Resistivity is a very important indicator for measuring the quality of pure water. Nowadays, the water purification system in the market is basically equipped with a conductivity meter. The conductivity can reflect the total ion content in the water to help us judge the water quality in time. In order to further ensure the accuracy of the measurement results, some laboratories use an external conductivity meter to measure water quality or perform metering, comparison, etc., but it is often found that there is a significant deviation between the external measurement result and the machine display value. Why?

To understand this problem, you need to start with the resistivity 18.2MΩ•cm.

1.What does it mean by 18.2MΩ•cm?

18.2 MΩ•cm is an important water quality test indicator for the concentration of anions and cations in water. When the ion concentration in water is lower, the detected resistivity will be higher; conversely, the higher the ion concentration in water, the lower the detected resistivity. Therefore, the resistivity is inversely proportional to the ion concentration.

2.Why the limit value of ultrapure water resistivity is 18.2MΩ•cm?

When the ion concentration in water is close to zero, why is the resistivity not infinite? To find out why, let's take a look at the reciprocal of resistivity - conductivity:

① Conductivity is used to indicate the conductivity of ions in pure water, the magnitude of which is linearly proportional to the ion concentration.

② The unit of conductivity is usually expressed in μS/cm.

③ Pure water with a conductivity of zero (indicating ion concentration) is not present, because technically, it is impossible to completely remove all ions in the water, especially considering the dissociation equilibrium of the following waters:

 H₂O≒H⁺+OH^-  

 c(H⁺)=c(OH^- )=1.0×10^-7

K_W=c(H⁺ )·c(OH^-)=1.0×10^-14（25℃）

From the above dissociation balance,

H+ and OH- can never be removed.

When there is no other ion in the water except [H+] and [OH-], the lowest conductivity is 0.055 μS/cm (this value is calculated based on the concentration of ions in the water, the mobility of the ions, and other factors). The calculation basis is based on [H+]=[OH-]=1.0×10-7), so theoretically, it is impossible to produce pure water below 0.055 μS/cm at 25 °C, and 0.055 μS/cm. It is well known that the reciprocal of 18.2MΩ•cm is 1/18.2=0.055.

Therefore, at a temperature of 25 ° C, it is impossible to have pure water having a conductivity lower than 0.055 μS/cm. In other words, it is impossible to produce pure water having a resistivity higher than 18.2 MΩ•cm.

3.Why is the water machine showing 18.2MΩ•cm?The results of your own measurements are difficult to achieve?

The ultra-pure water has a very low ion content and is highly demanding on the environment, operating methods, and measuring instruments. Any improper operation may affect the measurement results. When the laboratory measures the resistivity of ultrapure water by itself, the common operations are:

①Off-line monitoring: After taking out ultrapure water, it is placed in a container such as a beaker for measurement;

②The battery constant does not match: the conductivity meter with a cell constant of 0.1cm-1 cannot be used to measure the conductivity of ultrapure water;

③Temperature is not compensated: 18.2MΩ•cm of resistivity in ultrapure water generally refers to the result at 25°C. Since the water temperature during measurement is different, we need to compensate back to 25°C for comparison.

4. What should you pay attention to when measuring ultra pure water with external conductivity?

Refer to the contents of the resistivity test section in the latest GB/T33087-2016 "High-purity water specifications and test methods for instrumental analysis". For the resistivity measurement of ultrapure water, note the following:

①Instrument requirements: On-line conductivity meter with temperature compensation function, the cell constant of the conductivity cell is 0.01cm-1, and the temperature measurement accuracy reaches 0.1°C;

②Operation steps: Connect the conductivity meter cell to the water system during the measurement process, drain the air through the water, adjust the water flow speed to a constant, and record the water temperature and resistivity instrument after the resistivity reading is stable.

The requirements of the instruments and operating procedures mentioned in the above measurement methods are strictly observed to ensure the accuracy of our measurement results.