drops are of recent manufacture and don't expire for a long long time.

Why are they different and which one is it?

Thanks in advance!

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post #2 of 6 (permalink) Old 10-16-2015, 04:12 AM
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Measuring pH with accuracy looks easy, but it isn't. If you use the test kit, where you use the color of the water plus reagent to tell you what the pH is, it is very hard to judge the color accurately. You will be looking at a printed chart, with unknown color accuracy, as the standard, and comparing colored water in a round cross section tube to that. But, the color of the water will look different depending on how you view it. Getting a +/-0.2 pH accuracy almost takes a miracle with that test kit.

If you use a pH probe you have to calibrate the probe with standard pH solutions often, and do it correctly. The probe calibration can drift with time and usage, so you need to recalibrate it often. If you do that well you may be able to get a +/-0.1 pH accuracy.

I would trust the probe reading over the liquid test kit reading almost every time. But, I'm surprised that you got that big a difference. Something isn't right.

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post #3 of 6 (permalink) Old 10-16-2015, 05:40 AM
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Can you run the kit on the 7.0 standard and see what it says? Like Hoppy I'd be inclined to trust the probe, especially since you say it reads the standard as 7.
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post #4 of 6 (permalink) Old 10-16-2015, 09:41 AM
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You need to calibrate your probe separate from your storage solution.
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post #5 of 6 (permalink) Old 10-16-2015, 04:41 PM
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I had never thought to check the pH of the storage solution before reading this thread. I just checked, the storage solution I use is ~7.3.

These are what I'm using:
Not saying it's the best stuff to use, just what I happen to be using. And, I really don't know why you can't use calibration solution for storage, I just know they say you shouldn't long term.

My pH probe came with poorly written basic use instructions. Here is a more in depth write-up I found, Care and Storage of a pH Electrode.

FWIW - When I've checked the probe versus my API kit, they were both relatively close. I do believe the pH probe more so than telling the difference between shades of blue/green.
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post #6 of 6 (permalink) Old 10-17-2015, 03:45 AM Thread Starter
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So I recalibrated my meter with fresh standard and it was ~.2 off to my old standard.

I then took three samples of tank water and one sample of my RO/DI water and did the API test on it.

They all read pretty much the I'm not sure what happened or why there was such a discrepancy before. I tested a few times before I posted and got the same ~1PH difference over several days.

I haven't done a water change or added anything since.


Thanks for the replies you all!
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