Knowledge for the Sulphuric Acid Industry
Sulphuric Acid on the Web
Acid Plant Database
Boiler Feed Water
Materials of Construction
DKL Engineering, Inc.
Analytical Procedures - Arsenic in Gas Exit Drying Tower
December 19, 2002
Arsenic in the gas leaving of the drying tower will poison the vanadium containing catalysts in the converter. Analysis is required to determine is arsenic is present in the gas. The procedure involves absorbing arsenic in water and determination of the amount of arsenic present by Atomic Absorption technique as volatile hydride. The sample is treated with potassium persulphate and sulphuric acid in order to oxidize the arsenic to As(V), because in this oxidation state, the volatile hydride of the element produces a more time-constant signal. The reduced form of arsenic, As(III), produces a stronger but more random signal.
Gas sampling can be done using isokinetic sampling apparatus but is not absolutely necessary.
In the isokinetic sampling apparatus set up four impingers each containing 200 ml of water after the filter membrane. Draw about 100 litres of gas through the apparatus at a flow rate of 1 to 2 litres per minute allowing the gas to bubble through the water. Gather all the solution together for analysis.
Pour 100 ml of sample into a 250 Erlenmeyer flask and add 0.4 grams of potassium persulphate and 0.2 ml of sulphuric acid. Heat on the water-bath for about 90 minutes for to ensure all As(III) is oxidized to AS(V). Allow the sample to cool and pour into a 100 ml flask and make up to the mark with distilled water. The solution is ready to be analysed in Atomic Absorption by hydride evolution system.
If possible use a double beam spectrometer equipped with an automatic VHE system, like the Perkin Elmer MHS 20 or FIA one. Put 1 ml of the sample into the VHE system, add the proper solutions, as recommended by the instrument instruction manual and read the absorption peak (or directly concentration) at 193.7 nm. Determine, the arsenic concentration against previously defined calibration curves.