Printing Tips│What is pH? Is the hydrogen ion concentration index [printing and dyeing basic knowledge]

The pH is one of the most important physicochemical parameters of aqueous solutions. Where natural solutions are involved

pH and pH of the solution

Chemical changes and production processes are all related to pH. Therefore, pH values ​​need to be measured in the fields of industry, agriculture, medicine, environmental protection, and scientific research.

The pH of water is the negative logarithm of the activity of hydrogen ions in water. It is expressed as:

pH=-lg[H+]

The pH value is also sometimes referred to as the hydrogen ion index. Since the value of the hydrogen ion activity is often very small, it is inconvenient in application, so the pH value is used.

pH test paper

This concept serves as an indicator of the acidity and alkalinity of aqueous solutions. Moreover, the negative logarithm of the activity of hydrogen ions can indicate the magnitude of the order of magnitude of the change in acidity and alkalinity, which is very convenient for application and is thus obtained (at 25°C):

(1) Neutral Aqueous Solution, pH = 7

(2) acidic aqueous solution, pH <7, the smaller the pH value, the stronger the acidity;

(3) alkaline aqueous solution, pH> 7, the greater the pH value, the stronger the alkaline.

meaning

The basis for the determination of the acidity, neutrality or basicity of a solution is the relative size of the concentrations of [H+] and [OH-]. At any temperature, the solution [H+]>[OH-] is acidic, [H+]=[OH-] is neutral, and [H+]<[OH-] is alkaline. However, when the [H+] and [OH-] in the solution are small, it is inconvenient to directly express the strength of the solution with [H+] and [OH-]. In order to avoid the cumbersome calculations using the negative exponent of hydrogen ion concentration, Danish biochemist Sernsen (Sernsen) proposed in 1909 to replace the inconvenient numerical value with a logarithm and define it as "pH". Mathematically defined pH is the usual logarithm negative value for hydrogen ion concentration, ie: pH = -lg[H+].

At a standard temperature (25°C) and pressure, an aqueous solution of pH = 7 (eg, pure water) is neutral because of the product of the concentration of hydrogen ions and hydroxide ions naturally ionized by water at standard temperature and pressure. The (water ion product constant) is always 1×10^(-14), and both ions have a concentration of 1×10^(-7) mol/L. A small pH indicates that the concentration of H+ is greater than the concentration of OH-, so the solution is strongly acidic, and the increase in pH indicates that the concentration of H+ is less than that of OH-, so the solution is strongly alkaline. Therefore, the smaller the pH, the stronger the acidity of the solution; the higher the pH, the stronger the alkalinity of the solution.

Usually the pH is a number between 0 and 14 , when the pH is less than 7, the solution is acidic, when pH> 7, the solution is alkaline, when pH = 7, the solution is neutral. However, under conditions of non-aqueous solution or non-standard temperature and pressure, pH = 7 may not mean that the solution is neutral. It is necessary to determine the neutral pH by calculating the ionization constant of the solvent under these conditions. At a temperature of 373K (100°C), pH = 6 is a neutral solution.

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