[prdolino]

Hello,

I am trying to standardize a stock solution of formaldehyde and I am about to go insane. I am following the method to the tee and it is simply not working out. This method is very famous and is the following:

Reagents
1. Na₂SO₃ - 1 mol/L (eg. can be made by dissolving 126 g of anhydrous sodium sulfite per litre of water)
2. Thymolphthalein - 10 g/L of ethanol
3. H₂SO₄ - 0.01 mol/L (or 0.02 N)
4. 1500 ug/mL stock solution - 3.8 mL 37% v/v of formaldehyde solution made up to 1 L with water

Procedure
Pipette 50 mL of sodium sulfite (1.) into 150 mL Erlenmeyer flask. Add two drops of thymolphthalein indicator (2.). Add a few drops of sulfuric acid (3.) if necessary until the blue colour disappears.

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I follow the above. The problem starts when I place 50 mL of sodium sulfite into the E. flask and add the thymolphthalein. I add a couple of drops and the solution turns blue. I then add a few drops of sulfuric acid and nothing happens. I add maybe 20 more and the solution loses some of the blue intensity but is still clearly blue. After adding a few tens of mL of the acid, the solution finally turns somewhat clear. There is still no obvious endpoint as when phenolphthalein is used (in other titration experiments I have performed).

According to my stoichiometry, when 10 mL of the stock formaldehyde solution is added to the sodium sulfite solution (this time clear because I added a couple of drops of conc. sulfuric acid), about 25 mL of sulfuric acid (3.) is needed to reach the equivalency point. I added ~160 mL...

I am getting really frustrated with this. Anyone else encounter similar problems?

Thanks

[Borek]

I then add a few drops of sulfuric acid and nothing happens. I add maybe 20 more and the solution loses some of the blue intensity but is still clearly blue. After adding a few tens of mL of the acid, the solution finally turns somewhat clear.

So, this time you have added few tens mL of the acid to turn the solution clear.

(this time clear because I added a couple of drops of conc. sulfuric acid)

But this time you have added few drops of the acid to turn the solution clear.

No idea what you really did.

Then, what is the reaction equation?

[prdolino]

10s of mLs -> 0.02 N sulfuric acid used (as stated in the method)
few drops  -> concentrated sulfuric acid used

I was simply playing around at this point and used conc. acid to get that clear solution so I could add the 10 mL of formaldehyde and see if I get to the equivalence point when I add approximately the theoretical amount of 0.02 N sulfuric acid. I added something along the lines of 160 mL, which is absolutely ridiculous. I am assuming something is wrong with my anhydrous sodium sulfite.

However, I will be performing the standardization using a pH meter this time to see if I get better results.

[Borek]

OK.

I don't know the method, and your description is - at best - incomplete. Thus it is even hard to speculate on what is going on.

[wzhu]

Hi,

I am going to work on formaldehyde in the near future. I have no idea about HCHO standardization, but according to my experience on standardization, you may want to refer to some document with detailed introduction on the procedure including solution preparation, etc. Hopefully this is helpful.

[twinpeak]

From memory, the balanced reaction would be something like:

2CH₂O + 2Na₂SO₃ + H₂SO₄ --> Na₂SO₄ + 2CH₂(NaSO₃)OH

The few examples I've seen appear to employ a two-fold molar excess of sodium sulfite. You currently have around a one hundred-fold excess of this reactant.

In your first experiment, I wonder if the solution was initially hard to neutralise/decolourise as a result of the excess sulfite and relatively dilute acid.

Your second experiment I'm not sure about although I agree with your estimation of the titre. In the details we want to have the solution just neutralised/decolourised prior to adding any sample. If the solution was rendered acidic due to your use of conc acid this could have resulted in a lower titre. Can you confirm that the solution returned to an intense blue colour on addition of the formaldehyde?