Conductivity Measurements Vs. Refractive Index Measurement for Sulfuric Acid Applications

Accurate measurement of chemical concentrations is critical in industrial processes, especially when dealing with highly corrosive substances like sulfuric acid. The use of conductivity sensors and refractometers plays a significant role in ensuring precision and safety. However, each method has its own set of challenges and ideal use cases.

Conductivity Sensors: Versatile but Limited

Conductivity sensors measure the ability of a solution to conduct an electric current, which varies with the concentration of ions in the solution. These sensors are widely used due to their affordability and effectiveness in various applications such as monitoring drinking water, cooling towers, and processes involving less aggressive chemicals. 

The Nonlinearity Problem

When it comes to measuring sulfuric acid concentrations, conductivity sensors face significant challenges due to the non-linear relationship between concentration and electrical conductivity. This nonlinearity means that at certain points, the sensor may give the same reading for different concentrations, making it difficult to determine the actual concentration accurately.
For example, a sensor might show 1.1 micromho for both 20% and 50% sulfuric acid solutions. This ambiguity arises because the conductivity curve of sulfuric acid does not increase linearly with concentration. The result is that a conductivity sensor alone cannot reliably differentiate between these concentrations without additional information or correction methods.

Temperature Compensation

Temperature significantly affects conductivity readings. A conductivity sensor without proper temperature compensation can yield erratic results, as seen with substantial differences in readings at 70 degrees versus 180 degrees. Ensuring accurate measurements requires temperature compensation mechanisms, adding complexity and potential points of failure to the system.

Refractometers: Precision in Challenging Environments

Refractometers measure the refractive index of a solution, which changes with concentration. This method is particularly advantageous in applications where conductivity measurements fall short, such as with sulfuric acid.

Benefits in Sulfuric Acid Measurement

Refractometers provide a more reliable measurement for sulfuric acid due to their ability to handle the nonlinear conductivity curve. Unlike conductivity sensors, refractometers are not as affected by temperature variations, making them more stable in harsh conditions.

Limitations and Maintenance

However, refractometers are not without their challenges. They are generally more expensive and they require regular maintenance due to the aggressive nature of sulfuric acid, which can erode the prism seals. These seals, often made from Teflon or similar materials, are crucial for preventing the acid from entering the sensor. Once compromised, the sensor must be replaced to avoid contamination and ensure accurate readings. It is also important to make sure that the metallurgy of the refractometer probe is compatible with the process. Some highly aggressive acids require Hastelloy, or Alloy 20 for longer life. A product like Vaisala's Polaris^TM refractometer, which has prism washing capabilities, can be a great solution. 

Example applications

Cooling Towers and Cyanide Applications

Conductivity sensors excel in less aggressive environments such as cooling towers and gold mining operations. In cooling towers, conductivity sensors monitor the concentration of chemicals to prevent scaling on heat exchange membranes, thereby automating the process of maintaining optimal water quality. In gold mining, conductivity sensors facilitate the cyanide leaching process by monitoring the concentration of cyanide solutions used to dissolve gold from ore.

Sulfuric Acid Production and Copper Mining

In sulfuric acid production and copper mining, where acid concentration must be monitored meticulously, refractometers offer superior accuracy. For instance, copper mines capture sulfur dioxide from smelter stacks to produce sulfuric acid. Refractometers ensure precise measurement of the acid concentration, which is crucial for both safety and process efficiency.

Conclusion

While conductivity sensors are cost-effective and suitable for many applications, their limitations in handling nonlinear conductivity curves and temperature variations make them less ideal for measuring highly corrosive substances like sulfuric acid. Refractometers, though more expensive and requiring maintenance, provide the necessary accuracy and reliability in such challenging environments. Conductivity probes require periodic calibration to correct for sensor drift.

In industrial settings, the choice between conductivity sensors and refractometers depends on the specific requirements of the process. For applications involving aggressive chemicals or requiring high precision, refractometers are often the better choice despite their higher cost and maintenance needs. Conversely, for less demanding applications, conductivity sensors offer a practical and economical solution. The key is understanding the strengths and limitations of each method to ensure optimal performance and safety in industrial processes.

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Many thanks to Dave Lobach as our guest blogger!

To learn more about Vaisala liquid measurement solutions, contact us.