This buffer solution with a pH of 4,0 @ 20°C (68°F) is needed to calibrate your pH-meter. The use of calibration solutions is essential to ensure the measuring accuracy of the pH meter and thus the most accurate measured values. Depending on the frequency of use, calibration should be performed every 1-2 months.
The 20 ml sachet is an ideal solution on-the-spot calibration. Simply open the sachet, insert the tester or electrode into the solution and calibrate.