Range: 0 to 300 ppb. The HI 749 Checker®HC is a simple, accurate, and cost effective way to measure low ranges of chromium VI in fresh water. Designed as a more accurate alternative to chemical test kits, the HI 749 provides quick, accurate results in four easy steps. The HI 749 uses an adaptation of the ASTM Manual of Water and Environmental Technology, D1687-92, Diphenylcarbohydrazide method. The reaction between chromium VI and the reagent causes a purple tint in the sample.