Acidity and alkalinity are characteristics of substances in an acid-base reaction. Generally, acidic substances can turn purple litmus test solution into red, and alkaline substances can make them blue.

Later, with the development of acid-base theory, people give a more accurate and complete definition gradually touches on the nature of acid and alkali.

There are three measures of acidity and alkalinity:

The pH of the aqueous solution and pOH.

The pKa of the acid and the pKb of the base.

The chemical hardness of the acid and base.

Acidity and alkalinity are generally detected by pH test paper, litmus test solution and phenolphthalein test solution.


Acid and alkali have different definitions at different stages in history, some of which have long since been eliminated, while others have been used.


In general, acidity and alkalinity refer to the property of discoloring the acid-base indicator, but not all acids and bases can discolor the acid-base indicator, which requires accurate definition.

Acid and Alkali

I think that acid is a compound in which all of the cations ionized in water are hydrogen ions, and the alkali is a compound in which all of the anions ionized in water are hydroxide ions.

Acidity/alkaline corresponds to acid/base respectively. Acidity is the property of reddish purple litmus and neutralizes alkali to form water and salt.

Alkaline can make purple litmus turn blue, and can be mixed with acid. And the nature of the water and salt produced.

Since the concentration of hydrogen ions and hydroxide ions in the water is measurable, the strength of the acid and base can be quantitatively described, which has the concept of strong acid and weak acid.

It should be noted that although some substances are not acids/bases according to this definition, they may still have an acidity/basicity.

For example, sodium hydrogen carbonate is not a base, but its aqueous solution is alkaline.

Bronsted Acid and Alkali

Bronst and Lauren believe that acid is the donor of protons, alkali is the acceptor of protons, and the acid and base have a conjugate relationship.

Therefore, acidity is the property of a substance to provide a proton to a base, and alkalinity is a property in which a substance can accept a proton provided by an acid.


Similarly, in this theory, we can quantitatively describe the strength of acid and base. In addition, although some substances still do not meet the theoretical acid-base definition, they still have acidity/alkaline.

For example, pure sulfur trioxide does not give protons, but it has strong acidity.

Lewis Acid and Alkali

Lewis believes that acid is the acceptor of electrons and alkali is the donor of electrons.

This theory can interpret the source of acid and alkalinity of most substances, and it is extremely practical, but it does not give the quantitative relationship between acid and alkali strength, and sometimes it cannot be compared with acid and alkali.

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For example, both boron trifluoride and boron trichloride are strong Lewis acids, but in some acid-base reactions, boron trifluoride exhibits a higher acidity than boron trichloride, while in other cases, trichloro the acidity of boron is stronger than that of boron trifluoride.

The specific comparison of acidity and alkalinity is a major difficulty in the Lewis acid-base theory. Later, HSAB proposed to some extent to make up for this defect.

Strong and Weak Scale

In the Arrhenius acid-base theory, the strength of acid-base can be quantitatively compared with the concentration of hydroxide ions in the aqueous solution (the concentration here is exactly the activity, but the hydrogen ion in the dilute solution) The concentration is close to the activity, and the concentration can be replaced by a concentration that is easy to obtain data.

The hydrogen ion concentration is expressed as c (H + ), the hydroxide concentration is expressed as c (OH – ), and the hydrogen ion concentration is increased as the acidity is stronger.

The greater the oxygen ion concentration, the stronger the basicity.

At the same temperature, c (H + ) · c(OH – ) in the aqueous solution is a fixed value, so that the stronger the acidity, the weaker the alkalinity of the solution, and the stronger the alkaline solution, the weaker the acidity of the solution.

ph Scale Chart

In 1909, a Danish chemist proposed using pH to indicate the strength of acidity and alkalinity. pH is the negative logarithm of hydrogen ion concentration, namely:

pH = – log [H + ], the same as pOH is the negative logarithm of hydroxide ion concentration
One of the great benefits of introducing pH is that it is easy to write and to compare the acidity and alkalinity of the solution. At 298 K, c (H + ) · c (OH – ) in the aqueous solution is a fixed value of 10 -14 , so pH + pOH = 14.

The solution with pH <7 is acidic, the solution with pH = 7 is neutral, and the solution with pH >7 is alkaline.


The acidity, neutrality or basicity of the solution is based on the relative sizes of c (H+) and c (OH-). At any temperature, the solution c (H+)> c (OH-) is acidic, c (H+) = c (OH-) is neutral, c (H+) < c (OH-) is alkaline.

At 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 pressure and temperature. (Ion product constant of water) is always 1 × 10 ^ (-14), and the concentration of both ions is 1 × 10 ^ (-7) mol / L.

ph Value

The small pH indicates that the concentration of H+ is greater than the concentration of OH-, so the solution is acidic, and the increase of pH indicates that the concentration of H+ is less than the concentration of OH-, so the solution is 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 <7, the solution is acidic. When the pH is >7, the solution is alkaline. When the pH is 7, the solution is neutral.

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Under non-aqueous or non-standard temperature and pressure conditions, pH = 7 may not mean that the solution is neutral, which requires determining the pH to neutral by calculating the ionization constant of the solvent under these conditions. At a temperature of 373 K (100 ° C), pH = 6 is a neutral solution.


The acid and alkali can be measured with litmus test solution and phenolphthalein. The litmus test solution is neutral and does not change color.

When it encounters acid redness, it turns blue when it is alkaline. The phenolphthalein does not change color when it is neutral or acidic.

A more precise method of measuring acidity and alkalinity is pH test paper, acidity meter, and neutralization titration.

Among them, the accuracy of the pH test paper is poor, generally, only one bit, or there is no valid number, the accuracy of the acidity meter can reach 2~3 digits, and the titration can reach two decimal places.

As science advances, pH meters can also be used to measure pH, and pH meters can be used to better control chemical reactions for productivity, product quality, and safe production.

A pH measurement system with automatic recording can also provide evidence of litigation against pollution hazards. Some batch production processes (such as certain fertilizer production, food processing processes) can be converted to continuous production using a pH meter.

The use of pH meters in modern industry is more than the sum of other types of continuous analytical instruments.

A pH meter is required in almost every production department that requires water.

Applications range from industrial water and waste treatment to flotation in mining, including pulp and paper, metal processing, chemicals, petroleum, synthetic rubber production, power plants, pharmaceuticals, food processing and more.