[thedeadpoint]

Hi all,

I want to use DI water as a solvent for acidic and basic solutions for some experiments. We use the DI water faucets here in my building at my university. When I was preparing the different pH solutions, I noticed that the math wasn't adding up. I'd try to create a pH 5.5 solution and I couldn't get away from pH 5, regardless of how much I diluted it.

Then I decided to pH test the DI water itself. It's pH 5 out of the tap (the normal water is neutral). I figured this is a problem with our lab, so I went to another lab, tested their faucet and - ph 5 again. I even used different testing strips!

A professor helped do some troubleshooting. She found that CO2 disolves in fresh DI water and creates aqueous carbonic acid. We'd expect a pH closer to 6.5 or 6 rather than neutral.

Now, those pH's I can deal with. But we were getting pH 5!!!

An online source said we could boil the water to neutralize it. I tried that for an hour or so and no luck.


How can I neutralize my DI water? I'm going to use ICP to analyze the constituents of the solution before and after my experiments so I'd prefer to keep any chemical additions to a minimum (does that make sense?)


Thanks for any *delete me*

George

[Borek]

That's what buffers are for... Just select one that'll not interfere with ICP.

[thedeadpoint]

Thanks for the answer, Borek. But when I think of buffer, I think of high school chemistry titration labs - I can't remember what we actually used to buffer the solutions. What would I look for in a suitable buffer for me?

And to clarify, I'm a materials scientist - understanding of general chemistry but we get tripped up in situations like this.

[Borek]

Think what elements will interefere with your ICP - and look for buffers that don't contain them.

Hopefully someone more skilled in ICP will be able to add something.

Why do you need neutral pH?

[Arkcon]

If the buffer you chose has a pKa approx +/- 1 pH unit around the pH you want, in this case 5.5, and you prepare a solution of at least 10 mM, it shouldn't matter what the starting pH of water's is, or your chosen solute is, you should be able to titrate it to pH 5.5 with strong acid or base.  Assuming you've chosen a good buffer, your pH meter is in calibration, your chosen solute is chemically pure, etc.

[thedeadpoint]

Well, I only assumed I needed neutral ph before I made the solutions. Does it hurt if I used the DI water as is to make solutions ranging from pH 2 - 14 using HCl or NaOH?

[Borek]

Well, I only assumed I needed neutral ph before I made the solutions. Does it hurt if I used the DI water as is to make solutions ranging from pH 2 - 14 using HCl or NaOH?

Shouldn't matter at all.

[thedeadpoint]

Fantastic. Thanks for all the answers! I'll be sure to browse this forum further.

[nj_bartel]

Had fresh DI water reading 4.6 pH in a recent lab -_-

[Arkcon]

Typically, I've read DI water to be about 6.  But sometimes it's a little lower.  I knew on lab that found it's tap water to be routinely pH 10, I'd hate to be showering in that town.  The problem is, with so little charged species in DI water, you wait a long time with the pH meter in the water waiting for it to stabilize, and that deplete's the electrode's ions somewhat.  So it might be a little less reliable for a while, until it soaks up fresh ions from the storage solution.

[Borek]

Had fresh DI water reading 4.6 pH in a recent lab -_-

That's most likely pH electrode defect or miscalibration, or some other instrument error (fresh DI water has a specific resistance in the range of 18MΩ, which makes pH measurements almost impossible; that's other way of looking at the same problem Arkcon already signalled). IIRC around 5.6 is what you should expect once it gets saturated with atmospheric CO₂.

Unless you water was DI only by name

[nj_bartel]

The pH reader got the calibration solutions to within .01 pH of what they were supposed to be, so the only thing I could think of was some sort of impurity in the DI water.

[macman104]

That's most likely pH electrode defect or miscalibration, or some other instrument error (fresh DI water has a specific resistance in the range of 18MΩ

Interesting, we have a larger pump that is DI water, and then in a different room we have a special machine for the 18 mega-ohm water...

[Borek]

Well, I was perhaps a little bit hasty, DI water is not necesarilly the same as ultrapure 18MΩ water. But these are two sides of the same coin.

[Arkcon]

Every lab is different of course, but DI water can mean many different things, in many different places.  Depending on the application, water may be steam distilled, or run through exchange columns, I've worked in places that were either, or both, trying to get consistent water.  Either method can develop problems, that are transparent to some applications, that are eventually tracked down by trying to investigate some anomaly.

This all hinges back to what Borek: and I said -- properly made buffers should produce the correct pH, or close to it, no matter what the water is.  If you're still having problems, you may have to investigate this.  You can use any buffer system -- the CRC lists known pH's for mixtures of buffer salts, say, phosphate salts.  You can try your house distilled, buy a jug of the steam distilled water from a supermarket, or even tap.  Note, just any buffer and just any water will not likely make a clean buffer for use in an elemental analysis, this is just to see the source of the problem with incorrect pH for buffers.