Hi,
I am trying to measure sodium dodecyl sulphate after incubation with methylene blue and subsequent extraction with chloroform. I always find the bluish color of the organic layer to be inversely proportional to the concentration of SDS present in the sample. Normally my sample involve up to 2% of SDS. I wonder why the colour of the organic phase is not directly proportional and give the most intense colour for the higher concentration of SDS. I have tried this with pure SDS and not other additive in solution.
Best
Debashish
Hello,
The determination SDS is based on the creation of an ion pair between the cation of methylene blue and anion of surfactant. Following the methodology is primarily the content of organic sulphates (VI) and sulphonates, but may experience some positive and negative external influences disturbing. MBAS may be below the values obtained in the presence of a cationic such as quaternary ammonium compounds and proteins that make up the compounds with anionic surfactants, eg., cationic and anionic surfactants present in the sample can react with each other to form stable complexes which do not react with methylene blue. In theory, any compound containing a single, strong anionic group and a hydrophobic portion can form a compound of extractable ionic bonding with a cation methylene blue. Organic sulfates (VI), sulfonates, carboxylates, phenols and certain inorganic anions such as cyanates, nitrates (V) thiocyanate, and sulfides may be active against the methylene blue.
Have you tried indicate SDS by direct two-phase titration in the presence of indicator: dimidium bromide and Acid Blue I?
Hi Elwira,
Thanks. My concern is even after using pure SDS in dist. water (0.1-2%) I did not get a concentration dependent response with methylene blue and chloroform extraction as reported in the literatures. When I used another stain. Stains-All, I could find a concentration dependent response.
Is there any advantage of measuring SDS by direct two-phase titration in the presence of the indicator: dimidium bromide and Acid Blue I.
Hi Debashish,
SDS concentration of your samples is relatively low. Two-phase titration method may not be sensitive enough.
I dont have information about the location of the band methylene blue in chloroform. Position of the band of the complex: a cation methylene blue - SDS anion, generated during the assay is at 650 nm. If the band of pure dye was at a different position no dependence on the color intensity of SDS concentration could be explained by the fact that an excess of MB in relation to the low content of SDS results in the inability to bind the indicator ion pair. Excess free dye remains in solution.
Best,
Elwira
Hi Elwira,
Thanks. Actually, I found an inverse correlation of SDS concentration and the optical density when measured at 653 NM, which is quite surprising to me.
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