[Dustin100]

Hi all, I'm wondering if its possible to estimate pH of domestic tap water given a report from the water company?

I have values for Ca²⁺, Mg²⁺, Na⁺, K⁺, Fe, HCO₃^-, CO₃^2-, SO₄^2-, Cl^-, NO₃^-, NO₂^-, Fl^-

[Borek]

And the report doesn't contain information about pH? Quite strange.

In theory you can try to estimate pH from known concentrations of HCO₃^- and CO₃^2-, using Henderson-Hasselbalch equation.

[Dustin100]

And the report doesn't contain information about pH? Quite strange.

In theory you can try to estimate pH from known concentrations of HCO₃^- and CO₃^2-, using Henderson-Hasselbalch equation.

Thanks for the reply, I'll look into that. The reports I have do include pH, but they say its never been below 8. Whereas every time I test it using two different sets of Johnson indicator strips its always in the range 5.5 - 6. I've tested over the last 18 months and its consistent. I have tested my indicator strips with pH4 and ph7 calibration solutions, and the strips work fine for those pH values. The water company even took a sample direct from my tap and said it was pH 8.2. I don't know what's going on.

[Borek]

The reports I have do include pH, but they say its never been below 8. Whereas every time I test it using two different sets of Johnson indicator strips its always in the range 5.5 - 6.

5.5 and 6 are below 8, what am I missing?

The water company even took a sample direct from my tap and said it was pH 8.2.

That definitely contradicts what they say about not going below 8, no doubt about it.

[Dustin100]

5.5 and 6 are below 8, what am I missing?

I'm concerned that my pH strips are not correct. Is that possible? Could it be a bad batch? Or am I overlooking something like dissolved CO2 that the water company might eliminate before testing? I'm questioning the indicator strips rather than the water company because the water company runs a professional lab and I'm an amateur with amateur equipment who has forgotten most of the chemistry he ever learned at school. That said, I don't have full trust in the water company either because they changed our supply a couple of years ago and there have been many complaints, so much so that it was in the national press.

What I'd really like to do is borrow a decent pH meter, as that would answer my question straight away.

However I was wondering if it is possible to estimate pH from the water report ion analysis & alkalinity as CaCO3 values, to determine if the pH of 5.5 (my reading) is more likely, or the pH of 8 (water company measurement) is more likely.

[Borek]

I'm concerned that my pH strips are not correct. Is that possible?

Quite unlikely. Because of the way they work they are quite robust.

Or am I overlooking something like dissolved CO2 that the water company might eliminate before testing?

You may try to boil water before measuring the pH, just let it cool in a closed vessel (not too tightly, as you won't be able to open it, but try to minimize the overhead volume and amount of gases exchanged) before the measurement.

But IMHO they should not do such thing if what they check is the water sample collected at your house.

[Dustin100]

Quite unlikely. Because of the way they work they are quite robust.

Thanks for this information.

But IMHO they should not do such thing if what they check is the water sample collected at your house.

Exactly. I would expect them to have to measure it exactly as it is when it comes out of the tap.

I've contacted the indicator strip manufacturers and await their response. In the mean time, I'm busy reading up on alkalinity and pH relationships.

Thanks

[Corribus]

Are you reading the strips correctly?
Can you be there when the water company makes a reading? If so, measure at the same time and place the water company does. Maybe the pH is fluctuating.

[Dustin100]

Are you reading the strips correctly?
Can you be there when the water company makes a reading? If so, measure at the same time and place the water company does. Maybe the pH is fluctuating.

I don't think there's much I can get wrong in reading the strips. Immerse for 2 seconds, compare the colours within 40 seconds. I've had 2 other people check, and they got the same result as me. I've checked at a range of temperatures too with the same result. I took a reading at the same time the water company took the samples to, but they send them back to the lab, so I didn't actually see them measure the pH.

According to the readings I've been taking, the pH is always 5.5 - 6, so no discernible fluctuation, and I've been testing for around the last year or two.

[XeLa.]

Hmm...that's very interesting. It does seem very strange that the company and yourself and others are finding two very different results. If you could post maybe some of the data that you received in that water quality report that would be very helpful.

[Dustin100]

Hmm...that's very interesting. It does seem very strange that the company and yourself and others are finding two very different results. If you could post maybe some of the data that you received in that water quality report that would be very helpful.

See attached the water company reports. I had an interesting call with the test paper manufacturers last week who said that they might not be accurate when the ionic strength is too low. I'm sending water samples to them so they can test for themselves and see where the problem lies.

[XeLa.]

That's interesting what they said about the ionic strength. I did a bit of research, and generally, pH measurement methods such as reagents or probes are designed to provide accurate pH readings for highly buffered solutions (solutions containing a considerable concentration of a particular ion.) However, when the solution is less buffered, the accuracy of this equipment become less reliable. It can be considered that soft water (water containing fewer calcium and magnesium ions) is less buffered than what is considered hard water. Since soft water contains fewer ions, the ionic strength becomes less significant. This might be a stab in the dark, but my guess is that the water company might use a device such as a pH probe or a chemical reagent. However, since the water in your area is quite soft, there might be the possibility that their devices are rendered less accurate in comparison to your indicator strips.

It would be interesting to see what kind of reading you get if you were to use a pH probe instead.

EDIT: The one thing that I find to be very bizarre, however, it how your reading and their reading are 2 - 2.5 pH units apart. Generally, the maximum error of a pH probe or a reagent under such conditions (low ionic strength) is 1 pH unit I thought...