pH measuring equipment must be cleaned and calibrated regularly to ensure they are giving the user accurate readings. pHÿinstruments areÿcalibrated to pH 7.0 first (buffer)ÿthen to pH 4.0 (slope). Bluelab's EC/CF equipment does not need calibrating, as they are all factory calibrated. The CF27.7 (EC2.77)ÿsolution is used as a reference or checking solutionÿto ensure you have adequately cleaned the EC/CF probe. Sachets are sold in boxes of 25 pieces.