Mishulow and Krumwiede1 have recently suggested a colorimetric method for the estimation of the strength of bacterial toxins by means of titration with auric chloride, approximately as follows:
Two cubic centimeters of a 1:4 dilution of the toxin sample is added to each of a series of micro test tubes
containing graded volumes of 1 per cent auric chloride.
The contents of the tubes are mixed and warmed for forty minutes at 50 C.
At the end of this interval, the mixtures containing the smallest volumes of auric chloride are
observed to have undergone little notable change. Those representing the largest volumes of the reagent may have produced heavy flocculi. Intermediately, some of the mixtures may have developed a blue coloration (by transmitted light), and some a purple coloration, and some a green.2 The volume of auric chloride that has produced the most intense green color is presumed
LOCKE A, MAIN ER. INVESTIGATION OF PROPOSED COLORIMETRIC METHOD FOR TITRATION OF DIPHTHERIA TOXINS. JAMA. 1928;90(4):259-260. doi:10.1001/jama.1928.02690310011005