Why does my pH 10 buffer smell like Ammonia?

[Rudeofus]

I have bought buffer solutions for calibrating my pH meter, and have been wondering ever since why my pH 10 buffer solution is not just some Carbonate/Bicarbonate mix but actually smells like Ammonia. The biggest cause of pH changes in alkaline buffer solutions is, AFAIK, aerial CO2 which lowers pH, and I expect an even stronger effect if this pH 10 standard solution leaks NH3. And sure enough, the expiration date on this bottle is less than 6 months ahead, whereas the pH 7 buffer lasts more than twice as long.

Are there any advantages of using this buffer, or am I better off mixing my own Carbonate/Bicarbonate buffer?

 

[Gerald C Koch]

Do you know the composition of the buffer? Some amines have an odor similar to ammonia.

 

[Rudeofus]

It's based on Ammonia, Ammonium Chloride and EDTA-Mg. From looking at the manufacturer's webpage (product number 98213.260, sorry, no direct link) and their emphasis on "buffer solution for determining water hardness", I assume they try to avoid anything that causes Ca2+/Mg2+ precipitates, which excludes the usual suspects for buffers in that range (Carbonate, Metaborate). But why would they add EDTA-Mg ??? And why would a chem supplier (I mean my local chem store) sell such a specialty product as standard buffer solution for pH meter calibration?

I guess I will use the time until this bottle expires to find a suitable replacement based on either Carbonate/Bicarbonate or Borax/Metaborate ...

 

[Mr Bill]

Are you sure it's actually a pH buffer, intended for calibrating a pH meter?

Perhaps it's some sort of reagent for use in water hardness analysis, and the "buffer" effect is for something other than pH? Just guessing, but you might want to check the exact wording in the description. (I see from your link that it's from VWR, but the item number doesn't show up in a search.)

 

[Gerald C Koch]

At a pH of 10 any solution of ammonia and ammonium ion will release ammonia gas. It sounds like this is not intended for pH meter calibration purposes. A buffer solution should be stable over time. Recipes for buffer solutions can be found on the web. You do need a reasonably accurate scale though. But considering the accuracy of cheap pH meters they are not hard to make. I would never use an expensive electrode with photographic solutions. They are too easily damaged.

 

[Rudeofus]

Are you sure it's actually a pH buffer, intended for calibrating a pH meter?

It was sold to me as that, and is also described as buffer on its product page (I finally found a way to link to it, silly web 2.0).

Conclusion: it seems to be a specialty product which was sold to me as standard buffer solution. If that local store doesn't have anything more generic (and with longer shelf life, and for less money), I'll mix my own buffer from Borax/Boric Acid and NaOH. I can still use the specialty soup for establishing a proper mixing ratio.

 

[Rudeofus]

Recipes for buffer solutions can be found on the web.

I do have one that resolves milligrams and should give me repeatable results within 0.01g. What concerns me more is the purity of the chemicals I mix my buffer from. Anhydrous Carbonate takes on water given enough time, and I have no idea how pure and stable my Bicarbonate is, NaOH seems to be another source of trouble. I wonder whether there are buffer recipes made from compounds that have very long shelf life, that don't have varying amounts of crystalline water and that won't take on Carbon Dioxide or find another lame excuse to change their composition.

FWIW, it doesn't even have to have pH 10.0. My pH meter has one calibration knob for center (pH 7.0) and one for slope, so the non-pH-7 standard can have any pH, as long as it's stable and predictable.

But considering the accuracy of cheap pH meters they are not hard to make. I would never use an expensive electrode with photographic solutions. They are too easily damaged.

My pH electrode was not exactly cheap (around 100 Euros), but it lasted for over a year and broke rather from my clumsiness and inexperience than from normal operation. I would even stir liquids with it when fine tuning pH of some photographic solution. The biggest issues I had were unreliable pH standards, and that's something I would like to resolve once and for all.

 

[Alan Johnson]

I just use 5g/L borax pH 9.2.Would that do?

 

[Rudeofus]

I just use 5g/L borax pH 9.2.Would that do?

If that works reliably, I'm all for it, and since it's trivial to mix I don't care about shelf life of working solution. But how accurate is it, and how reproducible? Well, I have another 4 months to figure that out before my pH 10 Ammonia buffer runs past its expiration date. I shall report ...

 

[Gerald C Koch]

All the chemicals; borax, sodium bicarbonate, sodium hydroxide and sodium carbonate are stable if kept in well stoppered jars and not exposed to heat. Sodium hydroxide will absorb both water and carbon dioxide from the air. When this happens you will usually see the granules start to clump together.

 

[Rudeofus]

I just mixed two of the suggested buffers here and took a reading with my pH meter. The meter was calibrated with the commercial pH buffers as described in the original posting.

Here are my results:
5 g/l Borax: pHmeasured=9.31
0.05M NaHCO3 + 0.05M Na2CO3: pHmeasured=10.10

Both results deviate quite a bit from the suggested results (9.2 for the Borax buffer according to Alan, 9.9 for the Carbonate/Bicarbonate buffer according to Sigma Aldrich's buffer resource page), and I wonder whether both my test buffers are off (and in the same direction), or whether my calibration buffers have already gone off. Or should I already cheer the fact that I have come to within pH +/- 0.2 with my simple amateur setup (cheesy milligram scale, inaccurate volumetric measurements of liquids, ...)?

 

[Alan Johnson]

Here is my reference for the pH of Borax:
http://www.borax.com/docs/product_pdfs_data/pds-borates-boraxdecahydrate-us.pdf?sfvrsn=1
The pH of 5% sodium carbonate is about 11.6:
http://photo.net/black-and-white-photo-film-processing-forum/008y6d
I use these two together with a proprietary buffer 6.86 when measuring developer pH.
The pH probe is stored in a saturated solution of potassium chloride.
It's not up to lab standards but nor is my ebay fish tank pH meter.

 

[mrred]

I purchased a pint a year ago, ph=4.0. I have a friend (biologist) who tested it for me two weeks ago and it is still good. PM me if you want the ebayer I got it from.

 

[Rudeofus]

@Alan: that Borax table is very interesting, since the pH of Borax solution seems quite independent of actual concentration. As long as contaminations don't bring in their own acidity or alkalinity the pH 9.23 should be very reliable. Contrary to that I would not trust a pure Sodium Carbonate solution as it is not well buffered (see my observations with Sodium Sulfite in Michael's thread), that's why I chose a Carbonate/Bicarbonate buffer. I'll try some more buffers with well known pH and see what I get.

@mrred: I have no problem getting buffers, the problem is whether I can trust them. Right now I seem to get very strange results with my pH meter that has been calibrated with commercial buffers well within their expiration date. I wonder whether I get more reliable calibration by mixing a few standard solutions and doing an LMS fit. BTW a pH 4 buffer should be much more stable than a pH 10 or 12 buffer, because it is not affected by aerial CO2. On the other side, few photographic bathes except operate at pH 4, and the pH of these bathes (stop bath, acidic fixers and bleaches) is rather uncritical, so a calibration at or near pH 10 is much more important IMHO.

The question remains whether an amateur level pH meter can really reach the linearity and accuracy I'm looking for.

 

[Alan Johnson]

I would not trust a pure Sodium Carbonate solution

Me too.
It is just used to confirm the meter is working at high pH.
Calibration is with pH 6.86 and pH 9.2 +/- 0.1.

 

[Mr Bill]

@mrred: I have no problem getting buffers, the problem is whether I can trust them. Right now I seem to get very strange results with my pH meter that has been calibrated with commercial buffers well within their expiration date.
...
The question remains whether an amateur level pH meter can really reach the linearity and accuracy I'm looking for.

Hi, I don't know how well your gear performs, and that sort of thing. But I spent years closely involved with screening chemical mixes at a large photofinisher. Our in-house lab routinely checked processing machine samples as well as freshly mixed or regenerated chemical mixes, typically about 50 pH readings per day.

We found that maintaining the meter's electrodes sometimes seemed like a black art. (You do normal "maintenance" on the reference electrode, and if readings are still unstable, you just presume the the pH electrode is to blame.) We settled on a procedure of having two pH electrodes "active," one for reading and the other on standy. We exchanged them every day, and this procedure seemed to help the stability more than anything.

We nearly always used commercial buffers, generally from Fisher Scientific. Once or twice over the years, we found "jumps" in pH ~10 samples (our color developers) when opening a new buffer cubitainer. Although we kept some mix-it-yourself buffer packages around, we found that we were sort of helpless to troubleshoot the problem - the mix-it-yourself packs typically didn't agree (exactly) with anything else. Our eventual solution was to ALWAYS do a cross-check of the new buffer package before the old one was empty. They would generally read within 0.01 or 0.02, which was fine with us. But sometimes it would be more than that, whereupon we would just reassign our own value to the cubitainer.

Anyway, I'd feel pretty comfortable with a pH 10 buffer from Fisher. And I would tend to be more suspicious of your electrodes than the meter, but it's just a guess.

If you've never seen Kodak literature on this sort of thing, here's a link. http://motion.kodak.com/motion/uplo...n_motion_support_processing_h243_ulm191-2.pdf

 

[Alan Johnson]

IMO the quality of pH meter required depends on how accurate it needs to be.
In the thread about "Xtol concentrate" the writer was interested in pH approximately in the range 8.0 - 8.2, he needed a lab quality meter for that.
Usually the pH of developers run from about 7 (Perceptol) to about 12 (some Rodinal types).To get the pH within +/- 0.2 a low cost pen-type meter will do.

 

[Mr Bill]

IMO the quality of pH meter required depends on how accurate it needs to be.

I agree; I'm probably making it sound too difficult. In my application, if the color paper developer came in 0.05 pH units too high, we'd probably would have made a pH adjustment to it. So you need to have some confidence that the meter is accurate enough to be telling you this.

Lower pH developers are typically much easier to read. A "marginal" pH electrode, that just drifts and drifts in a color developer might stabilize right away in a lower-pH B&W developer.

In any case, even a lousy cheap pH meter is probably much better than pH test strips. On the other hand, why even bother to read the pH if you don't intend to do anything about it. And if you intend to do something, don't you need enough accuracy to do the job?

 

[Rudeofus]

See, this is why I keep procrastinating on getting a pH meter. :confused:

A pH probe, assuming it works as advertised, outputs a nearly linearly pH dependent voltage, and my calibration efforts revolve about establishing a straight line in 2D space. Even if that straight line is a bit off, as my case suggests, it still points in the right direction, i.e. higher readings correspond to higher actual pH, and the difference between two measurements deviates from the real difference a lot less than the measured pH values deviate from the real values. So it doesn't matter all that much if my numbers are off, as long as I don't ask anyone else with a different pH meter (with its own calibration error) to reproduce my results.

My next goal is this: prepare five different buffers: Trisodium Phosphate and Monosodium Phosphate around pH 11.5, Sodium Carbonate and Sodium Bicarbonate around pH 10, Borax at pH 9.23, Trisodium Phosphate and Monosodium Phosphate at pH 7.0, and finally Borax and Acetic Acid around pH 4.5. These five trivial buffers will give me five pH readings, which will, due to miscalibration, not line up with the expected values, but which will tell me with high confidence how to adjust calibration for optimal readings. With least mean square method I can derive two calibration parameters from these five readings and their difference to expected readings, and I should get much more robust and reliable calibration than with my current two point calibration, plus an estimate for my final calibration error. This sounds like a lot of effort, but the math is fairly straight forward, so hopefully I can post something soon.

 

[dburian]

In my work life, we use these commercially prepared buffers:

http://www.sigmaaldrich.com/analytical-chromatography/analytical-products.html?TablePage=9640159

The better pH meters are made to calibrate to pH 4.0, pH 7.0, and pH 10.0 using these buffers. Consult your owners manual.

If you don't know a supply house with a Sigma account, I know one I would be happy to recommend.
Dennis

 

[Rudeofus]

As I have mentioned, I used commercial calibration buffers so far, I can get them at a local pharmacy, but as my pH measurements indicate, they seem to have let me down.

 

[dburian]

As I have mentioned, I used commercial calibration buffers so far, I can get them at a local pharmacy, but as my pH measurements indicate, they seem to have let me down.

If you follow the link, you'll see that the pH10.0 calibration buffer does not contain ammonia.
D

 

[Rudeofus]

If you follow the link, you'll see that the pH10.0 calibration buffer does not contain ammonia.

I realize that I was sold a particularly strange animal/buffer here, but in the end all alkaline buffers suffer from the same fate: they pick up aerial CO2 which lowers their pH. Since the degree to which CO2 enters the buffer depends on many parameters, most of them barely controllable, you don't know what you have after a few weeks. Replacing these expensive buffers every other week may be viable for a full blown lab, but not for a casual amateur.