pH measuring equipment must be cleaned and calibrated regularly to ensure they are giving the user accurate readings. pH instruments are calibrated to pH 7.0 first (buffer) then 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) solution is used as a reference or checking solution to ensure you have adequately cleaned the EC/CF probe. Sachets are sold in boxes of 25 pieces.