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DI Water high pH reading

noodlesuk

noodlesuk

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Joined
21 Jul 2020
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370
Location
Oxfordshire
Have recently bought a cheap pH meter and was planning to run through the calibration procedure with the supplied reagents. I bought some standard Carplan DI from the garage, rather than wait till I'm next in work to get it free, (was a bit impatient) to mix with the powders.

Oddly, the DI water, from the bottle is reading a pH of 8.0, whilst tap water is coming in at around 7-7.5. I was expecting the DI to be <7?

Is it a case of leaving the water from the bottle to settle for a few hours? Or dodgy meter? With the DI reading so high, haven't used it for calibration yet.
 
I would mix the buffers with the DI water as per instructions, calibrate pH meter, then when you check the DI water is should be less than 7pH, more like 5.5pH, but also the DI water may need to breathe to take up the CO2 from atmosphere
 
Zeus. said:
I would mix the buffers with the DI water as per instructions, calibrate pH meter, then when you check the DI water is should be less than 7pH, more like 5.5pH, but also the DI water may need to breathe to take up the CO2 from atmosphere
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Ah ok, thank you, makes sense. Will give that a go. Cheers.
 
noodlesuk said:
Oddly, the DI water, from the bottle is reading a pH of 8.0, whilst tap water is coming in at around 7-7.5. I was expecting the DI to be <7?
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Hi @noodlesuk

Without specialized equipment, it is not possible to measure pH of deionized or distilled water. pH is a measure of the hydrogen ion concentration. But, you're trying to measure deionized water.

What pH buffers do you have for calibration? pH4, pH7 and pH10 are typical values.

I hope you struck lucky with your 'cheap pH meter'. They are often false economy but only time will tell.

JPC
 
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jaypeecee said:
Hi @noodlesuk

Without specialized equipment, it is not possible to measure pH of deionized or distilled water. pH is a measure of the hydrogen ion concentration. But, you're trying to measure deionized water.

What pH buffers do you have for calibration? pH4, pH7 and pH10 are typical values.

I hope you struck lucky with your 'cheap pH meter'. They are often false economy but only time will tell.

JPC
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Right that would explain it, am not fully understanding how the measurement is taken, so that's why the meter is struggling. Thank you.


Yes, the powders supplied are there abouts, 4,7,10. Have run through the calibration and it seemed ok. For now, but as you say, time will tell. I'll keep an eye on it and maybe look at a more expensive version. I Notice there was another thread, discussing these.


Interestingly I tried some pH test strips and these seem to be quite accurate in the calibration powders and also show the DI to be <6.
 
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Hi all,
noodlesuk said:
Oddly, the DI water, from the bottle is reading a pH of 8.0, whilst tap water is coming in at around 7-7.5. I was expecting the DI to be <7?
Click to expand...
You can just ignore the pH reading in the DI water. Theoretically the pH should be pH7 when the DI water is fully degassed. I say theoretically, because in practice pH is meaningless in DI water, pH measures the ratio of proton donors (H+) : proton acceptors (OH-) and in pure water you don't have any of either (I'm going to ignore the self-ionization of water).

In ultra pure water that pH value will fall to ~pH5.6 as the water absorbs CO2 from the atmosphere, and a tiny proportion of that CO2 goes into solution as carbonic acid (H2CO3). The carbonic acid disassociates into a proton (H+) and a bicarbonate (HCO3-) ion. Acids are defined as "proton donors" and the pH falls.

If you a minute amount of any base ("proton acceptor"), such as literally a drop of hard water mixed in with a litre of DI water, the pH will rise potentially to pH8. Two drops of an acid (proton donor) and back down to pH4 etc.

That is why conductivity is usually used as a measure of the purity of RO and DI water. Pure water (H2O) doesn't conduct any electricity, because it doesn't have any ions. RO water should read less than 10 microS.

cheers Darrel
 
jaypeecee said:
Hi @noodlesuk

What measurements did you get on your pH meter for each of the above? Does it have automatic temperature compensation (ATC)? And, does it measure to one decimal point, e.g. 7.0?

JPC
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So the calibration powders were not whole numbers something like 4.01, 6.61, 9.8. Guess they are a standard reagent that is that specific pH. My meter, like all cheap innacurate devices, reads to more places than it should! 2DP!. Readings were within 10% before calibration. No thermal compensation.
 
noodlesuk said:
So the calibration powders were not whole numbers something like 4.01, 6.61, 9.8. Guess they are a standard reagent that is that specific pH. My meter, like all cheap innacurate devices, reads to more places than it should! 2DP!. Readings were within 10% before calibration. No thermal compensation.
Click to expand...
Hi @noodlesuk

Thanks for the feedback.

JPC
 
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