What is the Difference Between pH and Alkalinity?
1. The concept of pH
The pH value, also known as the hydrogen ion concentration index and the acid-base value, is a scale of the hydrogen ion activity in the solution, which is also a measure of the acidity and alkalinity of the solution in the usual sense. The "H" in "pH" stands for hydrogen ion (H+), and there are many theories about the source of "p". The concept of citing the chemical world is to add p in front of the dimensionless quantity to represent the negative logarithm of the quantity.
The pH value is actually a "logarithmic unit". Each number represents a 10-fold change in the acidity of the water. Water with a pH of 5 is equal to 10 times as acidic as water with a pH of 6.
At standard temperature and pressure, an aqueous solution with pH=7 (such as: pure water is neutral, because the product of the concentration of hydrogen ions and hydroxide ions naturally ionized by water at standard temperature and pressure (the ion product of water) The constant is always 1×10-14, and the concentration of the two ions is 1×10-7moL. The pH value is less than 7, indicating that the concentration of H+ is greater than that of OH-, so the solution is strongly acidic, and the pH value is greater than 7. It means that H+ The concentration of OH- is less than that of OH-, so the solution is highly alkaline. Therefore, the smaller the pH value, the stronger the acidity of the solution; the higher the pH, the stronger the alkalinity of the solution.
2. The concept of alkalinity
Alkalinity refers to the total amount of substances in water that can neutralize strong acids. Such substances include strong bases, weak bases, strong bases and weak salts, and the like. Alkalinity in natural water is mainly caused by bicarbonate (bicarbonate, bicarbonate, the same below), carbonate and hydroxide, among which bicarbonate is the main form of alkalinity in water. The pollution sources that cause alkalinity are mainly waste water discharged from industries such as papermaking, printing and dyeing, chemical industry, and electroplating, and the loss of detergents, fertilizers, and pesticides during use.
Alkalinity and acidity are important indicators for judging water quality and controlling wastewater treatment. Alkalinity is also commonly used to evaluate the buffering capacity of water body and the solubility and toxicity of metals in it. The definition of total alkalinity is more commonly used in engineering, which is generally characterized as the concentration value equivalent to calcium carbonate.
3. The difference and relationship between pH and alkalinity
From a conceptual point of view, pH and alkalinity are not the same thing, and their actual meanings are also different. There is no clear correspondence between pH and alkalinity. Water (or solutions) with the same alkalinity may not necessarily have the same pH value. Conversely, water (or solutions) with the same pH value do not necessarily have the same alkalinity.
The reason is that the pH value directly reflects the content of H+ or OH- in water, while the alkalinity includes not only OH-, but also the content of alkaline substances such as CO3-2 and HCO3-. For example: NaOH solution with an alkalinity of 0.1mol/L, pH=13; NH3-H2O solution with an alkalinity of 0.1mol/L, pH=11; NaHCO3 solution with an alkalinity of 0.1mol/L, pH=8.3.
Although there is no clear correspondence between alkalinity and pH values, in practice, the higher the alkalinity, the higher the corresponding pH; the lower the alkalinity, the lower the corresponding pH; the higher the alkalinity, the higher the corresponding pH The greater the buffering help; the lower the alkalinity, the lower the buffering capacity of the pH solution!
