Pyrex Suspensions in Turbidimetric and Colorimetric Determinations—Standards of Comparison for Bacterial Suspensions and in the Resazurin Test

Abstract

IT has been difficult in general laboratory practice to prepare reliable standards of comparison for turbidity and turbidity with color, because, while it is possible to establish an adequate color-range with inorganic compounds in acid solution, materials for the simulation of turbidity are neither so readily available nor so satisfactory. In 1939 Brewer and Cook1 reported that a suspension of Pyrex glass had been found to be markedly superior in the standardization of typhoid vaccine to the suspensions of silica or barium sulfate ordinarily employed as standards of turbidity. Particles of from 0.5 to 3.5 microns in diameter were suspended in distilled water to simulate suspensions of typhoid bacilli; they did not dissolve or aggregate during a 5 year period of storage. The technic of Brewer and Cook has been adapted to the preparation of stable permanent standards of turbidity in the preparation of bacterial suspensions.2 Pyrex suspensions in colored media have also been used experimentally as color standards in the resazurin test used in the grading of milk.3 Other possible uses of the suspensions are as standards of comparison for milk of varying natural colors; alphanaphthoflavone-iodine adsorption compound, a colored suspension that is formed in a test for residual chlorine in water4; colloidal complexes of copper or zinc with sodium diethyldithiocarbamate; mixed suspension of silver chloride and silver chromate found at the endpoint of the titration of chloride in water; precipitate of barium sulfate encountered with turbidimetric tests for sulfate or barium. For this latter purpose it would be necessary to modify the procedure of decantation to select the larger particles.