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Analysis for Ferric Sulphate

Does anyone know of a safe and easy method of analysis for Ferric Sulphate where the answer is given in grams per litre. I have found it difficult to obtain an analysis procedure.

Nigel D Gill, B.Sc. MIMF AIEMA
Tehnologist. - Glasgow, Scotland
2004

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Nigel,

Is ferric sulphate the only dissolved agent within the solution you are analyzing? If this is the case, there are several titration procedures which offer accurate analytical procedures for iron ion. Cerric ammonium sulphate, potassium permanganate [on eBay or Amazon] and sodium thiosulphate [on eBay or Amazon] can be used as standardized reagent solutions for iron titration. The actual procedures are available within finishing and/or engineering textbooks. If the results are expressed in ounces per gallon (oz./gal.), just multiply by 7.49 to translate them into grams per liter (g/L). Calculating ferric sulphate from iron results can be done by multiplying the iron ion results by the molecular weight of ferric sulphate and dividing by the atomic weight of iron.

Randall Fowler - Fowler Industrial Plating, LLC
Cleveland, Tennessee, USA
2004

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2004

If all your iron is in the 3+ state, you can react it with an excess of potassium iodide to reduce it, then titrate the iodine generated by the reaction with calibrated sodium thiosulphate solution. Solution must be strongly acidic, and ferric ion complexants must be absent or rendered inactive by the high acidity. Best medium is HCl (sulfuric acid interferes). Iodide oxidation by oxygen should be prevented by a N2 or CO2 blanket (the latter can be generated by adding small amounts of bicarbonate). For your convenience, here is the procedure from Kolthoff and Belcher, Volumetric Analysis vol. 3 [affil link on Amazon], 1957, p. 343:

"Treat the ferric solution, 0.1 N in hydrochloric acid, with 1-2 g of potassium iodide and titrate with thiosulphate after 3 minutes. After the starch-iodine [indicator] is decolorized, heat the solution quickly to 50 degC and titrate directly to end-point. No iodine is lost with rapid heating because the small amount of iodine liberated on heating is held strongly by the excess iodide." The time needed to complete the reaction depends on Fe(3+) concentration and acidity; more dilute samples require more time. E.g., "Even 0.001 N ferric solutions can be titrated accurately provided that the solution is 0.1 N in HCl, at least 5 ml of 1 N KI is added per 50 ml of solution, and the reaction mixture is allowed to stand 30 minutes before the titration." (ibid.)

If your ferric iron is mixed with ferrous, or if you are working in a sulfuric and/or phosphoric acid solution, your best bet (titration-wise) is to reduce all iron to the ferrous (2+) state AND THEN to oxidize it quantitatively by titrating it with permanganate, dichromate or ceric sulphate (NOT thiosulphate). For an easy reduction procedure see my answer to letter 44029.

Emanuel Cooper
- Yorktown Heights, New York