API liquid pH test is useless with TDS

Our tap water is very soft at <10ppm

What I didn’t realize is this makes the API liquid pH test result very inaccurate.

Using a brand new, freshly calibrated Bluelab pH Pen, I got the following result for my tank: 7.3 pH
And for my tap: 7.3 pH

using api liquid test on the tank: 7.6 pH
Using api liquid test on the tap: 6.7 pH

(edit: hey mod’s please change title to read “…low TDS”)

 

The api test kits are so unclear sometimes and take some getting used to. Did you shake both test bottles well before use ? Is your pen calibrated accurately?

 

The api test kits are so unclear sometimes and take some getting used to. Did you shake both test bottles well before use ? Is your pen calibrated accurately?

I know how to use the API tests. The bottles were shaken. I follow the instructions precisely.
The pen was calibrated immediately before the test and calibration verified against reference solutions.

This is not a mistake. The tests are almost useless with low TDS, which is due to the way the tests work, they need more TDS to be able to accurately measure pH.

the pH pen will have the same inaccuracies with RO or distilled water but can still measure fairly accurate with much lower TDS than the API test can.

 

PH doesnt mean anything at near 0 TDS.

You are likely just measuring CO2 fluctuations

 

PH doesnt mean anything at near 0 TDS.

You are likely just measuring CO2 fluctuations

Yes, That’s exactly what I’m saying. Don’t trust the api liquid test if your TDS is low.

But also, I’m saying the pH reading in high tds water is off by 0.3 which is too much for me.

throw in the issues people have with getting the lighting right and you could easily be off by 0.5 for that particular test

I just don’t put too much trust in the test. It can only tell you if pH is far gone and will often fail to provide early warning of a dangerous change.

others have posted similar tests with a similar outcome using other probes

 

Hi@ MrMuggles,

I would be tempted to gradually add potassium chloride to each of the water samples. This will raise electrical conductivity/TDS without actually changing the water pH. This technique raises the ionic strength of the liquid under test. But, I can't be sure if it will work with the API pH test kit because I don't know what chemistry it uses. Be sure to add the same amount of potassium chloride to each sample of water.

I look forward to knowing the results.

Archaea

 

Oh well, maybe not!

Archaea

 

I know that pH meters are definitely affected by low ionic strength (they sell ionic strength adjustors) but I don't know if colorimetric pH measurement is affected.

I'd kind of suspect the pH pen is the one being affected...

 

Tap water needs to off-gas for a significant amount of time before a colorimetric test for pH or CO2 be taken to matter. The water also needs to be at/near tank temperature.

There is a direct relationship between pH/temperature/kH/CO2. When you take water from a pressurized pipe it is significantly different.

Also, you list your "hardness" as less than 10 ppm. I'm guessing you mean "general hardness". This number is very independent of pH in allot of water systems and it is the kH "carbonate hardness" that affects your pH directly. A conductivity based pen makes allot of assumptions that you have similair kH/GH and that is simply untrue in many municipal water districts, or in the situation of a water softener. A conductivity based test is also wrong if high CO2, peet, or other organics are being used to lower effective pH additively.

With a decent water softener in place it's fairly common to see less than one degree general hardness but 5-15 degrees carbonate hardness. This stops you from seeing calcium deposits, but keeps the waters effective pH from going acidic and destroying coffee pots, fridges etc.

Anyhow, sorry for the tangent, but without significantly more amplifying information I found the title of this thread very incorrect, and recommend differently than you summarized.

 

All drip tests I have ever used said in the instructions the test is not going to work reliably under a KH of 2°. Usually the accuracy is only +/- 0.5-1.0 points pH.
As KH is part of TDS, it is no surprise that at a TDS as low as 10 a test won't show good results. As that means conductivity is at roughly 20µS/cm, of course pH pens also don't work as they have a minimum conductivity requirement of 100µSI/cm usually.
But all that is nothing new.

So... what is the problem here?

 

I think you're asking too much here. An api test kit is about 30 usd. It has hundreds of tests for 5 parameters using reagents that must have close specifications. Everyone here spouts that this test kit is a must have for beginners and while that's true most of the time, the api test kit is not accurate, nor is it precise. It is, however, a billion light years better than, "run the filter set up in the aquarium with seachem stability for a week, then toss in those fish."

I use api and test strips bounced off each other, but I trust a calibrated dedicated piece of equipment much much more. You're trying to equate a tire pressure gage to a barometer here. One is for seeing if things are good enough to drive, the other can predict weather patterns. Anyone with a bit of experience or a lot of understanding can explain the difference.

To use my tire gage example: do you drive at 60 to 80 mph, then check your tire pressure within 30 seconds for both temperature and absolute pressure in order to determine proper pressure? No! You don't care! If it's within a psi or 3 or a kpi or 3 it doesn't really matter!

For most of the casual hobbyists, it will never really mean anything. It's better to worry about nitrates than perfect measurement of tds. Tds doesn't matter much to healthy fish getting monthly water changes in most of the world. Tom barr has equipment that will make any equipment you can find obsolete, and yet we all keep plenty of things happy. Leave perfection for the people looking to achieve it. It's a niche application. For most of the hobby, Api liquid tests are as good as they need. Are you ensuring your pur numbers are perfect for plants, micro organisms, and fish health? Buying a pur meter is prohibitive, as is even a decent par meter.