Boutron-Boudet’s reagent for determination of water hardness according to Boutron and Boudet The water hardness according to Boutron-Boudet is determined with various water samples of known and unknown hardness. It is a comparison of the amount of foam on different samples. HINT: A few drops of the solution are added to 40 ml distilled water and shaken. The samples with known and unknown water hardness are also mixed with the solution drop by drop and shaken until there is the same amount of foam with the same stability as in the distilled water sample. The water hardness can be calculated from this.