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Test method for gluconate in caustic etch baths

Q. My company is a supplier of metal treatment chemicals, and I am employed as a chemist in the laboratory, with functions of product development, quality control testing, customer service, and making tea.
We have a test method for caustic etch bath control using ceric sulphate to test for the concentration of sodium gluconate. However, we cannot work out where the factor 1.8 comes from. Its derivation is lost in the mists of time, and we do not know whether it is correct, wrongly calculated, or even comes about from a typing error.
Anyway, here is the method:
Put one ml of etch bath in a 250ml flask and add 50ml water and 10ml 50% sulfuric acid. Boil for 20 minutes to oxidise any sulfide and then add exactly 20ml 0.1N ceric sulphate solution. Boil for a further 30 minutes with stirring, presumably to oxidise the gluconate.
Cool and add ferroin indicator. Titrate the remaining cerium IV against 0.1N ferrous ammonium sulphate until the colour changes from blue/green to orange/red.
Sodium gluconate concentration = 1.8 (20 - FAS titre) g/l
BUT WHERE DOES THE 1.8 COME FROM?
There is a similar method for sodium oxalate determination where the equivalent weight of sodium oxalate is 67 (half the molecular weight), presumably on account of oxidation of both carboxylate groups. (See, for example, Vogel's Quantitative Inorganic Analysis [on Amazon affil links], publ. Longman.)
The molecular weight of sodium gluconate is 218. It contains one carboxyl group and five other hydroxyl groups.

Ken Gowers
Chemist, technical service and product development - Wrexham, Wales, United Kingdom
April 17, 2008

A. Ken

My guess is that this factor was determined empirically. But it's only a guess. If you can find some good purity sodium gluconate you could make yourself some standards and setup a calibration curve.

Terry

Terry Tomt
- Auburn, Washington

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Q. Hi. Can anyone explain to me how to determine concentration of sodium gluconate in a liquid sample by volumetric analysis? I work as a QC chemist. I know the assay test for Na gluconate using perchloric acid. I also want the calculation for this titration.

bhupesh mulik
- Mumbai, India
February 12, 2012