Anton Paar’s process density sensors are suitable wherever concentration measurements of corrosive media are required. Real time measurements will ensure product quality and lower production costs. The wetted parts of the sensor are applicable especially for corrosive media.

For efficient production of HNO3, H2SO4 and HCl, knowledge of the concentration is crucial. Control during the production can be enforced using Anton Paar’s process density sensors L-Dens 7400 and the correlation between density and concentration.

The sensors can be installed directly into the main pipeline, offering real time information about the product.

Inorganic acids like sulfuric acid, nitric acid and hydrochloric acid harm measurement tools with their corrosive behaviour. Therefore, special materials and instruments are required.

Anton Paar offers process sensors which are made out of suitable materials (e.g. tantalum) for corrosive media.

2 Applications


The synthesis of HCl is achieved by combusting of H2 and Cl2 gas. The emerged HCl (gaseous) and the excess hydrogen gas is absorbed in de-mineralized water in a falling film absorber to produce hydrochloric acid of 33 %w/w. Both reaction and absorption are highly exothermic.

With the process density sensor L-Dens 7400 TAN the HCl concentration can be measured right after it leaves the absorber.


Nitric acid is produced on industrial scales with the catalytic oxidation of ammonia. At high temperatures nitric oxide is produced, which reacts with the leftover oxygen to nitrogen dioxide. Then, the nitrogen dioxide is led into an absorption tower, where the nitrogen dioxide reacts to nitric acid.

The final product has an average concentration of 60 % and with distillation a concentration of 68 % can be reached. The concentrations at the end of the production can be monitored online with the process sensor L-Dens 7400 TAN.


The production of sulphuric acid starts with the combustion of sulphur to sulphur dioxide. The SO2 is then combined with oxygen to form sulphur trioxide. The final operation is the absorption of the gaseous SO3 in an absorption tower using water. The final product is a solution of 89 % to 99 % sulfuric acid.

The density, and furthermore the concentration, of the solution can be determined with implementing an L-Dens 7400 Sensor from Anton Paar.

Measurement steps

L-Dens 7400

The L-Dens 7400 density sensor uses the reliable oscillating U-tube measurement principle to measure the density. The sensor measures the frequency which appears when the sample flows through the U-shaped tube. Using the Anton Paar formulas, the density can be calculated.

The high accuracy and live response of the sensor deliver real time measurements for the wanted substances.

The L-Dens 7400 TAN has a U-tube made of tantalum specifically made for the measurement of corrosive acids. It is used in processes with sulfuric acids, hydrochloric acids and phosphoric acids.

For corrosive acids an accuracy of ±0.2 % can be guaranteed.

Conductivity vs Density

Conductivity is the ability of a substance to conduct electricity, heat or sound and it can be used to determine the concentration. However, there are many limitations to that method.

Firstly, the conductivity measures the concentration of the ions and gives information on the ionic strength. Thus, only the concentrations of inorganic liquids can be measured. With increasing concentration, the conductivity changes like a parable.

Consequently, the combination of the strong non-linear relationship and the before mentioned limitations lead to inaccurate results. In comparison to the conductivity, the density shows an almost linear change with increasing concentration (Figure 2). The density and conductivity are both influenced by the temperature.

Anton Paar devices have quick and reliable temperature sensors which provide results in less than one minute. Such temperature sensors are part of Anton Paar density sensors.

Process Integration

The compact and modular design of the L-Dens 7400 offers simple installation. The sensor can be installed inline directly at the product stream or in a bypass stream.


The concentration of the above-mentioned compounds can easily be measured using the tools from Anton Paar.

Density measurement is a reliable tool to measure the online concentration of the substances during the production. The real time monitoring reduces production and recovery costs.

Additionally, the L-Dens 7400 is maintenance-free and is easy to integrate. The sensor also includes automatic temperature compensation.


Wiaan Goosen
Anton Paar Southern Africa
Tel. +27 82 651 2308

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