The HI3820-100 are high quality reagents that are pre-measured, allowing for users to achieve fast and accurate acidity measurements with the chemical test kit. These reagents follow the methyl orange/phenolphthalein method to determine strong acid/total acid concentration.

For strong acid determination, also known as methyl orange acidity, the sodium hydroxide titrant reacts with acids present in the sample. With addition of bromophenol blue indicator, the solution turns from yellow to green/blue as it reaches the pH endpoint near pH 4.5. For total acid determination, also known as phenolphthalein acidity, sodium hydroxide is also used as the titrant. With addition of phenolphthalein indicator, the solution turns pink as it reaches the pH endpoint near pH 8.3. The number of milliliters of titrant used to reach the appropriate endpoint will determine the acidity as mg/L CaCO₃. These reagents are designed to be used with samples that have an expected acidity range of 0 to 100 mg/L and 0 to 500 mg/L CaCO₃.