The CF27.7 solution is used to ensure you have adequately cleaned the EC/CF probe.  The probe surface is where the instrument takes the reading of the solution. The information is sent back from the probe to the electronic brain of the instrument. A calculation is then done in the instruments brain or micro computer and a reading is then displayed. If the information sent back from the probe is inaccurate due to probe surface contamination then the reading will be inaccurate.

1 x 500ml bottle of CF27.7 Calibration Solution