A good starting point for ideas is Feigl's books Spot Tests in Organic/Inorganic Analysis (I don't have the inorganic one).

A simple test for 2- and 4-valent sulfur uses Raney nickel and HCl to give hydrogen sulfide, which can be detected (for example) with lead acetate paper or solution.

I once set this up in a flow system (post-column LC detection), using a phase separator borrowed from the hydride generator accessory of an atomic absorption spectrometer. Surprisingly, this doesn't seem to have been published, so please do so if you take up the idea.

Important safety warning: nickel salts are carcinogenic and particular care is required here because of aerosol formation.

Thanks Prof Lee, still am abit blur about the methods u just mentioned.

Hi, I gave a general kind of answer as I'm not familiar with the process and nearly all the literature is behind the paywall.

I did find a book chapter: edepotlink_t50c834c7_001.pdf

They measured only sulfide, using the ISO 10530-1991 test sold by Hach, which does not seem sufficient for your requirements. A full and specific analysis (of mercaptans etc) requires chromatographic or electrophoretic methods.

I understand that petroleum and natural gas are scrubbed with sodium hydroxide solution to remove sulfur comounds including hydrogen sulfide and mercaptans as the sodium salts of the anions. These products are then converted to sulfate by various oxidising reactions including the one you mention. Therefore, we need a simple method that will detect sulfur compounds other than sulfate.

In Spot Tests in Organic Analysis (Fiegl et al., Elsevier 1975, starting at page 78), several methods involving reduction of sulfur compounds under severe conditions  to hydrogen sulfide are given. I suppose there is more information relevant to your case in Spot Tests in Organic Analysis, but I don't have a copy.

I have used the Raney nickel method for organic compounds. Raney nickel is an alloy of aluminium and nickel. You add some of the powder to your solution and then acidify with HCl. The acid reacts with the aluminium to generate 'nascent' hydrogen in contact with nickel, which is a powerful catalyst that has become porous and clean because the aluminium content is being dissolved. Fiegl states that divalent and tetravalent sulfur compounds, but not sulfonic acids, sulfones or sulfate, are all reduced to hydrogen sulfide, which is entrained by the hydrogen and can be detected by any of the standard methods, including presumably the ISO one above.

In an old standard test for 'heavy metals' (deprecated for reasons that are not relevant here), the reference is obtained by adding H2S or a sulfide solution to a standard solution of a soluble lead salt. The resulting lead sulfide forms a colloidal coloured suspension that can be evaluated visually or by spectrophotometry.

It seems likely that this or one of the other reductive methods given by Fiegl might meet your requirements, being more general than the ISO one. However, they would have to be validated against methods currently used in the field.

Dear Prof Lee, It was so kind of you to put time for such an elaborated reply. I will try to find the book and go through. Highly appreciated.

Regards