Check the listing for details. General Hydroponics pH 4.01 Calibration Solution, 1 Qt.. Condition: New. Listed at 17.00 USD. General Hydroponics’ pH 4.01 Calibration Solution is a high quality solution that allows growers to calibrate many popular brands of pH, EC, and TDS meters. By Simply dipping the probe in a small amount of the balanced solution, adjust (calibrate) the meter to read 4.01 and you are ready to go. At General Hydroponics, growing is at the heart of who we are and everything we do. We live it and breathe it. The story of General Hydroponics spans more than 4 decades — with humble origins as a garage-based startup in Berkeley, CA and growing in hydroponic plant nutrition that it is today. A community built by those who wake up every day with the grit to create something extraordinary — and to do it on their terms. We take inspiration from those who refuse to follow the rules. We believe in the underdogs. The rebels. The nonconformists. Because that’s exactly how we got our start.