Disolving NaCl in deionized water gives high pH value

[mrandersdk]

Hello

I have a problem when I dissolves NaCl in deionized water. I have a pH probe in my deionized water (lets say 1 L). Then I add 40 g of NaCl, imidiately the pH-value rises to about 9.0 (i'm logging hte pH value as a function of time). Then slowly the pH-value lowers, and after 11 hours !, it has stabalized at 6.8.

The probe is calibrated, and talking with the manufacture they confirm that we are using the instrument correctly. When we transfer the probe to a calibration solution it reacts instantly to the correct value, and when we transfer it back to the NaCl solution it, goes back to the pH value we have reached for the solution at that time (we don't go back to the 9.0, but enter at for example 7.8 if that where we are in the process, and then it continues down to the 6.8).

Anybody has an idea why the pH value would rise when I add the NaCl?

Anybody has an idea why the pH value takes so long to stabalize? Does it really take so long for NaCl to dissolve?

Any ideas how we could get it to stabalize faster?

 

[Borek]

No clear idea about what is happening, but to start with: I wouldn't trust the probe in DI water (and I would rather avoid putting it in DI water at all). Correct readings require closing the circuit, DI water has a high resistance and makes it difficult (actually in UPW AKA 18MΩ water probes stop to work at all).

 

[mrandersdk]

Yeah I know the probe should not be in DI water. But I have dissolved a lot of NaCl so it should be ok I think.

 

[jim mcnamara]

@Borek is explaining that, before the NaCl is added, the diode already can have issues in DI water. You are getting bogus readings. Use another method to test pH.

 

[mrandersdk]

But it doesn't work even if I start with a recalibrated probe, and I start meassuring after the NaCl is added and ome have desolved. I also get the same effect if I use tap water.

If I meassure in only DI water, then I get false readings, but as soon as I add the NaCl the readings are consistent, and I also end up at 6.8 which I think is reasonable. It just takes a long time.

Also recalibrating the probe during the meassuering gives the same result. So I don't think it is a DI water problem.

My results are below. At the beginning I just added the NaCl. As you can se the pH value goes to arounf 8.5, then over the night it drops to 6.8.

Then at the next day i added some more NaCl (well below what can be dessolved in water), the pH values rises again. The while the pH value is dropping, I take up the probe and put it in the 7.01 pH calibration solution, it goes to 7 (roughly, a little below but it is still ok). When i put it back in the NaCl solution, it continuous to drop, and it even looks like I could continou the line from before smoothly. This is what suggest to me that it is not the probe, but it is actually the pH value that take hours to stabilize in the solution.

How do you read the data? Any ideas come to mind?

 

[DrClaude]

I haven't done electrochemistry for years, but couldn't it be a question of the electrolyte used in the electrode? Could the complete absence of the electrolyte in the water affect the functioning of the electrode?

 

[Borek]

Do you have another electrode to test?

pH of 6.8 looks a bit high to, unless you isolate the solution from the air, CO₂ dissolves quite fast and typically makes the pH drop below 6.

 

[Tom.G]

Litmus paper?

 

[mrandersdk]

We have another hand held probe, and it gives the same result. However, the point about litmus paper is good. It does not show a value near 8 - 9, but rather 6 - 7. So I guess it is the probe that takes a logn long time to stabalize. However, I don't understand why recalibrating the probe doesn't start the pH value at 9 again when i put it back in the solution, but instead it continues like it had been in the solution all the time (see image above). Futhermore if the solution stands over night with no probe in it, the probe will read 6.8 the next day imidiately.

So to me it seems like somhow the pH value is 6-7 (litmus paper), but some reaction/ions disturbs the measurement of the probe, and this stabalizes/stops over time.

 

[256bits]

I dissolves NaCl in deionized water

Is that pure NaCl?
Table salt has anti caking compounds that might, could, maybe explain the situation with the probe and falling pH over time.

 

[mrandersdk]

It is not table salt (however we tested with that also). It is this kind of salt:

https://www.eurowater.com/Admin/Public/DWSDownload.aspx?File=%2fFiles%2fFiler%2fCustomer+information%2fEnglish%2fY05A-42A-UK4_Salt+tablets_Leaflet.pdf

 

[BillTre]

Have not done this stuff for a while, but here are a couple possibilities that come to mind:

1)
DI water is exposed to atmosphere and absorbs CO₂ and other atmospheric gasses.
Add, NaCl and the water's ability to hold dissolved gasses (especially CO₂) goes down.
Until the excess CO₂ disperses to the atmosphere, excess carbonate (CO₃, an equilibrium product of CO₂ and H₂O) combines that the Na⁺ or Cl^- and makes a different buffer (NaCO₃ or Na₂CO₃ for example). This could drive a change in the pH (hopefully in the right direction).

Tests of this mechanism:

• degas the DI water before adding NaCl; should prevent pH shift if dissolved gas is involved.

or

• Heavily aerate the water after adding the NaCl to more rapidly remove any excess gas in solution; should speed up the pH return toward 7.0 if dissolved gas is involved.

2)
Adding large amounts of NaCl to the water may change its temperature.
I believe that pH is somewhat temperature sensitive. Perhaps buffer activity is also.

• Measure for any temperature change.
  
• To test control or change temperature.

I agree (as others have suggested) that another measuring approach would be a useful confirmation of the readings you are getting.

 

[Borek]

I believe that pH is somewhat temperature sensitive.

It definitely is, but the differences should be much smaller, in the tenths of a unit range.

 

[mrandersdk]

Interesting suggestion. I have already tried to agitate it, and also heat it to try and speed up processes. But it didn't help. But I will try to look more into it.

As I mentioned I tried to use litmus paper and it gave a ph of 6-7. That could suggest that it is a false reading of the probe, caused by something in the nacl, that somehow goes away with time. As I understand the price measures the H+ concentration. A false reading must mean that it reads a too low amount of H+. Since litmus says neutral the amount of H+ is not low. So something must make the probe think the H+ level is low. Something that stabilize over time.

Are there other ways I could measure the ph value? It needs to be precise enough so I with certainty can conclude that the ph is above 6.5 and below 7.2.

 

[BillTre]

Are there other ways I could measure the ph value? It needs to be precise enough so I with certainty can conclude that the ph is above 6.5 and below 7.2.

How about titration with a dissolved pH indicator?
You could do time points, but not continuous readings.

 

[Borek]

How about titration with a dissolved pH indicator?
You could do time points, but not continuous readings.

That's amount of neutralizable acid, not pH.

 

[Borek]

Are there other ways I could measure the ph value?

pH strips (not the universal type, there are ones for narrow ranges)

It needs to be precise enough so I with certainty can conclude that the ph is above 6.5 and below 7.2.

If you need pH from a given range it is better to use a buffer, just NaCl solution can change pH over the time due to atmospheric CO₂.

In general it is pretty difficult to keep pH more or less neutral without a buffer, as near pH 7 even traces of acid/base can change pH substantially.

 

[256bits]

DI water is exposed to atmosphere and absorbs CO2 and other atmospheric gasses.
Add, NaCl and the water's ability to hold dissolved gasses (especially CO2) goes down.
Until the excess CO2 disperses to the atmosphere, excess carbonate...

Should not CO2 dissolved in water make it acidic.

Here's my thinking, which is diametrically opposed to how the carbonates react:
An impurity in the salt, with the carbonic ion, combined with sodium or calcium, is present, which would make the solution basic. ( which is the basis for asking purity of salt ).
Exposure to the atmosphere would absorb CO2 and drive the pH down over time.
With a 40 g salt solution in water, impurity, 0.02 % seems to be about right for an initial pH of around 9.

 

[Nik_2213]

Um, you are correctly rinsing etc your pH electrodes between the daily-renewed buffer in its storage vial and your task ?
Then after ??

I remember having to finely adjust a 'buffered' HPLC component and discovering, after much exasperation, that the electrode's 'parking' buffer must have been contaminated by a previous user...

FWIW, a lab's pH meter should be able to cope with the low conductivity of distilled water, never mind de-ionised.
Is there a cleaning protocol for your electrode ?

( The only time I've seen purified water's low conductivity bite was when some-one had the bright idea to try an 'assist' level-detector, normally used to protect the vision-impaired from over-filling a pan or mug. Sadly for that notion, the 'domestic' sensor was at least a factor of 10 short of necessary sensitivity. We had to get a hinged float-sensor for the water-still's holding tank... )

 

[mrandersdk]

Great information about the narrow pH strips, didn't know them. We will try to get them.

About cleaning and care. The equipment is new, and I'm the only user of it. I have cleaned it and calibrated often, so I don't think that is the problem.

@256bits Interesting suggestion. If the pH value really is that high (above 9 to start with), wouldn't our pH strip show that?

 

[Borek]

If the pH value really is that high (above 9 to start with), wouldn't our pH strip show that?

You called them "litmus strips" - where they real litmus strips (they are a narrow range strips, blue means "8 or more") or universal ones (effective for the whole scale)?

 

[mrandersdk]

They are for the whole range. But according to the color scale it would be easy to see if it is 7 (green) or 9 (Blue)

We have just ordered some with a range of 6.2 to 7.8. To try and test in that specific range