The BSA standard curve was probably used for a protein assay to quantify the rhodanese concentration. The KSCN standard curve is for the product of the rhodanese reaction.

Reference: Oyedeji O. et. al (2013). Characterisation of rhodanes produced by pseudomonas aeruginosa and bacillus brevis isolated from soil of cassava processing site. African Journal of Biotechnology. Vol. 12(10. 1104 - 1114).

Here it says The unit of activity was expressed in rhodanese unit (RU). One rhodanese unit was taken as the amount of enzyme, which under the given condition produced an optical density reading of 1.08 at OD 460nm. The protein concentration were determined using standard curve of potassium thiocyanate (KSCN) - a plotted graph of OD against concentration.

From the above there seems to be a difference with your suggestion Mr. Adam B. Shapiro. Its like the KSCN standard curve is for rhodanese quantification, but you suggested that KSCN standard curve is for the product of the rhodanese reaction.

Please help me clarify these. and again can the calibration curve for FeSCN2+ be used in lieu of KSCN in the extrapolation of Rhodanese concentration?

Please your direction, help and advice will be most appreciated. if you can help with materials on KSCN standard curve, that will also be helpful. Thanks.

I think it is a typographical error in the article. Instead of "protein concentration" it should say "rhodanese activity." Check with the authors.

Thank you Sir. You have been helpful. But what about the other points I raised? 

I don't have personal experience with the assay, but I would guess that there might be a problem using the iron salt instead of the potassium salt if there is a difference in the absorbance caused by the absorbance of iron. Also, iron salts are usually less soluble than potassium salts, and Fe(II) oxidized]s in air to insoluble Fe(III).

OK. Thank you once again. I am deeply grateful for your assistance