pH Buffer 4 is a solution of a known and fixed pH (acidity) level that is used to calibrate pH meters. pH Buffer 4 can also be used to test the accuracy of pH meters. PH buffer solution should be used on a regular basis to ensure the accuracy of the pH meter’s reading, as an incorrect acidity level of the nutrient solution can result in your plants being unable to take up essential nutrients. pH Buffer 4 is commonly used when a pH meter requires 2-point calibration. For example, if you need to calibrate the pH reading on a Bluelab Combo Meter, both pH Buffer 7 and pH Buffer 4 are required. Available in 250 ml and 1 litre bottles.