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| UI colour chart |
I caught Evil Technician pondering how to sabotage a simple pH experiment today. She had test tubes of sodium hydroxide solution (NaOH) mixed with universal indicator solution (UI), which showed the students sodium hydroxide was very alkaline - a strong purple colour. However, as she pondered (and this took a while as Evil Technician can be a bit ditzy sometimes), the experiment seemed to sabotage itself - the UI and NaOH mixture turned blue!
Pleased with that result she carried on with her day as usual, but I just had to know why. It just didn't make sense. NaOH is one of the strongest alkalis we have in the lab, how did it go from pH 14+ to ph 9?
I tested the NaOH/UI mixture with UI paper, just to check, and it told me it was still pH14. Which could only mean the UI
solution was changing colour by itself - weird, eh?
Admittedly, it took me a good part of a afternoon to discover its secrets, but I managed to convince Senile Technician it was important scientific research so I think I've got away it. Here's what I found.
Universal indicator is in fact made of four other indictors mixed together!!! Who knew? (clearly James Kennedy did as I got this poster from
his website). But why would this mean it changes colour in NaOH?
The answer lies in
phenolphthalein, an indicator which is colourless in acid but turns bright pink in alkali. At least, it does for a while. In NaOH solutions of pH 10 and above, the pink colour fades over time. There is a
very confusing essay on the kinetics of it, but here's the gist - phenolphthalein is magic.
So with this magic, given time in NaOH, the purple end of UI spectrum disappears,
giving the impression NaOH is less alkali then it really is.
Uh-oh, Evil Technician's come back and she's grassed me up. I really have to get back and do some proper work anyway or no classes will get any practicals that work this week. Honestly, I have to do every thing round here.
At last, I got my own pair of safety specs. I've been on at Senile Technician for months that I needed some proper PPE, but she's got a memory like a sieve (and a very lax attitude to health and Safety in my opinion). Hopefully I'll get my custom made lab coat soon too, then we'll be in business.
