A Thiazole Yellow Method for Magnesium Determination in Human Balance Studies

The objectives of this work were: to study certain factors affecting the use of a modification of the Young and Gill thiazole yellow colorimetric method for quantitative estimation of magnesium made in the Nutrition Laboratory at the University of Tennessee to permit its use in the analysis of food, feces, and urine; and to compare magnesium determinations obtained by this method with those obtained on the same materials by an established gravimetric procedure. The thiazole yellow method involves the formation of a red magnesium-dye lake, which can be measured calorimetrically, between magnesium hydroxide and thiazole yellow in the presence of a protective colloid. The determination is made directly on the ash solution and interference of other ions is prevented by a compensating solution.

Techniques were devised for preparing completely anhydrous magnesium sulfate to be used in the magnesium standard solution and for weighing the anhydrous salt sufficiently rapidly to keep absorption of moisture at a minimum.

Recoveries of magnesium in standard solutions having a magnesium: calcium ration of 1:0.7, 1:4, and 1:10 and in fecal ash solutions with ratios of 1:6, 1:7, and 1:16 ranged from 98 to 102%, indicating that the modified compensating solution is effective in preventing interference of calcium over this wide range of concentrations. Interference which occurred in food and fecal samples having a high ratio of total mineral to magnesium was overcome by the use of smaller aliquots or greater dilutions of the test solutions.

Even though the color of the magnesium-dye lake faded with time in blanks, standards, and samples, apparent magnesium concentrations and percentage recovery did not change appreciably when optical density readings were taken at 10, 20, and 30 minutes after addition of dye and base. A time interval of 20 minutes proved most efficient and convenient.

Magnesium determinations on food, feces, and urine by the thiazole yellow method averaged 6, 4, and 6% higher respectively than those by a gravimetric method in which magnesium was precipitated from the calcium-free filtrate as magnesium ammonium phosphate and ignited to magnesium pyrophosphate. This difference was not sufficiently great to prevent comparison of human balance data obtained by the two methods. The fact that the modified thiazole yellow method appears reliable and is rapid seems to justify its use in routine analysis for magnesium in human balance experiments.