The Significance of Ultrapure Water 18.2

"Why is 18.2 MΩ·cm the maximum limit, and why can't there be a higher value?"

"What exactly does 18.2 MΩ·cm represent?

What is the relationship between it and a conductivity of 0.055 μS/cm?"

"At what level of impedance does the ultrapure water consumable need to be replaced?

18 MΩ·cm or 10 MΩ·cm?", and why?

First, we need to explain what 18.2 MΩ·cm represents!

18.2 MΩ·cm is a water quality testing data point. It measures the concentration of cations and anions in water and expresses it as resistivity. The lower the ion concentration in water, the higher the measured resistivity; conversely, the higher the ion concentration, the lower the measured resistivity. Therefore, resistivity is inversely proportional to ion concentration.

But why is the limit value 18.2 MΩ·cm? If the ion concentration in water approaches zero, why isn't the resistivity infinite?

To understand why, first understand the reciprocal of resistivity >>

Conductivity! What is conductivity?

'Conductivity is measured by placing a cathode/anode electrode with a fixed low AC voltage in the water to be tested and measuring the current. Because the current is directly proportional to the concentration of cations and anions in the water—the fewer the ions, the smaller the current, and vice versa—conductivity is linearly proportional to the ion concentration.

The unit of conductivity is usually expressed in μS/cm. "μ" stands for 10⁻⁶, "S" is the first letter of Siemens, equivalent to the unit of electrical conductivity, Mho, and cm means centimeters.' Interestingly, pure water with zero conductivity (meaning zero ion concentration) does not exist. It's impossible to completely remove all ions from water, especially considering the following water dissociation equilibrium equation:

H₂OH⁺ + OH⁻ [H⁺] × [OH⁻] = 1 × 10⁻¹⁴ (25℃)

From the above dissociation equilibrium equation, H⁺ and OH⁻ can never be removed! When there are no other ions in the water besides [H⁺] and [OH⁻], the lowest conductivity is 0.055 μS/cm (this value is calculated based on the concentration of individual ions in the water, as well as the ion's mobility and other factors, based on [H⁺] = 1 × 10⁻⁷ M and [OH⁻] = 1 × 10⁻⁷ M). Therefore, under this theory, it is impossible to produce water with a conductivity lower than 0.055 μS/cm at 25℃. The 0.055 μS/cm is the reciprocal of the well-known 18.2 MΩ·cm.

1/0.055 = 18.2 or 1/18.2 = 0.055

In electrical engineering, conductivity and resistivity are inversely related. Therefore, at 25°C, it's impossible to produce pure water with a conductivity lower than 0.055 μS/cm; in other words, it's impossible to produce water with a conductivity higher than 18.2 MΩ·cm. To reiterate, the unit "μS/cm" and the commonly used "MΩ·cm" are reciprocals. In other words, "μS/cm" and "MΩ·cm" refer to the same thing, like two sides of a coin. With data in either unit, you can easily convert between "μS/cm" and "MΩ·cm" using simple mathematical operations. In the pure water industry, μS/cm is typically used to express the water quality of wastewater, tap water, and RO water (based on the concentration of ions in the water), while MΩ·cm seems to be used only for ultrapure water. Generally, we use 18.2 MΩ·cm to indicate that the purity of ultrapure water has reached its limit (total salt concentration below 1 ppb). Under these conditions, the only conductive cations and anions remaining in the water are 1*10-7 M [H+] and [OH-]. However, it's important to note that some inorganic substances, such as silicates, have low electrical conductivity, so conductivity cannot truly reflect the actual concentration of silicates in water!

Finally, let's clarify, "At what level should the impedance value drop before ultrapure water consumables need to be replaced?" Generally, when the total ion concentration in water exceeds 1 ppb, the 18.2 MΩ·cm value will drop. If it drops below 10 MΩ·cm, it indicates that the total ion concentration exceeds 50 ppb. Furthermore, if it drops below 1 MΩ·cm, it indicates that the total ion concentration exceeds 500 ppb. Different laboratories have different water quality requirements. If the required resolution is in the single digits of ppb, it is recommended to replace the consumables immediately once the 18.2 MΩ·cm value drops.