Acidity and alkalinity

Note: This is an entry that describes acidity and alkalinity, if you want to get more information about acidity and base, please go to the article Acidity and Base Theory. Acidity and alkalinity are the characteristics of substances in the acid-base reaction, generally speaking, acidic substances can make purple litmus solution red, alkaline substances can make it blue, and later with the development of acid-base theory, people have given a more accurate and perfect definition, and gradually touched the essence of acid-alkaline causes. There are three scales of acidity and alkalinity: pH and pOH of an aqueous solution, pKa of an acid and pKb of an alkali, and chemical hardness of an acid or base. Acidity and alkalinity are generally tested by PH test paper, litmus test solution, and phenolphthalein test solution.

The Chinese name is acid-alkaline

The foreign name is acidityandbasicity

Meaning: The properties that a substance presents in an acid-base reaction

Identify acid-base indicator discoloration

Category: pH and pOH of aqueous solution

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pH test strip acidity and alkalinity correspond to the color

definition

Acidity and alkalinity have had different definitions at different stages of history, some of which have long since been phased out, while others have remained in use. Generally speaking, acidity and alkalinity refer to the properties that discolor acid-base indicators, but not all acids and bases can change the color of acid-base indicators, which requires an accurate definition.

I think of the acid-base

In my opinion, acids are compounds in which all cations ionized in water are hydrogen ions, and bases are compounds in which all anions ionized in water are hydroxide ions. Acidic alkalinity corresponds to acid-alkaline, acidity is the property that can make purple litmus red, and can neutralize with alkali to form water and salt, alkaline is the property that can make purple litmus turn blue, and can neutralize with acid to form water and salt. Since the concentration of hydrogen ions and hydroxide ions in water is measurable, the strength of the acid and base can be quantitatively described, which gives rise to the concept of strong and weak acids. It is important to note that although some substances are not acid-bases according to this definition, they can still possess acidic alkalinity, for example, sodium bicarbonate is not a base, but its aqueous solution is alkaline.

Bronster acid base

Bronster and Lauren believe that acids are donors of protons, bases are acceptors of protons, and that acid-bases have a conjugation relationship. Thus, acidity is the property of a substance that can donate protons to a base, and alkalinity is a property of a substance that can accept protons provided by an acid. Similarly, in this theory we can quantitatively describe the strength of an acid-base. In addition, although some substances still do not meet the definition of acid-base in this theory, they can still have acidic alkalinity, such as pure sulfur trioxide, which does not give protons, but is more acidic.

Lewis acid-base

According to Lewis, acids are acceptors of electrons and bases are donors of electrons. This theory can explain the source of the acidity and alkalinity of most substances, and the practicality is extremely extensive, but it cannot give a quantitative relationship between the strength and weakness of acidity and alkalinity, and sometimes even the acidity and alkalinity cannot be compared. For example, boron trifluoride and boron trichloride are both strong Lewis acids, but in some acid-base reactions, boron trifluoride is more acidic than boron trichloride, while in others, boron trichloride is more acidic than boron trifluoride. The specific comparison of acidity and alkalinity is a major difficulty in Lewis's acid-base theory, and the HSAB proposed later makes up for this deficiency to a certain extent.

Strength scale

In Arrhenius' acid-base theory, the strength of acidity and alkalinity can be quantitatively compared by the concentration of hydrogen ions and hydroxide ions in the aqueous solution (the concentration here is the activity accurately, but the concentration of hydrogen ions in the dilute solution is close to the activity, and the activity can be replaced by the concentration that is easy to obtain data), the hydrogen ion concentration is expressed as c(H+), the hydroxide concentration is expressed as c(OH-), the greater the hydrogen ion concentration, the stronger the acidity, and the greater the hydroxide ion concentration, the stronger the alkalinity. At the same temperature, c(H+)·c(OH-) in the aqueous solution is a fixed value, which shows that the more acidic the solution is, the weaker the alkalinity, and the more alkaline the solution is, the weaker the acidity.

In 1909, a Danish chemist proposed to use pH to express the strength of acidity and alkalinity, and pH is the negative logarithm of the concentration of hydrogen ions, that is:

pH = -log[H+], in the same way pOH is the negative logarithm of the concentration of hydroxide ions

One of the great advantages of introducing pH is that it is easier to write and to compare the acidity and alkalinity of the solution. At 298K, c(H+)·c(OH-) in the aqueous solution is a fixed value of 10-14, so the solution with pH + pOH=7 is acidic, the solution with pH = 7 is neutral, and the solution with pH 7 is alkaline.

The acidic, neutral or basic nature of a solution is determined by the relative size of c(H+) and c(OH-). At any temperature, the solution c(H+)c(OH-) is acidic, c(H+)=c(OH-) is neutral, and c(H+)c(OH-) is alkaline.

At the standard temperature (25 °C) and pressure, the aqueous solution (such as pure water) with pH = 7 is neutral, because the product of the concentration of hydrogen ions and hydroxide ions (the ion product constant of water) naturally ionized by water at standard pressure and temperature is always 1×10^(-14), and the concentration of both ions is 1×10^(-7), indicating that the concentration of H+ is greater than the concentration of OH-, so the solution is highly acidic, and the increase of pH means that the concentration of H+ is less than the concentration of OH-, so the solution is alkaline. So the lower the pH, the more acidic the solution; The higher the pH, the more alkaline the solution becomes.

Usually pH is a number between 0 and 14, when pH 7, the solution is acidic, when pH 7, the solution is alkaline, and when pH = 7, the solution is neutral. However, pH = 7 may not indicate that the solution is neutral under conditions of non-aqueous solution or non-standard temperature and pressure, which requires the value of pH neutrality by calculating the ionization constant of the solvent under such conditions. For example, at a temperature of 373K (100°C), pH=6 is a neutral solution.

Test

Litmus test solution and phenolphthalein can be used to measure acidity and alkalinity, litmus test solution does not change color when neutral, turns red when acidic, and turns blue when alkalin; Phenolphthalein does not change color when neutral and acidic, and turns red when alkaline.

The most accurate methods for measuring acidity and alkalinity are pH test strips, acidity meters and neutralization titrations. Among them, the accuracy of pH test paper is poor, generally only one place, or there is no significant figure, the accuracy of the acidity meter can reach 2~3 significant figures, and the titration can reach two decimal places.

With the advancement of science, pH meters can also be used to measure pH, and pH meters can better control chemical reactions, so as to improve productivity and product quality, as well as safe production. pH measurement systems with automatic recording also provide evidence for litigation against pollution hazards. Some batch production processes (e.g. some fertilizer production, food processing processes) can be converted to continuous production using a pH meter. In modern industry, more pH meters are used than other types of continuous analytical instruments combined. pH meters are required in almost all production sectors that require water. Applications range from industrial water and waste treatment to flotation processes in mining, including pulp and paper, metal processing, chemicals, petroleum, synthetic rubber production, power plants, pharmaceuticals, food processing, and more.