Sciencemadness Discussion Board

Quantitative analysis of aldehydes

jenijjun - 6-8-2021 at 04:03

Does anyone know any method how to quantitatively measure the amount of an aldehyde in a sample?

Antigua - 6-8-2021 at 04:05

Tollens test and check the weight of silver precipitated. Or bisulfite adduct and calculate on it's basis.

unionised - 6-8-2021 at 04:32

Or reaction with DNPH.
If three's lots of aldehyde, you can weigh the derivative.
If there are only traces, you can measure it by HPLC or some such.

karlosĀ³ - 6-8-2021 at 05:08

Second the DNP.

Bisulfite adducts can have a certain solubility even in water.

Bedlasky - 6-8-2021 at 15:13

Water soluble aldehydes (C1-C4) can be easily determined by acidimetry. You need 5% sodium sulfite solution neutral to thymolphtalein. Make a stock solution of your aldehyde, pipette some of it in to the titration flask, add thymol blue and add 0,1M NaOH until indicator turns blue. Then add 10 ml of 5% Na2SO3 solution. Aldehyde react with sodium sulfite according to equation:

Na2SO3 + RCHO + H2O --> RCHO.NaHSO3 + NaOH

NaOH formed in reaction is titrated by 0,1M HCl until indicator become colourless. I tried this method on formaldehyde and propionaldehyde and it worked well. If you don't have thymolphtalein you can use phenolphtalein.

I also found little bit different method which can be applied even on insoluble aldehydes:

Hydroxylamine method is another option, but it is more time consuming:

[Edited on 6-8-2021 by Bedlasky]

[Edited on 7-8-2021 by Bedlasky]

unionised - 7-8-2021 at 03:25

In my experience the sulphite/ thymolphthalein titration sucked badly. Phenolphthalein was even worse.
Even following the pH with a meter it was hard to determine the end point.

The titration with hydroxylamine/ hydroxylammonium chloride was much better.

The best precision I could get was by gravimetry using a modification of Tollens reagent.
You dissolve silver oxide in an aqueous solution of tertiary butylamine (this has the great advantage that it doesn't produce explosive silver compounds on standing.)

Add a weighed amount of your aldehyde solution and when the reaction finished, weigh the silver produced.

The big advantage to DNPH and HPLC is that it lets you analyse a mixture of aldehydes (and, of course it offers much better sensitivity than weighing).
It's not great with unsaturated aldehydes like acrolein.

And the derivatives are pretty colours.
The big disadvantage is that the stuff is potentially explosive.

My view on the ISO committee which decided that the best thing to do with it was to add perchloric acid is probably unprintable.

Bedlasky - 7-8-2021 at 05:25

What problems did you have? Not enough sharp end point? I never tried it with indicators. I used potentiometric determination and this surely work without problems. You adjust pH to 9,5 and titrate back to 9,5.

unionised - 7-8-2021 at 06:19

It's 15 or 20 years since I did this but as I recall, the "fade in" from clear to blue took about 2 ml of NaOH soln.

I also looked at the method using an alkaline solution of a mercury complex- reduced to metallic mercury- but I didn't see any advantage over silver, and I didn't fancy the toxicity.

Other colorimetric options are "purpald"
and 3-methyl-2-benzothiazolinone hydrazone hydrochloride

The N methyl derivative of DNPH is interesting- you don't get the same problems of cis/ trans isomers with aldehydes and unsymmetrical ketones.
I don't know if MDNPH is on the market as a reagent.