General Hydroponics pH 4.01 Calibration Solution is used to adjust your pH meter using the highest quality reference solutions, allowing growers to calibrate many popular brands of PH, EC and TDS meters (NaCl scale or the 442 scale). Easy to use, just dip your electrode into a small amount of the buffer and adjust the meter to read 4.01. Do not reuse solution. Suitable for all makes and models of pH pens and meters. Each batch is carefully checked by General Hydroponics technicians to ensure that it is precisely calibrated and yields reliable readings every time they are used.