info on measuring pH in DI water

[DJ88]

Well I did some reading after a comment was made that something was wrong with my RO/DI as my pH readings from my meter were high.

Well after sitting down over several days and measuring the effluent from my RO/DI with the DI on and bypassed, I have been doing some additional research.

In fact it turns out that with the equipment we as hobbyists use we cannot accurately measure the pH of DI water.

A pinpoint meter operates by measuring the electric potential(voltage difference) between two different sensing electrodes. One which is sensitive to H+(Hydrogen) Ions and a reference electrode. If we run our water through a De-Ionizing resin what is there to measure? Nothing.

Keep this in mind those of you using a Pinpoint meter. If you measure your RO/DI water for pH it will show erratic numbers. Over a period of an hour I watched mine swing from 8.3 to 10 to 6.2 to 8.9.

Well my curiosity is satisfied and I wanted to share.

Once again. this is with a pinpoint meter. Not solutions.

Hope others find that interesting. I sure did. I had remembered that a meter couldn't properly measure DI water but not why. Now I have a better idea.

If you want to know more.. go visit this site.
ph-measurement.co.uk

[ 14 January 2002: Message edited by: DJ88 ]

[StirCrazy]

what do you get on your RO/DI witha dropper kit Darren? good to hear that I wwasn't losing it when I thought your readigns were high [img]images/smiles/icon_biggrin.gif[/img]

Steve

[Aquattro]

Not sure I understand that Darren. DI water still has H+ ions and the reference electrode is regulated by the meter.

[DJ88]

Brad,

What do you mean the electrode is regulated by the meter?

The meter doesn't regulate anything with them. All the meter does is sense a potential difference between the two electrodes. The same things is done with a temperature sensor that you place in the water for an electronic thermometer. Nothing is regulated. All that happens is two different meterials react differently in the solution. The reaction of the probe sensetive to Ions in a solution is so different to the reaction that the reference probe has that an electric potential is created. This potential is measured by a meter and then displayed as a pH reading.

DI water is of such a low Ionic strength that you can't get a proper reading with the pinpoint monitor.

A pinpoint meter reads the Ionic strength of a solution. If you remove the Ions(De-Ionisation) how do you read the strength? You can but it has to be done with a probe made to measure in pure(Low Ionic strength) water.


Here is what Randy Holmes-Farley just passed onto me.
<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR> Measuring the pH of pure water is problematic for a variety of reasons. These include real pH changes caused by just about any contaminant around (including atmospheric CO2) and phantom changes (e.g., water motion around the electrode). The root of all of these problems is that there is very little buffering capacity in pure water, and the pH is free to easily drift over the range from about 5-9.

So the short answer is that measuring the pH of pure water is difficult. However, if you put a pH electrode in "pure" water and get a pH reading that is stable, it is likely a real reading, and can be used for most purposes.

I've measured my RO/DI water and the pH is on the high side. Something real is in the water that is raising the pH, though I've not tried to determine what it is, or whether it comes through the RO/DI, or is only coming into the water as it sits in my plastic trash can. <HR></BLOCKQUOTE>

[ 15 January 2002: Message edited by: DJ88 ]

[DJ88]

Steve, I thought I was losing my mind as well. [img]images/smiles/icon_biggrin.gif[/img] Oh wait.. I am.. Or I had serious problems with my RO/DI. Thing is I changed my DI as I thought it was toast.. GRRRRRRRRR....

[Aquattro]

Darren, I meant that the second electrode provides the difference rating from the detected ions. Although ionic activity is low in pure water, there is an abundance of hydronium and hydroxyl ions...that's what water is. Even distilled water has a [H+] of -log of 1 x 10^7. I figured this could be detected. My thinking is that without ionic interference from other species, it would be easier. Not saying I was right, just didn't make immediate sense to me. Gonna have to dig up the chem books [img]images/smiles/icon_confused.gif[/img]

[Aquattro]

It's coming to me now <g> Although deionized and distilled water both have the same concentrations of H+ at any given pH. That can't change. Water = H2O = H+ + OH-. I'll have to read up on the exact definition of pH, whether activity and concentration are interchangeable. As a trivia point, pH comes from the latin phrase "pondus hydrogenii" or "weight of hydrogen".
I graciously back away from this topic until I can re-read my chem books again [img]images/smiles/icon_razz.gif[/img]

[DJ88]

Brad,

I am going on what a chemist(Randy Holmes-Farley) has passed onto me and a few sites I have been reading that go into DI water and measuring the pH of that water.

I am not a chemist but am just trying to pass on what I has been shared with me so others can understand it.

And also keep in mind.. All of this I am talking about is using an electrode to measure pH. Not solutions or other means. This is with using the pinpoint pH meter or other probe type measuring methods using similar techniques.

[Aquattro]

Darren, I am in no way dsagreeing with what you've said. I'm just trying to understand it better. I've read many things by Randy Holmes-Farley and I think he might know more than me (big sarcasm). I just want to understand why the H+ isn't detected normally with an electrode in this situation.

[DJ88]

I haven't quite figured it all out.. all I know is that everytime I read about the H+ wih reference to a pH probe I keep getting the same thing. It measures the activity of the H+ ions or lack there of. It does make sense to me WRT the potential created between the two probes. Luckily I just did a lab in my physics course which created a similar voltage potential to measure heat transfer.