When using a color block comparator for water testing, one should hold it up to what?

Using a color block comparator for water testing requires optimal lighting conditions to accurately compare the colors of the test sample against the standard color blocks. Holding the comparator up to the horizon opposite the sun achieves the best results because it provides diffuse, natural light that minimizes glare and harsh shadows. This type of lighting allows for a more consistent and even illumination of the color blocks and the sample being tested, enhancing the ability to discern subtle differences in color.

Natural light is often favored for color comparison as it aligns with the human eye's sensitivity and provides a balanced spectrum. By positioning yourself with the sun behind you, you avoid any direct light that could distort the colors you are trying to analyze. Therefore, this practice is vital for obtaining reliable and accurate test results when conducting water quality assessments.