What is Pure Water?
Ultrapure Water (UPW), Demineralised Water (Demi), High-purity Water and Deionized (DI) are all terms describing basically the similar property. They refer to water which has been purified to the highest standards by removing all contaminants such as organic and inorganic compounds; dissolved and particulate matter; volatile and non-volatile, reactive and inert; hydrophilic and hydrophobic; and dissolved gases. The purified water has very low conductivity which means it is high in resistance because all the conductive components have been removed.
Carbondioxide (CO2)
Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms. It is often referred to by its formula CO2. It is present in the Earth’s atmosphere at a low concentration and acts as a greenhouse gas. In its solid state, it is called dry ice. It is a major component of the carbon cycle.
The atmospheric carbon dioxide concentration has increased from an estimated 280 parts per million (ppm) at the beginning of the industrial revolution (around the year 1750) to a measured 406 ppm in 2017.
CO2 Solubility in Purified Water
The solubility of carbon dioxide in pure water is a function of pressure and temperature, as shown in graphic below. To summarize, increasing CO2 pressure increases the solubility of CO2 in the fluid. Increasing temperature decreases the solubility of CO2 in the fluid unless the pressure and temperature is approximately above 30 MPa and 65 °C respectively, where solubility begins to increase.
For reference, the molecular weight of CO2 is about 0.044 kilograms/mole. At ambient conditions (25 °C, 1 bar total CO2 pressure), the maximum solubility is in the order of several moles.
The solubility of CO2 decreases as the amount of dissolved material in the water composition increases. Fluid type refers to the dominate cation(s) and anion(s) in the fluid. For example, CO2 solubility in sodium bicarbonate brines is different than in sodium chloride brines.
Given enough time and surface area between a gas and a fluid, the partial pressure of CO2 in a gas phase will equal the calculated partial pressure of CO2 in the fluid phase. If the CO2 partial pressure in the gas increases, CO2 will dissolve into the fluid. If the CO2 partial pressure in the gas decreases, then CO2 will evolve from the fluid.
How CO2 effects pure water?
However, once water is exposed to the external environment, it begins to absorb CO2 and its conductivity increases (according to the atmospheric concentration of CO2, temperature, etc.). Therefore, if conductivity is displayed as 0.055 μS/cm (18.2 MΩ-cm), it is likely that the conductivity of the dispensed water has risen to 1 or 2 μS/cm (or dropped to 1 or 0.5 MΩ-cm) within seconds of collection. For ambient air, 300 – 500 ppm CO2 is typical, which causes the conductivity of water at 0.055 μS/cm (18.2 MΩ-cm) to rise to approximately 1 μS/cm (1 MΩ∙cm). Small, crowded areas can have CO2 concentrations between 1000 and 3000 ppm, in which case the conductivity will possibly increase to around 2 μS/cm (0.5 MΩ-cm).
One might expect the pH of the water inside a UPW system to be 7.0; however, as with conductivity, once water is dispensed and exposed to CO2 in the air at typical ambient conditions, the pH is expected to drop to approximately 5.7, a significant change from the predispensed UPW. Once CO2 is dissolved, it forms carbonic acid, which dissociates to hydrogen and bicarbonate ions, leading to changes in conductivity and pH. Measurements of pH can be unstable and inaccurate due to the low waterionic strength and low buffering capacity of UPW.One might expect the pH of the water inside a UPW system to be 7.0; however, as with conductivity, once water is dispensed and exposed to CO2 in the air at typical ambient conditions, the pH is expected to drop to approximately 5.7, a significant change from the predispensed UPW. Once CO2 is dissolved, it forms carbonic acid, which dissociates to hydrogen and bicarbonate ions, leading to changes in conductivity and pH. Measurements of pH can be unstable and inaccurate due to the low waterionic strength and low buffering capacity of UPW.
As mentioned above CO2 gas in atmosphere has very critical effects on purified waters. This is especially crucial for facilities which are treating raw water to ultrapure water levels and uses it in their processes. Purified water should be delivered into operational use right after treated or kept under isolated conditions from atmospheric CO2. CO2 absorber systems are one economic way to ensure keeping purified water storage tank away from CO2 contamination during atmospheric air intake.