# Ultraviolet acid base indicator

Is there a chemical(s) that provides good discrimination between absorbance curves for different pHs in the UV light spectrum (100-400nm?)

• Why this unusual wavelength range? Oxygen absorption cuts in at ~ 200 nm, water stops being UV-transparent at ~ 175 nm. – Abel Friedman Oct 20 '14 at 16:06
• 100 nm is tough! In terms of emission wavelength, the closest light source that comes to my mind is an argon excimer lamp (or laser) with $\lambda_{em}$ = 126 nm. – Klaus-Dieter Warzecha Oct 20 '14 at 20:01
• Let's ask the other way around: Which pH range do you have to cover, which other compounds (with relevant absorption in the UV/VIS) are present in your solutions? You might want to chose your indicator accordingly and perform UV absorption measurements at two different wavelength, i.e. at the absorption maxima of the protonated and unprotonated form of your indicator. – Klaus-Dieter Warzecha Oct 20 '14 at 20:07
• My motivation is curiosity. Universal indicator is a good broad range indicator in visible light but the discrimination between colours at pH 7 is quite poor. The light range doesn't have to be 100-400nm, it could be 200-400 or 700nm+ (infrared) it just needs to be good at highlighting differences in pH. Typically we use visible light indicators because we can see them but if you have a spectrophotometer that does UV, why not use it? – user2679447 Oct 21 '14 at 10:27

## 2 Answers

There are some good examples in Multiwavelength Spectrophotometric Determination of Acid Dissociation Constants Pharaceutical Research Volume 16, Issue 3, pp 374-381.

Benzonic acid (pka = 3.98), phenol (pka 9.73), phthalic acid (pkas 2.70 and 4.86), nicotinic acid (pKas 2.1 and 4.63) and p-aminosalicylic acid (pKas 1.79 and 3.58) would be good in the ~220nm-300nm range.

Red cabbage indicator absorbs the uv and visible radiations from520nm to600 nm range.For every pH the colours as well as the absorptions are also different. It shows deep red in pH 1,light red in 2,pink in 3,light pink in 4,5,6,purple in7,light purple in 8,light violet in 9,violet in 10,blue in 11,green in 12,yellow in 13 and deep yellow in 14.

• A bunch of links and references would be nice. Unreferenced material may be challenged and removed. – M.A.R. Apr 19 '16 at 16:00
• The main problem with this answer is "uv...radiations from 520nm to 600 nm" – DavePhD Apr 20 '16 at 14:50