The HI3811-100 are high quality reagents that are pre-measured, allowing for users to achieve fast and accurate alkalinity measurements with the chemical test kit. These reagents follow the phenolphthalein/bromophenol blue method to determine phenolphthalein alkalinity/total alkalinity concentration.

For determination of phenolphthalein alkalinity, the dilute hydrochloric acid titrant neutralizes the sample to the pH endpoint near pH 8.3. With addition of phenolphthalein indicator, the solution turns pink to visually indicate it is near the pH endpoint. For total alkalinity determination, dilute hydrochloric acid is also used as the titrant. With addition of bromophenol blue indicator, the solution turns yellow to blue/green as it reaches the pH endpoint near pH 4.5. The number of milliliters of titrant used to reach the appropriate endpoint will determine the alkalinity as mg/L CaCO₃. These reagents are designed to be used with samples that have an expected alkalinity range of 0 to 100 mg/L and 0 to 300 mg/L CaCO₃.