% iron - sulphuric acid pickling - steel wire rod?

[larc69]

I would like to know if the following testing method for iron % in sulfuric acid - steel wire rod pickling is correct and if any other exist?

first - % acid

- Add 5 mls of acid solution in a 250 mls becker.
- Add 100 mls of water.
- Add 4 drops of methyl orange
- use carbonate sodium until color change to yellow
keep same solution
second - %iron
- add 5 mls of sulfuric acid
- use potassium permaganate until color change (light pink).

NOTE: number of mls (1.on) used of permanganate potassium x 1.44= % iron


thanks

 

[kenvlach]

You've omitted some necessary details for the acid analysis; the iron analysis is partially correct but may still be useful.

I have added the conversion for sulfuric acid to procedures for hydrochloric acid pickling solution; H⁺ is H⁺.
It's probably more common to use sodium hydroxide solution than carbonate solution for acid titrations, but either works as long as you know the normality. The procedure I use for HCl-containing solutions is this:

Hydrochloric (or sulfuric) acid:
1. Pipet a 2 ml sample into a 250 ml beaker and add 100 ml DI water.
2. Add 5 mLs of 20%(w/v) KF (avoids Fe⁺³ precipitation).
3. Titrate to pH = 3.8 using 1 N NaOH.

[HCl (36 wt%)], vol.% = 4.356 x mls of 1 N NaOH.
or [H₂SO₄ (98 wt%)], vol.% = 1.33 x mls of 1 N NaOH.

Using a 5-mL sample & methyl orange indicator with 1 N NaOH titrant, the HCl concentration is given as
[HCl (36 wt%)], vol. % = 1.74 x mls of 1 N NaOH
or [HCl (32 wt%)], vol. % = 1.96 x mls of 1 N NaOH
or [H₂SO₄ (98 wt%)], vol.% = 0.532 x mls of 1 N NaOH.
A 1 N carbonate titrating solution should give the same results.

IRON analysis. Your oxidation titration only gives [Fe⁺²], not [Fe⁺³] ( the predominant Fe species present in yellow solutions).
Try adding hydrogen peroxide to a sample to see all Fe as Fe⁺³, or sodium thiosulfate for entirely Fe⁺². Also, I suggest not using wt% unless also measuring the specific gravity (SG).

[Fe⁺²], g/L = 11.1 x mLs 0.1 N KMnO₄ or 1.11 x mLs 1.0 N KMnO₄

For total [Fe], you can first titrate all Fe⁺³ to Fe[sup+2[/sup] by reduction with 0.1 N sodium thiosulfate (no excess, can do by ORP or color), then titrate all the Fe⁺² by oxidation with 0.1 N KMnO₄. Or, calculate the [Fe⁺³] from the reduction titration and add it to the [Fe⁺²] from your original oxidation titration.

Maybe only necessary to do the [Fe⁺²] analysis as you already have data on the pickling behavior. Also, some people determine [Fe] by careful SG measurement after adjusting both [acid] and temperature to standard values, but this is a bit crude.

 

[unclesyd]

Adding a little to kenvlach's I would use a polyethylene or teflon beaker for the first step. The HF is hard on glass.