The video linked below shows how to titrate for the determination of formaldehyde. I don't understand the chemistry behind this, ie, how does Na2S
react with formaldehyde. Can anyone please enlighten me?
The H2S reacts with the formaldehyde to give trithiane (and probably some other gunge) leaving NaOH to be titrated with sulfuric acid.
I'm also supposing that (most) all of the Na2S dissociates and some of the NaOH reacts with the formaldehyde. The remaining NaOH would be titrated. I
imagine a quantitative result being calculated by difference and by assuming that the amount of Na2S is exactly (within the error of the method) the
same every time.
I suppose this would be OK for a fixed (and high) range of formaldehyde, say a final product. Much less useful, I think, for looking at a wider range
of concentrations (without modification of sample amounts, titrant/Na2S strength, etc.).
Or, derivatization with DNPH at pH 4.5 (citrate buffer), extraction with DCM, solvent exchange into MeCN, and quantitation via HPLC (C18, MeCN:H2O
gradient, ABS 360 nm). TLC works, too, but it is much harder to get good chromatograms by densitometry (although some freeware is available that can
work with photos, IIRC).
The H2S reacts with the formaldehyde to give trithiane (and probably some other gunge) leaving NaOH to be titrated with sulfuric acid.
But how does this change the amount of acid/base to be titrated?
Unless you mean it decreases the amount of the acid H2S.
I was wondering if somehow the formaldehyde was oxidized to formic acid - but I don't see how.clearly_not_atara - 8-2-2016 at 14:29
First note: in the video H2SO4 is added to a solution of Na2S. While this can be done without releasing H2S (up to a point), a single mistake can
kill you, so don't do it without a hood.
Anywho the relevant equilibria are:
H2S <> H+ + SH-
3H2CO + 3H2S <> (H2CS)3 + 3H2O
H2O <> H+ + OH-
Since H2S is a stronger acid than H2O it follows that increasing the concentration of H2CO decreases the concentration of protons.
[Edited on 8-2-2016 by clearly_not_atara]Magpie - 8-2-2016 at 14:38
I see now if trithiane is formed this consumes the H2S and formaldehyde, thus reducing the acidity.Dr.Bob - 8-2-2016 at 17:11
I would agree that forming a hydrazone (NHPH) might be easier to do as a colormetric type assay.
Perhaps you could add some urea or phenol and heat to form a polymer and then weigh the glob of goo that you get after drying it. I made Bakelite as
a kid that way.
The H2S sounds like it would work, but I found several other methods with a quick search:
The sodium sulfite method. Add an excess of sodium sulfite solution to the formaldehyde solution, then titrate with sulphuric acid and
phenolphtaleine. The following reaction applies:
CH2O + Na2SO3 + H2O → NaOH + Na[H2CSO4]
Walker, J. F.; Formaldehyde; ACS Monograph Series, 1964
---------
Abbas Afkhami and Mostafa Rezaei, Determination of Formaldehyde by Inhibition of the Malachite Green–Sulfite Reaction, Microchemical Journal 63,
243–249 (1999).
---------------
The hydroxylamine method for determining formaldehyde in my researches
The oximation reaction is carried out with hydroxylamine HCl which is very stable.
Each mole of oxime formed results in the formation of one mole of HCl which can be titrated with standard alkali to give a measure of the carbonyl in
the sample.
------------
The best way for determining formaldehyde is by using colorimetric determination using Florescence/UV-vis spectroscopy. You need to do
standardization first (using titration as mentioned by Oliver which is available in Walker, Formaldehyde book) and using known formaldehyde
concentration solutions that you prepared from your standard formaldehyde solution, you need to make your calibration curve using Florescence/UV-vis
spectroscopy.
In order to detect formaldehyde with spectroscopy method, you need to react your formaldehyde solution with acetylacetone reagent, in this reaction
each mole of formaldehyde will turn into a compound which has double bond and can be detected with spectroscopy methods. you can review this method in
the following pubs:
Nash, T. 1953. The colorimetric estimation of formaldehyde by means of the Hantzsch reaction. Biochem J 55: 416-421.
Belman, S. 1963. The fluorimetric determination of formaldehyde. Anal Chim Acta 29: 120-126
[Edited on 9-2-2016 by Dr.Bob]Ozone - 8-2-2016 at 17:56
Pretty much. But, the dependence of NaOH on the quantity of formaldehyde present would only be proportional if the formation of trithiane drove the
dissociation of the Na2S. Because I know this dissociation in water will happen spontaneously, the proportionality would only exist for some short
while--that is, you must mix the stuff and titrate it immediately. I imagine a reagent blank would also be required.
Anyhow, yeah, the sulfite titration is probably easier, but I still like the chromotropic acid assay: Make a standard curve with formaldehyde in
water, treat with chromotropic acid (see NIOSH method) and add 75% H2SO4 (we used 98% into a small sample aliquot in Hach tubes). The complex is
bright purple and linear with concentration. The standard curve is good for months (you can keep the sealed set in a drawer or box in the hood), and
you can read Hach tubes in most (cheap, VIS only) spectrophotometers (with appropriate adapter). Easy, accurate, and reproducible if you can get the
reagent (Hach sells the pillows/reagents, but it's always cheaper to make your own) and the acid.
Apparently, you can also get away from the concentrated H2SO4 with a mixture of HCl and H2O2:
I also think the Hantzsch rxn with acetylacetone looks good, too. Nice find.
Cheers,
O3
blogfast25 - 8-2-2016 at 18:07
Wouldn't it be simpler to titrate CHO with KMnO4 in acidic conditions, using the permanganate's colour as a natural indicator?Magpie - 8-2-2016 at 19:01
I could do the sulfite and hydroxylamine HCL methods with the chemicals on hand. I don't have a spectrometer so the colorimetric methods are out.
Use of KMnO4 sounds like a possibility.
Actually I couldn't do the Na2S method either as I have no thymolphthalein indicator. BTW, that looks like it might be fun to make if thymol is OTC
at the grocery store in oil of thyme.
Thanks to all for the help.blogfast25 - 8-2-2016 at 19:07
I could do the sulfite and hydroxylamine HCL methods with the chemicals on hand. I don't have a spectrometer so the colorimetric methods are out.
HCL!!!! Aaarghgh!!!!
You don't need a spectrometer for titrations involving KMnO4, unless you call your eyes spectrometers.
My only concern is reaction speed but aldehydes are so easily oxidised, I can't see a problem there. Oxalic acid (to CO2) is used to
standardise KMnO4, so 'go figure'! Ozone - 8-2-2016 at 20:03
So long as your solutions contain only formaldehyde, then KMnO4 is a great idea. See also, Jone's reagent (Dichromate in H2SO4). While I agree that
acidic pH will help with selectivity, I've got to recommend a matrix blank to rule out oxidation of other components that might be present.
Technically, if the matrix is sound, you could also use H2O2, add KI with H2SO4 and titrate with thiosulfate. You'd have to standardize the peroxide
(KI/H2SO4 with molybdate catalyst, or other, with thiosulfate titrant) first.
[Edited on 9-2-2016 by Ozone]UC235 - 8-2-2016 at 22:08
The problem with using KMnO4 is that formalin solutions contain a considerable amount of methanol.Magpie - 9-2-2016 at 15:56
I titrated 25mL of my aqueous formaldehyde solution made with anhydrous formaldehyde using the Na2SO3 method. The result was 1% formaldehyde in 125
ml. This is less than I had hoped for but then I only sparged for an hour.
Thanks Dr Bob for suggesting the Na2SO3 method - it's quite easy. I had perused the formaldehyde ACS monograph in the forum library before but wasn't
looking for analytical information at that time.
If you live in a salmon egg curing area you may be able to buy anhydrous Na2SO3, two lbs for $12.50 at Ace.blogfast25 - 9-2-2016 at 18:40
@Magpie:
Nice to see a plan work out.
But what the hell is Na[H2CSO4]?? clearly_not_atara - 9-2-2016 at 19:24
He means sodium methanolsulfonate, CH2OHSO3Na. He's off by a hydrogen atom.blogfast25 - 10-2-2016 at 07:11
He means sodium methanolsulfonate, CH2OHSO3Na. He's off by a hydrogen atom.
Thought so. Thanks!alexchim - 20-2-2016 at 11:30
Formaldehyde could be titrated by using aqueous solution of iodine in potassium iodide in basic media, iodine disproportionate into iodide (I-) and
hypoiodite (IO-) which oxidize formaldehyde into formiate (HCOO-) ions. Bring the mixture back into acidic media lead to the transformation of I- and
IO- ions into iodine, which could be titrated back with sodium thiosulfate. Redox equations could give the concentration of formaldehyde into a
diluted solution, and by the way give the concentration of the pure solution (if obtained from dilution).unionised - 21-2-2016 at 02:43
I'm pretty sure that the video is wrong. It should be sodium sulphite.
The mass taken is exactly half a mole of sodium sulphite but some odd looking fraction of a mole of sulphide.
And I have done the analysis of aldehydes by titration of the hydroxide released by reaction of sulphite with aldehyde using thymolphthalein as an
indicator.
I looked at a few titration methods and the sulphite one was the least friendly.
Even with a pH meter the endpoint is indistinct and creeps due to atmospheric CO2.
For formaldehyde the iodine based titration was the best.
For other aldehydes the reaction with hydroxylamine HCl was OK.
For the best precision I'd use gravimetric assay by precipitation of silver from a solution of silver oxide in aqueous tertiary butylamine (which
avoids the possibility of producing explosive silver amine compounds).
meohmix - 19-12-2017 at 11:56
Having performed a catalytic oxidation of methanol vapor with air over copper, I now have a solution of formaldehyde in methanol and water. I would
like to determine the weight % of formaldehyde I have produced.
Can I assume the presence of methanol would render sulfite, H2SO4 method of titration unusable/inaccurate?
How does such a unknown mixture behave under atmospheric and vacuum distillation?
Ultimately my goal is a 37% solution of formaldehyde in H2O, varying levels of MeOH are perfectly acceptable.
Can I assume the presence of methanol would render sulfite, H2SO4 method of titration unusable/inaccurate?
The methanol won't upset the titration much.meohmix - 19-12-2017 at 20:30
Thanks unionised.
I went to the local university library to have a look at the 3rd ed of Walker's Formaldehyde text and made a few scans. See PDF for it's description
of the sulfite method.
(I also scanned the chapter on distillation of formaldehyde solutions. I will post the PDF in another thread)
Again, member's thoughts are appreciated!
Attachment: FormTit.pdf (221kB) This file has been downloaded 528 times
