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Synthetic indicators seem to be exclusively used when determining the pH of a substance with an indicator (with the exception of that school experiment where you boil cabbage to demonstrate natural pH indicators) or in acid-base titrations etc. over natural indicators. Why is this the case when naturally occurring indicator can be easily prepared from accessible and sustainable methods, such as boiling cabbage?

Intuition seems to suggest that perhaps synthetic indicators are much more accurate and reliable, being specifically designed for the task, is this the case? Or are there other reasons why natural indicators are regularly used as a cost-effective, environmentally friendly and easily prepared alternative to synthetic pH indicators?

What are the advantages/disadvantages of using synthetic indicators over natural ones (or vice versa)?

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    $\begingroup$ Probably it's still cheaper. Also why waste good cabbage ;) $\endgroup$ – Mithoron Mar 24 at 0:17
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    $\begingroup$ Scientists like to minimize unknown factors. If you can give me a top 3 list of what's in red cabbage, I will be impressed. $\endgroup$ – Zhe Mar 24 at 1:51
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    $\begingroup$ Another point is that any kind of analysis is about achieving consistency. It is often better to synthesize a pure compound than to try to extract it from some biological and purify it. $\endgroup$ – MaxW Mar 24 at 2:07
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    $\begingroup$ Most of the common synthetic pH indicators were probably discovered somewhere around 1850-1920, when organic chemistry really started to take off with simple aryl compounds. This means the indicators are very easy to make with modern knowledge/facilities/supply lines, so they can be made pure in metric ton scale for pennies. Hard to beat those economics. $\endgroup$ – Nicolau Saker Neto Mar 24 at 5:15
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    $\begingroup$ As a side note, you shouldn't equate "natural" and "environmentally-friendly." Extracting a bunch of hydrogen cyanide from bitter almonds and dumping it in a lake has the same effect as synthesizing a bunch of hydrogen cyanide using Andrussow oxidation and dumping it a lake. $\endgroup$ – probably_someone Mar 24 at 15:10
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In acid-base titrations, synthetic indicators are exclusively used to find accurate end-point determinations because they always have a highly defined color change at certain pHs. For example, phenolphthalein ($\mathrm{p}K_\mathrm{a} = 9.7$ at $\pu{25 ^{\circ}C}$) is colorless in acidic solutions (precisely $0 \lt \mathrm{pH} \lt 8.2$), but it is pink in basic conditions when the pH of the solution goes above 8.2 (precisely $8.2 \lt \mathrm{pH} \lt 12.0$), which makes it ideal for strong acid-strong base titrations (Wikipedia). On the other hand, bromothymol blue ($\mathrm{p}K_\mathrm{a} = 7.1$ at $\pu{25 ^{\circ}C}$) is yellow in acidic solutions (precisely $0 \lt \mathrm{pH} \lt 6.0$), but it is blue in basic conditions when the pH of the solution goes above 7.6 ($7.6 \lt \mathrm{pH}$). However, it shows greenish-blue color in a neutral solution (precisely $6.0 \lt \mathrm{pH} \lt 7.6$; see the picture below).

Bromothymol blue_pH_Range

It also change color from yellow to pink in high acidic conditions such in concentrated hydrochloric acid ($0 \gt \mathrm{pH}$; see most left test tube in the picture below), thus it is ideal to use as a universal indicator (Wikipedia). These two examples would clearly show you, as you correctly put it, synthetic indicators are much more accurate and reliable since they are specifically designed for the task.

Bromothymol blue_pH_0

Sure, natural indicators could be easily prepared from easily attainable row materials, and hence, could be regularly used as a cost-effective and environmentally friendly alternative to synthetic pH indicators, but I regret to say that it can be used only in high school science activities to fascinate science-loving kids. Since those natural sources contain several color changing substances (e.g., flavanones, flavones, flavonols, and anthocyanidins), the color change at the end-points is not sharp, perhaps, due to the difficulty of determining $\mathrm{p}K_\mathrm{a}$ of relevant indicator(s). For example, although boiled red cabbage water can be used as a universal indicator in high school projects and show the end point of an acid-base titration (see the following figure), it cannot be used in a day-to-day analytical lab to do accurate calculations.

Natural-Indicators

Also, another reason for not using natural indicators is that most of these dyes tend to decompose in higher pH values. The best example is most common natural color indicator, anthocyanidins. The stability of anthocyanidins is dependent on pH. At a low pH (acidic conditions), colored anthocyanidins are present, whereas at a higher pH (basic conditions) the colorless chalcones forms are present (see scheme below) (Wikipedia).

Anthocyanidins

Most importantly, aside from the above, flavonols including anthocyanidins have been studied extensively in drug development due to their plethora of biological activities (Ref.1). Thus they have been targetted in much more important tasks than a usage as pH indicators.

Reference:

  1. M. Rahnasto-Rilla, J. Tyni, M. Huovinen, E. Jarho, T. Kulikowicz, S. Ravichandran, V. A. Bohr, L. Ferrucci, M. Lahtela-Kakkonen, R. Moaddel, “Natural polyphenols as sirtuin 6 modulators,” Scientific Reports 2018, 8, 4163 (11 pages) (DOI:10.1038/s41598-018-22388-5).
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The main issue with natural indicators is their purity, long term chemical stability and shelf-life. Note that some natural indicators are pretty good for example the famous litmus paper is obtained from lichens. It is a mixture of several pigments like the red cabbage. Sometimes titrations are carried out at warm temperatures, do you think the pigments in cabbage survive heat in acidic and basic solutions?

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