Physical properties of sodium silicate
The composition of sodium silicate can be defined by the formula: Na2O(SiO2)n, where n is the number of SiO2 silica moles for each mole of Na2O sodium oxide, also called the Molar Ratio. This value can be between 0.4 and 4.0, but in most industrial applications it varies between 1.6 and 3.5. The atomic mass of sodium, Na=23, Oxygen O=16, Silicon Si=30. Na2O weighs 60 units of atomic mass, SiO2 weighs 62. Since a Na2O molecule weighs more or less as much as a SiO2 molecule, for sodium silicate the molecular ratio is similar to the weight ratio. In critical applications, however, it is necessary to specify whether a molar or weightful SiO2 /Na2O ratio is referred to. To switch from molar to weight ratio and vice versa just multiply or divide by the constant 62/60. For example, 2:1 molar ratio corresponds to a weight ratio of 2.06:1.
Silicate solutions have very variable viscosities; viscosity generally increases as the concentration of solids increases, and with the same dissolved solids, the higher ratios (with more SiO2 than Na2O) are more viscous. Viscosity also depends on temperature; at higher temperatures the viscosity decreases. Some types of silicate solution are delivered in insulated tankers and at a heated temperature (about 70°C at the start) to facilitate the unloading of tankers.
In the International System the density is measured in kg/liter; water has density 1. In the production of silicate, it is used to express the density in Baumé degrees (°Bé). The two units are bound by the following conversion formulas:
Density Kg/l= 145/(145-°Bé) °Bé= 145-(145/Kg/l Density)
For example, a silicate at 38° Bé will have densities of 1,355 Kg/l; 1520 g/l corresponds to 49.6 °Bé
The density changes with the temperature; a standard temperature of 20°C is usually referred to. The density of the silicate decreases as the temperature increases and increases almost linearly with the solid parts in solution.
The viscosity of sodium silicate should be considered as a function of concentration, density, molar ratio, and temperature.
The viscosity of silicate with a higher molar ratio (greater amount of silica than soda) increases faster with an increase in concentration than does the viscosity of silicates with lower molar ratio (more alkaline).
The viscosity of the sodium silicate (in commercial concentrations) almost decreases with the increase in temperature according to an almost exponential law; we can indicate with a fair approximation that an increase of 20°C in temperature corresponds to a halving of viscosity.
The pH of sodium silicate solutions is closely related to the concentration and molar ratio. The pH decreases as the molar ratio increases. The pH of sodium silicate solutions is kept high until the alcaline component is almost completely neutralized. This buffer action – the ability of the pH to withstand changes – increases as the molar ratio of the silicate increases; diluted silicate solutions also maintain a relatively constant pH, despite the addition of acid.