The reason that the pH meter's reading is negative

Acidity meter measured at 25 °C acidic solution measurement <7, alkaline solution measurement> 7;

One, misunderstanding

The pH meter's reading will be negative because the pH is 7 and the acidity meter will measure the acidity. The pH meter reading will be zero. A positive number means that your measured object is acidic, and a negative number means that your measured object is alkaline;

Second, the PH meter reading is a negative condition

Acidity meter measured at 25 °C acidic solution measurement <7, alkaline solution measurement> 7;

Acidity meter PH value =-lg(H)--As annotated see below, when your H concentration is greater than 1mol/L, the negative number is displayed, and the greater the negative absolute value is, the stronger the acidity is, and the PH value shows 1 - The range of 14 is only when the H concentration is small, and it is not suitable for more than 1 mol/L!

Note: pH=-lgC(H)--under the impact of water molecules, the acid will ionize hydrogen ions. At this point, the hydrogen ion is excessive and the solution is acidic. The more hydrogen ions, the stronger the acidity. Similarly, if the alkali ionizes excess hydroxide ions in water, the solution is alkaline. The more hydroxide ions, the less hydrogen ions and the stronger the alkalinity. Therefore, as long as one value is given, the pH of the solution can be expressed, and in order to avoid the tedious calculation of the negative meditation index using the hydrogen ion concentration, biologist Sorensen proposed this inconvenience in 1909. The value used is replaced by a logarithm and is defined as "pH". Mathematically defined pH is a common log negative value for hydrogen ion concentration.

If the pH of your solution is clearly not negative, there may be a problem with your pH meter or electrode.