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I make soft pretzels using a hot (190 °F) and dilute (2-4% w/v) sodium hydroxide solution. The bath is kept hot in a open-top stainless steel vessel like a stockpot. I make my pretzel dough and dip the dough in the hot solution for about 20 s prior to baking. As you can imagine, there is a fair amount of evaporation, as well as absorption by the dough of the solution.

Can someone help me theorize what happens to the sodium hydroxide concentration over time? pH is already 14, so pH is not a good measure of concentration. I don’t need to measure precisely, but I would like to understand if concentration is increasing as water evaporates, or if concentration stays the same because the solution is evaporating, or if there is a decrease in concentration by absorption of the pretzel dough.

This will help me understand what I need to do to keep my production going without running out of solution. In other words, do I just add water back to the bath as it evaporates, do I add water and the same concentration of sodium hydroxide I did in the beginning? Or other?

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    $\begingroup$ If you measure the density, that might give you a better metric. $\endgroup$
    – A.K.
    Commented Apr 9, 2019 at 21:44
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    – A.K.
    Commented Apr 9, 2019 at 21:44
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    $\begingroup$ One posting (npr.org/sections/thesalt/2014/08/09/338591194/…) suggests lye is a browning agent; so perhaps as long as the pretzels come out brown it is working. $\endgroup$
    – Luquettm
    Commented Apr 9, 2019 at 22:08
  • $\begingroup$ pH may still work for you. Take a sample of the fluid. If pH is 14, then dilute it, take pH, and do the math. $\endgroup$
    – Luquettm
    Commented Apr 9, 2019 at 22:18
  • $\begingroup$ Measuring pH 14 may be tricky, as most of glass of pH electrodes does not like it. Using pH papers may not provide needed resolution. $\endgroup$
    – Poutnik
    Commented Apr 10, 2019 at 4:28

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You need both.

  • Adding water, as water evaporates, but hydroxide does not.

  • Adding the hydroxide solution, as it is taken away by pretzels

But I suppose the evaporation is the major effect.

Probably the most convenient way is measuring density, with areometer $\pu{1.0-1.2 g / ml}$

Additionally, hydroxide gradually absorbs carbon dioxide, becoming sodium carbonate. pH paper strips may do good here, if solution looses it's alkality.

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