Malachite Green test method

Another AOAC approved method for sulfites in meat is the qualitative test based on decolorization of malachite green solution when mixed with a sample containing sulfites. This may be used to screen samples prior to quantitative determination as described above. 

Two other methods have been approved by AOAC for determination of sulfites in food. One is a quantitative assay based on malachite green decolorization using flow injection analysis. Sulfite is released from a sample slurry with alkali, then the test stream is acidified to produce sulfur dioxide gas which diffuses across a Teflon membrane into a flowing stream of malachite green, and the extent of decolorization is measured at 615 nm. The other method is based on ion exclusion chromatography with sulfur dioxide being released by alkali extraction, and the diluted filtrate is injected onto an anion exclusion column linked to an electrochemical detector. 

Another test for phosphorus (as phosphate) is based on the fact that the phosphomolybdates (and phosphotungstates) form deeply coloured insoluble lakes with triphenylmethane dyes such as methyl violet or malachite green (Chapter 12.8). The procedure is as follows a drop of test solution is placed on a filter paper which is then sprayed with a 1% solution of methyl violet. After about half a minute the paper is then sprayed with a solution of acidified ammonium molybdate and a blue spot develops. This method is sensitive to 1 part in 500,000. 

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