# Household items: pH standards



## grak70

So your SMS122 is due for calibration but oh noes! You don't have any Milwaukee standard solutions left! What else could you possibly use? I talked at length with some folks in IRC #ptchat about this and I'd like to get people's feedback.

Here are some candidate household materials that might be useful as "standards" for pH in the absence of a buffer solution. DI/RO water is not listed among them as it has no buffering capacity. Since none of these have a near-neutral pH (7.0), it is a very good idea to do both an acid and a base. The farther apart your standards are in pH, the more difficult it will be to calibrate your meter. Standards that are too close together in pH will not be able to operate accurately over a very large range. Ideally, you should choose at least one standard close to the pH range in which you will operate.

Bases (pH>7)

-Sodium bicarbonate (baking soda) - ~8.5g dissolved in 1L of DI/RO water: pH=8.3-8.4
-Sodium borate (Borax)- ~1wt% solution in DI/RO water: pH=9.2
-Ammonia cleaner: pH=11

Acids (pH<7)

-Potassium bitartrate (cream of tartar) - ~19g dissolved in 1L of DI/RO water: pH=3.6
-5wt% acetic acid (distilled vinegar): pH=2.4-2.5

Additions and criticism welcome. Remember: to be a good standard, a substance needs to be simple (one pure solute), aqueous (pH probes we use are only relevant in water), and have low vapor pressure and reactivity (soda water is acidic, but H2CO3 is unstable at standard temperature and pressure).


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## Diana K

If the goal is to calibrate the pH meter with something close to the range you want to test, is there something as simple as these that will create a solution that is closer to neutral? Or a couple of somethings, say about... 6.5 to 7.5?


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## grak70

Diana,

That is a big part of my request for feedback. I honestly cannot think of anything household accessible that has a pH in the 6.5-7.5 range. Cream of tartar is very useful as a substitute for 4.0 standard, but I'm at a loss for something with neutral buffer properties.


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## grak70

Unlikely as it sounds, pure MSG (monosodium glutamate) is reported to have a pH of 7.0. It is the conjugate base of glutamic acid. This is available in any grocery store spice aisle (though I'd recommend against eating it lol). I will need to do some experiments to confirm its usefulness as a buffer.


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## Zapins

Excellent thread grak! One of the best thread's I've seen in a long time. Finally something new on APC!

By the way the MSG... do they still sell that with all the laws against using MSG in food?

What about our aquarium chemicals? I'm sure they have slight acid or slight basic natures, perhaps we can use our fertilizers to calibrate our pH probes?


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## grak70

I was thinking about that too. The problem with our fertilizer materials is that except for phosphate salts, they are all conjugate salts of strong acids (H2SO4, HNO3). These are poor buffers by definition. Their equilibrium lies far to one end of the acid scale and their titration curves are extremely sharp because the anion is so stable. Phosphate is complicated. It's hygroscopic for one thing and I'm not entirely convinced that our little baggies of KH2PO4 are pure. I've read online that KH2PO4 is supposed to have a pH of 4.1, but when I measure it, I get 5.1-5.2. I just recal'ed my probe with commercial standards so I'm pretty convinced of the measurement. Must be something else going on...

Contrary to popular belief, sensitivity to MSG is poorly attested and mostly anecdotal. In fact, if you eat any cooked vegetable protein or meat at all, you're eating glutamic acid and glutamate salts. It's much more likely that people who say they're sensitive have a food allergy in a related ingredient or a different sensitivity. Still, it's a source of a lot of sodium and I like to avoid it...probably an irrational fear. 

You can buy MSG in the spice aisle, sometimes near the asian food section. It's also labeled "flavor enhancer".


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## grak70

A quick guide to preparing phosphate buffers (any pH between 5.8 and 8.0)

http://www.cnr.berkeley.edu/soilmicro/methods/phosphate buffer.pdf

There is a problem with this approach in that you'll have to know whether your phosphates have taken on water as a complex. Otherwise you will weigh out too little material when making the stock solutions.

I'll keep looking into the glutamate/MSG solution.

Edit: doesn't look very good. To be a good buffer, the target pH should be near the pKa. Additional deprotonation of MSG has a pKa of 9.47. Back to the drawing board.


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## bobbittle

If you can muster up enough of it, a typical persons saliva has a ph of 6.3-6.6. A bit more morbid, human blood has a ph of 7.35-7.45.

Tomato juice - 4.2
Lemon juice - 2.3
Egg whites - 7.6-8.0
Milk - 6.3-6.6


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## Diana K

So... Milk, Egg Whites, blood and saliva seem to be in the right range, but are they a bit too variable? What is the range for a calibrating solution? Shouldn't it be more accurate than the figures for these materials? Blood doesn't seen to vary as much. 

With all of these is the added complication of temperature, too. If they sit at room temperature until they have warmed up or cooled off does the pH change? Will the blood unload it's CO2?


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## JeffyFunk

I have to admit, I've been following this thread for a while and I find it kinda disturbing because of my chemistry background... 

First of all, all of these solutions you are talking about are not "standards". Instead they're called "Verification Solutions". By definition, a standard is a solution used to calibrate the electrode. If the same solution is used to verify the accuracy of the electrode, then it's a "Check Standard". If it's a different solution of a different pH, then it's a "Verification Solution". 

Second of all, the pH meters that I have worked with in my laboratory can only be calibrated with solutions of specific pH values. The most common calibration solutions used are pH 4.0, 7.0 & 10.0 (@ 25oC) but there are other calibration solutions commonly used as well. Given that I work with a "laboratory" pH meter and am unable to change the pH values that it can be calibrated with, I doubt that a pH meter for home use would have the ability to change your calibration standards. 

Third of all, it's good laboratory practice to only use pH calibration solutions once. Therefore, when you calibrate (or verify) your pH probe, dispense a little of the solution into a clean, dry container and then dispose of the solution after you're done. 

Overall, I think that there are two good ideas that can come out of this thread, however. First of all, the idea of an independent pH Verification solution is a good idea and good laboratory practice. However, calling it a "standard" is misleading. Second of all, if you're "REALLY" trying to save money, why not just buy the salts that these pH standards are made up from and make your own pH standards? The chemicals themselves aren't that expensive (relatively speaking), non-toxic and stable. The equipment necessary to make accurate pH calibration solutions may be expensive (i.e. volumetric flasks, analytical balance), but that seems like an afterthought.

p.s. Diana K - Yes, the pH of solutions is temperature dependent. Most pH meters also have a temperature probe in order to make the necessary pH adjustments due to changes in temperature. For the "Calibration Solutions", the change in pH value with temperature is well documented and automatically taken into consideration during the pH calibration process. Yet another reason not to use random solutions to calibrate your pH meter with since their temperature dependancy is not as well known.


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## grak70

Jeffyfunk:

Thanks for your input. I'd be very interested to hear more about the specific pH meter you are using and why/how it is limited to 4,7,10 pH standards. Setting aside acid/base errors at very high or low pH, there is no apparent reason why other "standards" cannot be used. Are you sure this is not just built-in idiot-proofing? The probe you are using at work, while more sophisticated and sensitive, operates under the same principles as the ones we are using in the hobby.

Buffer pKa temperature dependence is an issue if your prepared buffer differs from your tank temperature, true. But it's not difficult to measure this. All you need is a microwave, a thermometer, and cold dilute (~0.1M) solutions of several buffers at different pH using the same ion pairs (NaH2PO4/Na2HPO4 for example). Measure pH and temperature after various times in the microwave.

pH = pKa(T)+log([A-]/[HA])

[A-] and [HA] are calculated from how much of each material you added. A plot of pH vs. log ([A-]/[HA]) for each temperature will have y-intercept pKa(T). Replotting these pKa values vs. T will give you the temperature dependence of pKa on T. If the pH of the solutions you are testing are close to the pKa at 25C/77F (say +/- 1) and the temperature variations you input are not severe (say +/-20F), the ratio of acid to conjugate base will remain nearly unchanged.


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## JeffyFunk

grak70 said:


> Thanks for your input. I'd be very interested to hear more about the specific pH meter you are using and why/how it is limited to 4,7,10 pH standards. Setting aside acid/base errors at very high or low pH, there is no apparent reason why other "standards" cannot be used. Are you sure this is not just built-in idiot-proofing?


In my laboratory, I use a Mettler Toledo SevenMulti System with the pH expansion unit. Oddly enough, the first unit we bought could have "custom" standards put into it while the second unit we purchased could not. I'm sure it is "idiot-proofing", but the system does come with 5 sets of predefined calibration buffers from which to choose from including the pH 1.68 / 4.00 / 7.00 / 10.01 set.



grak70 said:


> Buffer pKa temperature dependence is an issue if your prepared buffer differs from your tank temperature, true. But it's not difficult to measure this. All you need is a microwave, a thermometer, and cold dilute (~0.1M) solutions of several buffers at different pH using the same ion pairs (NaH2PO4/Na2HPO4 for example). Measure pH and temperature after various times in the microwave.
> 
> pH = pKa(T)+log([A-]/[HA])
> 
> [A-] and [HA] are calculated from how much of each material you added. A plot of pH vs. log ([A-]/[HA]) for each temperature will have y-intercept pKa(T). Replotting these pKa values vs. T will give you the temperature dependence of pKa on T. If the pH of the solutions you are testing are close to the pKa at 25C/77F (say +/- 1) and the temperature variations you input are not severe (say +/-20F), the ratio of acid to conjugate base will remain nearly unchanged.


I'm curious how you are deriving this formula and how to measure what you're talking about. From what I can see, the original equation looks a lot like the Henderson-Hasselbalch equation except that "T" is not a variable in the Henderson-Hasselbalch equation since T is assumed to be a constant. I'll be the first to admit that physical chemistry wasn't my favorite chemistry class, but I don't think can just stick T into the equation like that. Could you please explain how you derived that equation then? I'm sure I've forgotten something but I obviously can't remember....

Second of all, even if that *IS* a valid equation, I'm not following your logic for how to graphically determine the values from that equation. I'm familiar with the idea of plotting equations in the form of Y = mX + b, but I'm just not seeing how you can do what you say you can do. (Sure, you can do that with dG = dH - TdS as in the Gibbs' Free Energy equation where a plot of dG vs T will yield a y-intercept of dH and a slope of dS (where d = delta in all cases since I can't put greek characters in this message...)). If you're using the concept of Y = mX + b, then why wouldn't you just make up a solution of known [Base] and [Acid], then measure the pH as a function of T so that a plot of pH vs T would yield a y-intercept (b) of log([Base]/[Acid]) and a slope (m) of your pKa? Besides, shouldn't you know the pKa of the compounds you're using anyways since they're well defined?

I stand by my original assertion that it's just easier to buy premade standards or purchase the chemicals that are used to make pH standards than risk using other chemicals to calibrate a pH electrode with. Can other chemicals be used to verify the calibration of an electrode? Sure. But calibration is a different thing completely in my eyes...


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