The slope is a key indicator to determine whether the life of the electrode is exhausted. Generally there are three levels of electrode life, the slope of new electrode after calibration is between 95% and 105%, if the slope is lower than 90 it is recommended to replace the electrode, otherwise it will affect the accuracy of its measurements.PH meter calibration with PH calibration fluid. The slope is calculated in relation to the potential of the electrode to measure the solution you have prepared. Three standard buffers, the calibration of the general choice of two on the line, if all three are involved in the calibration, the slope will have a corresponding difference.

What is pH? pH is an abbreviation of the Latin term "Pondus hydrogenii" (Pondus=pressure, pressure hydrogenium=hydrogen), which measures the activity of hydrogen ions in a substance. This activity is directly related to the acidity, neutrality and alkalinity of aqueous solutions. Water is chemically neutral, but it is not free of ions, and even chemically pure water is dissociated in trace amounts: strictly speaking, the hydrogen nuclei do not exist in a free state until they are hydrated with water molecules.

H2O+ H2O=H3O++ OHˉ, and since the concentration of hydrated hydrogen ions (H3O+) is treated as equivalent to the concentration of hydrogen ions (H+), the above equation can be simplified to the following common form: H2O=H++ OHˉ.
The positive hydrogen ion here is expressed as "H+ ion" or "hydrogen nucleus" in chemistry. The hydrated hydrogen nucleus is expressed as "hydrated hydrogen ion". The negative hydroxide ion is called "hydroxide ion".