Talk:PH indicator
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Methyl violet Could someone please clarify the behaviour of this pH indicator? According to methyl violet it is the opposite to what is shown in the table. Either pH Indicator or methyl violet needs to be corrected.--Paiconos 22:14, 29 Apr 2005 (UTC)
- Fixed. Good catch. --TenOfAllTrades (talk/contrib) 15:22, 30 Apr 2005 (UTC)
is there a reson the listed indicators arent linked? --Herzog 04:11, 6 October 2005 (UTC)
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[edit] H+ is incorrect
There are no such species in solution as H+, the correct species representing an acidic proton in the hydronium ion (H3O+).
[edit] anthocyanin ph indicators
why in any of the articles has no-one metioned garlic? this turns green/blue when exposed to warm acid, eg; vinegar at approx 40 degrees celcius.
[edit] Range of an indicator
Can someone please explain to me how one might determine the pH at which an indicator changes color?
That is simple. You simply have to find the pKa (that is, the -log(Ka)) of the indicator. The pKa is when the two colours are at equal concentration, concentrations over that shift the indicator to the colour listed in the table. This can be proven using the equilibrium formula and allowing [HIn]=[In-]*, meaning that the two forms are in equal amounts. The two should then cancel leaving you with Ka=[H3O+], meaning that two forms are equal when the pH is equal to the pKa(definition stated above).However, usually when one colour is within 10X in concentration to the other, both forms of the colour will exist. Over that, one colour is definitely more visible than the other. Therefore, a transition pH (usually between +/-1, -- 10X or 1/10th concentration --, of the pKa) is given in tables.
That said, we should add a table of pKa values to this article.
- HIn stands for H+ ions binded to the indicator, where In = indicator. In- stands for the form missing the hydrogen ion. Therefore, the equilbrium of the two forms, the same as an acid/base equilbrium is: HIn<--->In- + H30+. The equilibrium formula can be used to find the pH at which the indicator changes colour, as seen above.
[edit] Value not matching
The pH range given in this article (4.4-6.3) does not match with the value given in Methyl red (4.4-6.2). Could somebody check the real figure and make the corresponding correction? Thanks. - LR4087 13:37, 24 October 2006 (UTC)
- Specified pH ranges are usually approximate; they rely on the appearance of a colour change, and there's definitely some uncertaintly at the edges of the range. A discrepancy of a tenth of a pH unit in these values probably means that the numbers were drawn from different reference texts; it's most likely that neither figure is more 'real' than the other. TenOfAllTrades(talk) 15:19, 24 October 2006 (UTC)
