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We're a small homestead cheese manufacturing company. Recently we've done R&D for cheese that's salted by means of a brine.

Problem: we have had inconsistent pH readings and we are left confused as to the cause.

Brine recipe:

  • 12 L drink water
  • 3 kg sea salt
  • 14 g calcium chloride
  • 10 g white alcohol vinegar (this is the nominal recipe, in effect we had to adjust this amount to arrive at the desired pH).

The target pH for this brine is 5.20 with an acceptable ~0.05 deviation.

We use a not-top range but pretty good food-grade pH meter, electronic with a hard pointy tip that is able to measure liquids as well as solids. We calibrate the meter before measurements and the pH readings of water and milk are consistent with expected readings. So we are quite confident in our pH meter's accuracy (i.e. accurate enough for our purpose).

However, we have had very inconsistent results when adding the vinegar. To arrive at 5.20 had to add 10 ml (first time), then 15 ml, then 5 ml and as far down as 3.5 ml.

For instance, one time we added 10 ml and arrived at 5.2 and the next time adding 10 ml gave us 3.8 and another time 4.2.

So we find ourselves improvising the vinegar quantity instead of following a standard recipe.

What could be the cause of this? Salt, I read, could affect pH reading accuracy, but when we measure salt + water we find that the pH reading isn't affected (or not significantly enough to explain the huge variance we observed above).

As I am writing this, I realize we never measure the pH of the vinegar over time, could its acidity change over time?

Can you please help me think this through?

Bonus Question: we often prepare the brine the day before actual use, and we noticed an increase of +0.10 to +0.20 pH overnight. What could explain that?

Your help is much appreciated.

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    $\begingroup$ It might be your water. CO2 dissolves in the water and alters the pH. The vinegar is presumably a standard commercial (12% or other) version and you used the same bottle (batch) throughout? I cannot imagine the acidity of the vinegar changing significantly if the bottle is properly stored. What time period was the experiment performed over? Is the room temperature relatively constant? How accurate are your volume and weight measures? OTOH 0.05 (pH 5.20->pH 5.25) is not a large error for a low quality meter. I'd suggest repeated small scale measurements to check the performance of the meter. $\endgroup$
    – Buck Thorn ♦
    Commented Jul 30 at 8:45
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    $\begingroup$ See eg chemistry.stackexchange.com/questions/149486/… $\endgroup$
    – Buck Thorn ♦
    Commented Jul 30 at 9:17
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    $\begingroup$ Aside of the final brine, try also regular measurement of water alone // water + vinegar // water + calcium chloride // water + sea salt // water + vinegar + calcium chloride // water + vinegar + sea salt. This way you could determine the major source of pH variability. As Buck says, CO2 release can play its role, so time patterns may be significant too. $\endgroup$
    – Poutnik
    Commented Jul 30 at 10:14
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    $\begingroup$ P.S.: .. and the high sodium issue is the thing, as ACR says, I have forgotten to mentionit. $\endgroup$
    – Poutnik
    Commented Jul 30 at 13:44
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    $\begingroup$ Thank you. Vinegar: same source used throughout our trials. It is a 5% distilled vinegar. Water: we used bottled drinking water for the time being for this process. Temperature is relatively constant in our work area and cold storage area. The trials spanned over 6 weeks now, once a week. We use 0.00 g precision scale for vinegar and calcium chloride. We reconciled vinegar in g and ml too. $\endgroup$
    – neydroydrec
    Commented Jul 31 at 13:31

2 Answers 2

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Regular glass pH electrodes are known to give incorrect results in the presence of high sodium concentrations. It is called sodium error, because sodium ions begin to compete with hydrogen ions when their concentration is too high on the electrode surface. In 12 kg of water you have 3 kg of sea salt. You have about 4 M $\ce{Na+}$ (mol/L) ions. Now you wish that the electrode respond to 10$^{-3}$ M hydrogen ions! Think of this comparison. The pH readings are certainly not reliable in such a high ionic strength solution.

Try this experiment: Dip the calibrated electrode in your test solution with vinegar and salts, and measure the pH after letting it become stable. Electrodes can take about 1-2 minutes to stabilize. Quickly wash the electrode and dip it in a separated calibration solution of pH=7, and see if the electrodes shows the correct reading. Notice how long it takes to read pH=7 or close to it. Discard that test calibration solution.

Also, when you calibrate, the pH-meters show the slope of calibration. It should be 95% or higher. This indicates the status of the electrode.

Perhaps the correct procedure would be that the pH of water and vinegar mixture be measured before the addition of all these salts. If the vinegar contains a portion of alcohol, pH values are also unreliable then in a strict sense as pH is only defined and calibrate for purely aqueous solutions. Measuring the pH of milk is also a "cloudy" area. We have an emulsion and fats there and the pH reading is not the true pH. For practical purposes, it may be just a number for consistency.

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    $\begingroup$ Na+ ion error is pertinent at high pH. $\endgroup$
    – jimchmst
    Commented Jul 30 at 16:54
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    $\begingroup$ Also, in high Na concentration. 4 M NaCl is too much. It is brine. $\endgroup$
    – ACR
    Commented Jul 30 at 17:40
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    $\begingroup$ Thanks for the rich answer, this is why I cam to this platform :) My pH meter does not show a slope value unfortunately. And I just learned about what a slope of calibration is thanks to your response. For pH of milk, we know it is not accurate, but we get good enough results and use it for consistency of our product between lots. Most of our products allow for a 0.15 variance in pH readings. $\endgroup$
    – neydroydrec
    Commented Jul 31 at 13:49
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This seems to be a mixing problem, especially if everything checks out before the addition of the acetic acid and the deviations seem to be towards low pH. Alcohol Vinegar, is this a consistent source? What is the concentration and composition [just curious]. Carbon dioxide is almost saturated in the tap water, adding salt and CaCl2 will reduce the solubility raising the pH slowly. Check the tap water and possibly consider using reverse osmosis water especially if the water is chlorinated.

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    $\begingroup$ Thank you. Vinegar: same source used throughout our trials. It is a 5% distilled vinegar. Water: we used bottled drinking water for the time being for this process. $\endgroup$
    – neydroydrec
    Commented Jul 31 at 13:28

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