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This question stems from one I just asked over at Seasoned Advice, where my recipe tasted decidedly off because the pH was wrong.

The recipe called for $\pu{16 g}$ bicarb of soda (sodium bicarbonate, $\ce{NaHCO3}$) and $\pu{15 g}$ of vinegar (mine was 5% acidity, and I assumed a density of $\pu{1 g ml-1}$). I just googled the corresponding pHs of these two, and apparently vinegar is more acidic than bicarb is basic: vinegar has a pH of 2.4–3.4 depending on concentration, and bicarb is pH 9.5.

How can I calculate whether I put too much vinegar or too much bicarb into my recipe? The ratio in the recipe is almost $1:1$.

What ratio in grams do I need of bicarb vs vinegar in order for the result to be neutral? I know there are other contributing factors to the pH in the recipe but I just want to focus on these two first. Assuming the vinegar is 5% which mine was.

Edit:

So I found the explanation for the chemical reaction between bicarb and vinegar here. Relevant bit:

$$\ce{CH3COOH + NaHCO3 → CH3COONa + CO2 + H2O}$$

$$\frac{\pu{1 g}\;\ce{NaHCO3}}{\pu{84.0068 g mol-1}} \times \frac{1}{1} \times \pu{60.0522 g mol-1} = \pu{0.715 g}\;\ce{CH3COOH}$$

What this formula tells you is that every gram of baking soda requires \pu{0.715 g} of acetic acid. But the remaining problem is that vinegar is not pure acetic acid. In fact, most vinegars vary from 4% to 18%.

Using those numbers, you would need somewhere between $\pu{0.715 g}/0.04 = \pu{17.9 g}$ and $\pu{0.715 g}/0.18 = \pu{3.97 g}$ of vinegar, for each gram of baking soda.

If you're lucky you can get some vinegar with the percent acetic acid listed on the label, then you can substitute that number into the equation above to determine how much vinegar you'll need.

So if I plug in my 5%, I get $\pu{0.715 g}/0.05 = \pu{14.3 g}$ vinegar for $\pu{1 g}$ bicarb.

Yikes! This is way more than $1:1$. But I'm not sure if it answers my question. Is the result, $\ce{CH3COONa + CO2 + H2O}$, pH neutral? If yes, then my vinegar does hardly anything to cancel out the base, is that right?

Edit 2:

Okay so according to Wikipedia, $\ce{CH3COONa}$ is a base. So even though my reaction formula shows what happens when all the bicarb/vinegar is used up, the result is not neutral (how much does the water and carbon dioxide contribute to bringing the pH down?). I need more acid to make my sodium acetate neutral (or at least neutral enough to be palatable?). This means that the needed ratio is higher than 14:1!? So almost certainly way too much bicarb?

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Your calculation is correct (14.3 grams of 5% acetic acid completely reacts with 1 g sodium bicarbonate). But you're not interested really in "completely reacts"; you want something like pH 7.

You've got it just about right with your mix; you're at the endpoint, and this is a point where the pH will drop very rapidly with even a small addition of vinegar. So it would only take a few additional drops to make the pH go down to 7... but that may not be what you really want.

Sodium acetate has a mild salty taste by itself. So I'm not sure adding extra vinegar will be worthwhile, it'll make it a bit more vinegary.

Here's what I'd do: Mix vinegar and baking soda in the ratio you've figured out, and then see how that tastes. Make adjustments at that point according to taste, rather than by "titrating" to an arbitrary pH.

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  • $\begingroup$ Why does the ph drop very rapidly with even a small addition of vinegar? $\endgroup$ – user13121 Feb 2 '15 at 7:32
  • $\begingroup$ When both acetic acid and acetate are present, the equilibrium between the two can shift when you add any more acid or base to the mix so the pH doesn't change much---this is called "buffering". When you only have one form or the other present, the mix can't buffer so any added acid or base can change the pH a lot. $\endgroup$ – Fred Senese Feb 2 '15 at 13:06
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I am a professional baker. A good ration in volume is for every 1/4 tsp Baking Soda, use 1 Tablespoon white vinegar. But you can try this for yourself. in a small bowl, measure 1/4 tsp Baking soda, pinch of salt, and a TBS sugar and 2 TBS water (because your baking formula will have similar ingredients. At this point there will be NO reaction (bubbles). Very slowly and incrementally add the 1 TBS white vinegar while stirring. As you add the vinegar, you will observe a reaction. You should also notice the reaction begins to weaken toward the last drops of vinegar. At this point you can taste the liquid. It will taste like flat soda like sprite or 7 Up but not have any objectionable acid or base taste. in all reality, the mix will still react with more vinegar,but this is good because you don't want to burn out all your leavener while mixing. You may increase these amounts in your recipe as required... hope this helps

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    $\begingroup$ I tsp a tea spoon, or a table spoon, or is that TBS? It would be preferable if you could use units of measurement that are also know outside the English-speaking world. $\endgroup$ – Martin - マーチン Sep 15 '17 at 1:14

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