pH Swing in Tap Water - The Planted Tank Forum
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post #1 of 7 (permalink) Old 07-08-2015, 11:28 PM Thread Starter
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pH Swing in Tap Water

So I need a chemist buff or at least one who is just smart enough to be dangerous.

My pH has constantly been at 7.2-7.4 with a KH of 4, GH of 5, and TDS close to 190.

Anyway, today I did a water change and shortly after was running my every other week tests and realized my pH in my tank is now 9.0.

Quickly I tested pH meter on distilled water and it read at 6.9 (I just calibrated this meter this weekend).

Anyway, I ran the pH from tap water and am getting 9.9.

I checked TDS, KH, and GH in the tank and got TDS of 167, KH of 3, and GH of 5.

I'm assuming I'm ok but am curious what is missing that is being put into the h20. We have received a ton of rain in St. Louis so I am questioning if there is some groundwater leaching or something going on.

Just wanting some thoughts, or help to troubleshoot this situation.
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post #2 of 7 (permalink) Old 07-08-2015, 11:46 PM
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"Just enough to be dangerous" ;-)

Lots of rain...
Less carbonates... (and other minerals)
pH drops...
Water company does not want acidic water in the pipes, so they add something like sodium hydroxide or other material to make the water more alkaline.
Net result:
Softer water when you measure KH, and lower TDS, but higher pH.
I am not sure why the GH did not also drop, but I suspect it is hobby grade kits? Test might be off a bit.
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post #3 of 7 (permalink) Old 07-08-2015, 11:53 PM Thread Starter
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They are hobby grade kits. I knew there must be a simple solution

Am I correct in the assumption it's not going to be severely perilous to my fish? I have fish who prefer softer water (but not die hard softies) - Neon, Glowlight, and Bloodfin Tetras, Otocinclus, and GBR.
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post #4 of 7 (permalink) Old 07-09-2015, 12:16 AM
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Depending on what part of the St. Louis area you are in, the local water may vary widely during high water as when the Missouri is flooding. At times the river is good, easy to access, water but during floods it can be totally impossible to use.
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post #5 of 7 (permalink) Old 07-09-2015, 01:39 AM
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It is still very possible that your pH meter isn't reading accurately at that high a pH. You should calibrate with a known pH solution that is at least close to the reading you want to take. No meter of any kind, measuring anything, is always perfectly accurate. They can only be assumed to be accurate if you calibrate them carefully. And, a 9.9 pH is way above the normal range where a 7.0 pH calibration is good.

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post #6 of 7 (permalink) Old 07-09-2015, 01:41 AM Thread Starter
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Just Sunday I calibrated with three solutions, a 4.0, a 6.9 and a 9.2.

That was done Sunday and was accurate at that time. I also compared my pH meter reading to the High Range pH solution test, which also showed a nice purple color compared to the usual light blue.
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post #7 of 7 (permalink) Old 07-09-2015, 05:06 PM
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For a pH meter to work properly the water has to have a minimum conductance of 100 uS/cm. Minerals (salts) in the water is what increases the conductance. It is possible to measure the pH of low conductivity water but requires a probe with a high flow rate from the reference cell through the junction or the addition of high purity KCl to the sample to increase the conductivity.

I can almost guarantee that your tapwater is not pH 9.9. The EPA sets a secondary limit for the pH of water.

More than likely something is not right with the meter or solutions. It is possible to calibrate a bad pH electrode. The electrode needs to have a good offset (mV in pH 7) and slope (mV difference between pH 7 - pH 4). Unless you have a meter with mV mode it will be difficult to test. If the probe is less than 1 year old then it will most likely be okay.

Another concern is the age of the buffers. The pH 9.18 buffer is not as stable as pH 7 or 4. Buffers are made with water. Water reacts with carbon dioxide to form carbonic acid. The acid will cause a decrease in pH. H20 + CO2 --> H2CO3 --> H+ + HCO3-

The pH 9.18 might actually be pH 8.5. When calibrating the meter will adjust to the pH 9.18 buffer value. All readings will be off. I once had a municipality that was a whole pH unit off from field to lab. The problem was the probe has bio film on the glass from storing in groundwater that was causing an offset error (+60 mv in pH 7 when it should theoretically be 0 mv. +/- 30 mV is acceptable). The other problem was that they were calibrating to pH 10. The pH 10 buffer was not 10. The slope of the electrode was 85% or -150 mV difference from pH 7 to 10. Theoretically a slope is +/-180 mV @ 25 oC. The difference of 30 mV represents a 1/2 pH unit. If the probe actually has 100% slope that means the pH 10 buffer was actually pH 9.5. For this customer there was two problems, a dirty electrode and bad buffer.

Moral of the story, rinse probes between buffers and samples with purified water, use fresh buffers, and store the electrode in a storage solution. If no storage solution is available then use pH 4. If you do not have pH 4 then use pH 7. NEVER STORE IN PURIFIED WATER. pH 4 is recommend over pH 7 since bacteria and other stuff do not grow well at low pH's.
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