The reason why there is a big difference between pH meter and pH test paper!
PH test paper: the principle is based on the color reaction of indicator, that is, the test paper contains a certain amount of indicator, and when it changes from one color to another, it will consume a certain amount of H+ or OH-. When H+ or OH in the solution is not enough to change the indicator from one color to another, this will cause the pH test paper to be unable to accurately display the pH of the solution.
Buffer solution
Buffering theory points out that within the buffer range of the buffer solution, the pH of the solution is basically stable when a small amount of H+ or OH- increases or decreases, that is to say, enough H+ or OH- can be provided to discolor the pH test paper, but the pH of the solution is basically unchanged, and the pH test paper does not produce errors.
When the actual pH of the solution is beyond the buffer range, a little H+ or OH- will cause a great change in the pH of the solution. When testing with pH test paper, a little H+ or OH- which makes the actual pH of the solution exceed the buffer range is not enough to change the color of the test paper to the actual pH, while the original actual pH of the solution (soaked with the test paper) has changed (returned to the buffer range), and the pH test paper often still shows the value near the buffer range, resulting in errors.
summary
PH test paper consumes a certain amount of H+ or OH-, so pH test paper is prone to errors!
For strong acidic solutions and strong alkaline solutions, pH test paper will generally produce errors.
The pH test paper reflects the properties of the buffer solution more: within the buffer range of the buffer system, the pH test paper shows the actual pH of the solution well; Outside the buffer range, the value of the pH test paper will deviate from the pH of the solution, which can reach several pH units seriously.
