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Are there alternative ways to determine $\mathrm{pH}$ of a liquid without using $\mathrm{pH}$-meter or litmus paper? In case if we didn't cook the liquid ourselves and don't know concentration of acid in it.

I want to see if it's below or above 5.5, the closer it's going to be to 5.5 the harder it would be to say if it's 5.4 or 5.5 or 5.6 using visual methods.

If yes, how precise it's going to be if compare with those two, what accuracy a method has?

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    $\begingroup$ Aquarists routinely use the sets to determine water pH. A solution of pH indicator mixture is dropped into water, resulting color then compared with color of the set of test tubes or paper. I guess accuracy can be within 0.2. $\endgroup$ – Poutnik Jul 10 '20 at 11:55
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    $\begingroup$ If you search the internet, you can find many suggestions for common food items that can be used as pH indicator dyes. For example, this list: thoughtco.com/home-and-garden-ph-indicators-601971 $\endgroup$ – Andrew Jul 10 '20 at 12:02
  • $\begingroup$ A single roll of 5m universal indicator paper costs less than five Euros. How precise do you want it? $\endgroup$ – Karl Jul 10 '20 at 12:12
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    $\begingroup$ BTB has transition 6.0-7.6, AFAIK. $\endgroup$ – Poutnik Jul 10 '20 at 17:10
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    $\begingroup$ Yes you will. Unless your solution has it´s own colour, methyl red or BTB are well suited to make that distinction. With universal indicator, a difference of 0.6 is at the limit. You still haven´t said what kind of precision you need. Which leads me to conclude you don´t know exactly. (I understand that´s hard to find out, no sweat!) Please update your question if you can be more specific. $\endgroup$ – Karl Jul 11 '20 at 6:48
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from https://de.wikipedia.org/wiki/Indikator_(Chemie)

Take four test tubes with sample, add bromothymol blue in two of them, methyl red in the rest. Add a few drops of HCl to one of the BTB tubes, and a few drops of NaOH in one of those with methyl red.

One of the tubes will have a noticable change of colour.

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  • $\begingroup$ I want to see if it's below or above 5.5, the closer it's going to be to 5.5 the harder it would be to say if it's 5.4 or 5.5 or 5.6 looking at colors, this method seems couldn't provide precision of 0.1 in that case. But still looks great, thanks. $\endgroup$ – R S Jul 11 '20 at 14:49
  • $\begingroup$ If you buy a precision pH meter, and it shows you 5.50, what will you do then? That´s like buying a slide rule and asking wether the 50cm mark is more or less than 50cm from zero. It sure is one or the other, but nobody would care! $\endgroup$ – Karl Jul 11 '20 at 20:54
  • $\begingroup$ Then I'll assume it's 5.50 and not less or more :) $\endgroup$ – R S Jul 11 '20 at 23:03
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By using a indicator, you cannot achieve $\mathrm{pH} \ \pm 0.1$ accuracy. To get that accuracy, as Karl suggested, you must buy a precision $\mathrm{pH}$ meter (you still has to calibrate it before use). The best indicator to get fairly accurate reading within the $\mathrm{pH}$ you are interested is bromothymol blue indicator. The following is a display of color change according to increasing $\mathrm{pH}$ values:

Bromothymol Blue Indicator

The color starts to change from yellow to green when $\mathrm{pH}$ reaches the value of 5. At $\mathrm{pH} \approx 6.4$, it is more green than yellow. You can make a method to determine how close it is to $\mathrm{pH \ 6.4}$ such as counting the $\ce{NaOH}$ drops added until the yellowish solution becomes light green. Other than that, there is no other way you can reach needed accuracy without a $\mathrm{pH}$ meter.

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