Acid-base equilibria
Read our general notes on Risk Assessment
Just a quick question about something brought up in class that confused my teacher; What pH is pure ethanol? We argue that because it is pure and there is no reason for the C-O-H bonds to break down, there should be no H+ ions and therefore it shouldnt be acidic. However, when Universal Indicator was added it was shown to be acidic. This is also the case when Methyl Orange was added. So why has this happened, and what is the true pH of pure ethanol?
Ulex replies
The concept of pH usually refers to solutions in water: pH = -log H+(aq). You should therefore have added your ethanol to water before testing, having first checked to see that the water is sufficiently pure to have a pH of 7 or something close to 7. The question is then a matter of whether ethanol donates protons to water or not. Like you, I would have thought it does not – at least, not enough to register with methyl orange, which would only turn red if the pH is less than about 4.5.
If your mixture of ethanol and water does have a pH of less than 4.5, I would suspect the purity of the ethanol. Pure, or so-called ‘absolute ethanol’, free from virtually all contaminants, is a rarity in school labs and very expensive. I suspect you were using rectified spirits or even industrial methylated spirits (IMS), which are fine for the majority of laboratory experiments but which do have other substances in them. A likely culprit could also be ethanoic acid, resulting from air-oxidation of the ethanol.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
Rate this page or react
Share your views on this page, 0 ratings so far
updated: 13 December 2006
