Two brand new pH tests disagree. One high range, one low range. Which to trust?

[knifegill]

Alright, I confess that I don't understand the testing chemistry as well as I probably should. But I do have a clue, so here are the players:

API liquid drop high range pH test kit reading at 7.4
API liquid drop 'normal' pH test kit reading at 7.8

Huge limestone (sandstone? flat, orange, granular, soft) rock has been in this tank to maintain the hardness the catfish was used to. I removed the stone just before this water change.

Sodium bicarbonate was added on day one to bring the pH to 8.0 to match the tank of origin, at least ten days ago. I've been changing 1/6th of the water almost everyday to gradually bring the pH down (and to accommodate a 9" eigenmanni in a 30 gallon quarantine tank!). By now there is probably very little sodium bicarbonate left, but the hardness from the 'stone' is still in good effect.

My tapwater is highly gassed. It comes out around 7.3 and drops below 6.0 within a day. The tapwater also has 0 hardness.

A neutralized piece of driftwood, the larger pea gravel from Lowe's, and a tiny fruit container full of java moss. There is also a fossil stone in the tank, but I've used it successfully in small softwater tanks without any hardness leeched.

I did a 50% change just now in anticipation of moving him to the main tank within a day or so. But my pH tests disagree. I'm guessing it's either the gas from my tapwater or the mineral hardness interfering with one of them. Any ideas?

 

[Zander_The_RBP]

high range tests from 7.2-8.8 while low range tests from 6.0 - 7.6

I don't know how you got 7.8 on a low range API test as mine only goes to 7.6.

Anyway your test results are impossible. if it is indeed 7.4 both will show up as such if it's 7.8 the low range one will be 7.6 (as high as it will go) and the high range will be 7.8. I suggest retesting.

Basicaly use the high pH one when you know your pH is usualy above 7.6 and use the low pH one when you know it's usualy below that.

 

[knifegill]

high range tests from 7.2-8.8 while low range tests from 6.0 - 7.6

This is almost what my cards display. The high range doesn't go any lower than 7.4 on the master test card. But the low range is the same here. The 7.6 is a certain blue, but my result was a richer blue than the "7.6 blue". To be honest, I mistook that teeny 6 for an 8 and thought that my result was just a little higher than the scale, but since that was the highest (apparent) number, I took it as the result. In short, it could have been anywhere above 7.6 for all I know, out of the range.

I don't know how you got 7.8 on a low range API test as mine only goes to 7.6.

As above.

Anyway your test results are impossible. if it is indeed 7.4 both will show up as such if it's 7.8 the low range one will be 7.6 (as high as it will go) and the high range will be 7.8. I suggest retesting.

Yes, the low range was as high as it would go. I will happily retest, as I am stumped as to an explanation for this discrepancy. It does seem impossible, that's why I'm asking. Perhaps the reagents in one test react differently with the minerals or dissolved gas than the other test does, skewering the readings?

Basically use the high pH one when you know your pH is usually above 7.6 and use the low pH one when you know it's usually below that.

I was bringing it down from what the high range told me was 7.8, so I used both tests to confirm. They are brand new and recently manufactured. I doubt they are defective. I'll test again now, with both tests, and let you know what I get.

 

[Just Toby]

I would not worry about the difference between 7.4 or 7.8 etc... it is within the correct range.

What I would worry about is why the ph drops so quickly, this is probably due to the hardness of the water (buffering) I would test for GH and KH as I suspect these are low due to fairly soft water.

I hate adjusting water from tap as it causes too may issues....I would focus on buffering up the water with hardness.

Adding crushed coral / coral gravel to the filters or sump will add to this buffering. You could also add coral gravel to the substrate under a gravel tidy and just keep it clean with regular vaccing?

 

[Zander_The_RBP]

This is almost what my cards display. The high range doesn't go any lower than 7.4 on the master test card. But the low range is the same here. The 7.6 is a certain blue, but my result was a richer blue than the "7.6 blue". To be honest, I mistook that teeny 6 for an 8 and thought that my result was just a little higher than the scale, but since that was the highest (apparent) number, I took it as the result. In short, it could have been anywhere above 7.6 for all I know, out of the range.

As above.

Yes, the low range was as high as it would go. I will happily retest, as I am stumped as to an explanation for this discrepancy. It does seem impossible, that's why I'm asking. Perhaps the reagents in one test react differently with the minerals or dissolved gas than the other test does, skewering the readings?

I was bringing it down from what the high range told me was 7.8, so I used both tests to confirm. They are brand new and recently manufactured. I doubt they are defective. I'll test again now, with both tests, and let you know what I get.

just wen't and checked yeah my high range dosen't go below 7.4 as well i was mistaken.


it could be different gases reacting with the reagents in the tests that's always a possibility but i'd figure the tests are made to deal with the normal gases present in fish tank water.

 

[knifegill]

I would not worry about the difference between 7.4 or 7.8 etc... it is within the correct range.

I'm not exactly sure why you think the difference is a small one. It matters a lot, especially right now as I'm bringing a fish back into softwater conditions.

What I would worry about is why the ph drops so quickly, this is probably due to the hardness of the water (buffering) I would test for GH and KH as I suspect these are low due to fairly soft water.

You are correct. My water has absolutely 0 hardness. I have to maintain a low level of constant buffering in all of my tanks to keep the pH in testing range.

I hate adjusting water from tap as it causes too may issues....I would focus on buffering up the water with hardness.

Yup. I'm good on that front.

Adding crushed coral / coral gravel to the filters or sump will add to this buffering. You could also add coral gravel to the substrate under a gravel tidy and just keep it clean with regular vaccing?

I don't usually use substrate, the crushed coral goes into my AC filters, or in a nylon bag into the tank. The only reason there is substrate in the tank in question right now is that it previously held a jurupari which has now moved into my 55g tank. I saw no need to remove it since it's round and large, and in a thin enough layer to easily vacuum thoroughly. I suppose that this pea gravel may be contributing to hardness as well. Now that the "sandstone" is gone, the hardness ought to be dropping. The filter only contains a small, porous tube filled with coral gravel, not enough to raise the pH, just enough to soften the crash.

just wen't and checked yeah my high range dosen't go below 7.4 as well i was mistaken.


it could be different gases reacting with the reagents in the tests that's always a possibility but i'd figure the tests are made to deal with the normal gases present in fish tank water.

It's all good. Maybe API didn't account for certain minerals or gases when formulating one of the tests? Either way, they are adding up now.

I just retested. The results are:

Low range = 7.6 (now the exact same blue, not any darker. The water has been degasing for a bit now)

High range = 7.8. A very spot on color, difficult to misread. The bold retro-orange at 7.8 looks nothing like the mustard of 7.4 or the tan of 8.0.

And yes, I was very very careful to hold the bottle vertically, add the correct number of drops, shake well, etc.

Based on these readings, it looks like my pH can safely be stated as 7.8 and dropping. Agree? Disagree?

 

[squint]

Both are within the range of error. There are better colormetric test kits but pH is something that lends itself to electronic testers.

If your water is very soft, it might not just be dissolved gases lowering your pH but the nitrification process (which consumes alkalinity).

I've used reverse osmosis water for nearly a decade and am constantly measuring pH and add baking soda when needed. I ended up getting a digital pH meter and pH calibration solutions.