Titration procedure for oxalic acid anodizing bath

A. Dear Marc: From Langford and Parker, Analysis of Electroplating and Related Solutions [affil link on Amazon, ... on AbeBooks], dilute a sample 10ml in a 100 ml volumetric flask [100 mL flasks on eBay or Amazon] using water. Pipet 10 ml of that diluted sample into a boiling flask, add 50 ml water and 25 ml of 50% sulfuric acid. Heat to 65-70 degrees C and titrate with N/10 KMnO4. 1 ml N/10 KMnO4 x 6.3 = g/l oxalic acid. You can test in your own laboratory if additions of oxalic acid will affect your sulfuric acid titration.

A. I would add to Tom's remarks that oxalic and sulfuric acid will titrate together in a simple acid-base titration, so your best bet is to determine oxalic acid with permanganate and then subtract it from your total acid result.

As for the acid-base titration: If the concentration of oxalic acid is high, you may be able to see a small inflection point between the first part of a potentiometric-end point titration curve - which includes all of the sulfuric acid and half of the oxalic acid - and the second part, which is due to just the second half (2nd proton) of the oxalic acid.

You may be able to get better separation of the two parts by adding methanol [affil links] or ethanol to your sample. I don't think an indicator method has any chance. A thermometric titration may work though (see my answer to letter 4626).

A. Hello,
here is my method to make the weekly analysis of my anodizing bath

Sulfo - Oxalic anodization bath

Preliminary :
Pipette [pipettes on eBay or Amazon] 10 cc of the bath in a gauged balloon flask of 100 cc. Complete up to 100 cc with demineralized water. = The mother solution

1) Oxalic Dosage:
pipette: 10 cc from the mother solution and poor it in an Erlen of 300 cc
+ 50 cc of demineralized water
Add + 3 à 5 cc .pure sulfuric acid.(96%) Attention hot!.
Titrate with potassium permanganate [on eBay or Amazon] N/10 -> rose persistent color

2) Free Sulfuric dosage :
10 cc of the mother solution -> Erlen. of 300 cc. + 50 cc demineralized water.
+~ 1 g. potassium fluoride pure. Mixed for complete dissolution.
+ 3 à 5 drops of bromothymol blue. (if powder : some grains)
Titrate by NaOH N/1 -> bleue color.

3) Dosage of dissolved Aluminium :
10 cc of the mother solution -> Erlen of 300 cc.
+ 50 cc demineralized water + Bromothymol blue
Titrate with NaOH N/1 -> persistent blue color

Calculation :

Oxalic A ..... cc x 6.3 = _____ g/l


Free Sulfuric(10 x B ... cc - A ... cc) x 4.9 = _____ g/l

Aluminium (C ... cc - B ... cc) x 9 = _____ g/l

Good luck
Best regards,