Various indicators of water quality-pH value, suspended matter
1. The pH of water
The pH value of water is a negative logarithm of the concentration of hydrogen ions in the water, expressed as: pH=-lg[H+]
The pH value is sometimes referred to as the hydrogen ion index. It can be known from the concentration of hydrogen ions in the water whether the aqueous solution is alkaline, neutral or acidic. Since the value of the hydrogen ion concentration is often very small, it is very inconvenient in application, so the pH value is used as the judgment index of the acidity and alkalinity of the aqueous solution. Moreover, the negative logarithm value of the hydrogen ion concentration can just show the number and size of the change range of acidity and alkalinity, so it is very convenient to apply. And therefore get:
- Neutral aqueous solution, pH=-lg[H+]=-lg10-7=7;
- Acidic aqueous solution, pH<7, the smaller the pH value, the stronger the acidity;
- alkaline aqueous solution, pH>7, the greater the pH value, the stronger the acidity;
If the water quality is further classified according to pH value (acid, alkaline), you can get
- Strong acidic aqueous solution, pH<5.0;
- Strong acidic aqueous solution, pH=5.0-6.4;
- Neutral aqueous solution, pH=6.5-8.0;
- Weak alkaline aqueous solution, pH=8.1-10.1;
- Strong alkaline aqueous solution, pH>10.0;
2. Suspended substances in water
Suspended substances in water are particles with a particle diameter of about 10-4 mm or more, which are visible to the naked eye. These particles are mainly composed of sediment, clay, protozoa, algae, bacteria, viruses and high molecular organic matter, etc., often suspended in the water flow, causing water turbidity. These particles are very unstable and can be removed by precipitation and filtration. When the water is allowed to stand, heavy particles (mainly inorganic substances such as sand and clay) will sink. Light particles (mainly organic compounds such as animals, plants and their debris) will float on the water and can be removed by separation methods such as filtration. Suspended matter is the main source of turbidity, color, and odor. Their content in water is also unstable, often varying with seasons and regions.
3. Various indicators of water quality-water hardness
There are some metal cations in the water, which are combined with some anions. When the water is heated, due to evaporation and concentration, scale is easy to form. With the influence of heat conduction on the heated surface, we call the total concentration of these metal ions in water as water Of hardness. For example, the most common metal ions in natural water are calcium ions (Ca2+) and magnesium ions (Mg2+), which interact with anions in water such as carbonate ions (Co32-), bicarbonate ions (HCO3-), and sulfate ions (SO42 -), chloride ions (Cl-), and nitrate ions (NO3-) combine together to form calcium and magnesium carbonate, bicarbonate, sulfate, chloride, and nitrate hardness, iron in water , Manganese, zinc and other metal ions will also form hardness, but because their content in natural water is very small, it can be omitted. Therefore, the total concentration of Ca2+ and Mg2+ is usually regarded as the hardness of water.
The hardness of the water has a great influence on the boiler water. Therefore, the water should be softened or desalted according to the requirements of the boiler for the water quality.
The unit of hardness commonly used is mmol/L or mg/L. The equivalent concentration N commonly used in the past has been disabled. When converting, 1N=0.5mol/L
Because the hardness is not formed by a single metal ion or salt, in order to have a unified comparison standard, it is necessary to convert to another salt. It is usually expressed by the mass concentration of Ca0 or CaCO3. When the hardness is 0.5 mmol/L, it is equal to 28 mg/L of CaO, or 50 mg/L of CaCO3. In addition, some countries also use German degree and French degree to express hardness. 1 German degree is equal to 10 mg/L of CaO, and 1 French degree is equal to 10 mg/L of CaCO3. 0.5 mmol/L is equivalent to 208 German degrees and 5.0 French degrees. The relationship between commonly used hardness units is as follows:
4. Various indicators of water quality-resistance value, conductivity
1. Water resistance
When measuring the conductivity of water, it is related to the resistance value of water. The resistance is large, the conductivity is poor, and the resistance is small. The conductivity is good. According to Ohm’s law, at a constant water temperature, the resistance R of water is inversely proportional to the vertical cross-sectional area F of the electrode and proportional to the distance L between the electrodes.
The resistivity of water is related to the amount of salt in the water, the concentration of ions in the water, the number of ions’ charge and the speed of movement of ions. Therefore, the resistivity of pure water is very large, and the resistivity of ultrapure water is even greater. The purer the water, the greater the resistivity.
2. The conductivity of water
Because the water contains various dissolved salts and exists in the form of ions, when a pair of electrodes are inserted in the water, after being energized, under the action of the electric field, the charged ions will move in a certain direction, and the anions in the water will move to the anode. The cation moves to the cathode, making the aqueous solution conductive. The degree of conductivity of water is called electrical conductivity S (or electrical conductivity). Electrical conductivity reflects the amount of salt in water and is an important indicator of the purity of water. The purer the water, the less the salt content, the greater the resistance, and the lower the electrical conductivity. Ultrapure water can hardly conduct electricity. The size of the conductance is equal to the reciprocal of the resistance value. That is, S=1/R, S=(1/ρ)·(F/L). 1/ρ is called electrical conductivity, and its international unit is Simi-1 (S·m-1)