Boffis
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Neutralization Equivalence (NE) value
The NE value of a carboxylic acid is in effect the molecular weight if the acid is a mono-protic acid or half the molecular weight if it is a diprotic
acid. It is a valuable tool in examining reaction products that may be carboxylic/sulphonic etc acids.
It is easily measured by titrating a known weight of acid against standard sodium hydroxide solution. The problems with this technique are two fold.
The sodium hydroxide solution absorbs CO2 and most aromatic acids are sparing soluble in water. The former is overcome by simply using a freshly
prepared and standardised solution while the latter by the addition of a co-solvent. However, with some acids like halobenzoic acids and phthalic
acids even this is not possible and back tritration is often required.
I am having real problems getting accurate values though. For instance I have recently been examining terephthalic acid and chlorobenzoic acids. For
control I ran a standard, Analar 2-chlorobenzoic acid, but even this gave a poor result, NE=134 instead of 157 which is way outside normal analytical
error. Can anyone offer any help as to why back titration should give such poor results?
My NaOH solution is freshly prepared each time and standardized using recently purchase 2M hydrochloric acid which claims to be +/-0.5% of the stated
value. I ran an experiment using this acid to dissolve Analar BaCO3, boiled out the CO2 and then weighed the undissolved carbonate and got a value of
2.02M which is pretty good. I use the acid diluted to 0.2M using standard pipettes and volumetric flasks. The indicator is as stated in the procedure,
phenolphthalein. The values I am getting are highly reproduceable just too low.
I have attached a copy of the basic procedure but like I say I am having to use a back titration where I add 25ml of standardised approx. 0.1M NaOH
solution and then tritrate the excess NaOH against 0.2M HCl.
Attachment: Neutralisation equivalence (NE) value.docx (13kB)
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unionised
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What indicator are you using?
You may get better results if you follow the titration using a pH probe (You may not- especially with dilute solutions).
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Boffis
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I am using, as I said, phenolphthalein as indicator. The pKa of p-chlorobenzoic acids is 3.99 about the same as acetic acid while 2-chlorobenzoic acid
is significantly lower at 2.94. This means that the neutral point for o-chlorobenzoic acid is about pH 7.8 and for the p- isomer 8.25. So
phenolphthalein should give reasonable result. In fact the NE value is specifically derived using this indicator regardless of the acid. The only
modification I have made is the back titration, using an excess of alkali and then titrating the excess alkali against a known acid. The end point now
is theoretically at about pH 7 which should still be high enough not to protonate the carboxylate ion. In fact 4-chlorobenzoic acid is so insoluble
that deprotonation results in immediate precipitation of the free acid which would be visible.
I don't have a pH meter so that's not an option.
I am mystified as to why I am get such inaccurate end points inspite of them being very sharp and reproduceable.
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