The hemoglobin content in the blood is determined by colorimetric methods, one of which (the Sahli hematin method) is based on the formation of a stable brown solution when hemoglobin interacts with hydrochloric acid. The principle of the colorimetric method is that if the test solution is diluted to the same color as the standard solution, then the concentration of dissolved substances in both solutions will be the same, and the amounts of substances will be correlated as their volumes.