A reference solution used for adjusting nutrient Instruments. This PH Nutradip PH Meter Calibration Solution of 4.0 is for calibration of a PH pen. This fluid is at a set 4.0 ph and you use this to calibrate all PH pens just like the 3 in 1 Tri-Meter™. Read the calibration instructions for your instrument before calibrating. Comes in a 500 ml Bottle.