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On-line Analysis of Concentrated Sulfuric Acid

Supplier: B-R Controls
31 October, 2007

Sulfuric acid (H2SO4) is a colour and odourless liquid and it is important in the chemical industry as a raw material or produced as a by-product in different chemical processes such as: Minerals processing Production of fertilizers Production of acids, e.g phosphoric acid production of titanium oxide (paint industry) battery acids pickling baths in steel industry.

The most important industrial process for the production of sulfuric acid is the "double contact process" where H2SO4 is produced by dissolving sulfur trioxide (SO3) in concentrated sulfuric acid. For the control of the H2SO4 production and dilution an on-line measurement of the sulfuric acid concentration is necessary.

Concentration determination of sulfuric acid

The concentration of aqueous solutions of sulfuric acid is quoted as % w/w H2SO4. For high concentrations between 90 to 100 % w/w Sound velocity should be used to determine the H2SO4 concentration since it is strongly influenced by a concentration change.

The new Anton Paar on-line sound velocity transducer SPRn L Gold was developed to measure the concentration of sulfuric acid in the highly corrosive environment. It offers the possibility of an accurate concentration determination of H2SO4 in the range of 90 to 100 % w/w for process control with high reliability and low maintenance.

Measuring Principle

Sound velocity is measured with an SPRn sound velocity transducer. The active parts of the SPRn consist of an ultrasonic transmitter and receiver. The electronics measures the propagation time of the ultrasonic pulses through the sample. By knowing the distance between the transmitter and receiver and the propagation time, sound velocity can be calculated.

Since the sound velocity is highly temperature dependent, a temperature sensor is also built in. Sound velocity and temperature information are transferred to an evaluation unit with a preprogrammed high order polynomial to convert the measured sound velocity into concentration units.

The SPRn L Gold transducer is coated with a porefree Gold-layer (40 µm thick) which is chemically resistant to the high concentration acid sample and prevents corrosion. The SPRn L Gold sensor is mounted into a specially designed PVDF T-piece which is also resistant to the effects of concentrated sulfuric acid

Measuring specifications

Measuring Range:

Temperature: 20 to 90 °C, Concentration: 90 to 100 % w/w H2SO4


Sound velocity: 0.1 m/s
Corresponding variations in H2SO4 concentration < 0.05 % w/w H2SO4 (typical).

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