Analytical Procedures - Arsenic in Gas Exit Drying Tower
December 19, 2002

Introduction Equipment Reagents Procedure Calculation

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Introduction

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.

Equipment

• Isokinetic sampling apparatus
• PTFE membrane (teflon)
• Stainless steel holder for membrane filter
• Atomic Absorption spectrometer equipped with volatile hydride system (VHE)
• Water bath heater
• 250 ml Erlenmeyer flask
• 100 ml flask
• 250 ml plastic bottle

Reagents

 

• Aqua regia solution
• Sulphuric acid concentrate
• Potassium persulphate grade reagent
• Arsenic(V) standard solution

Procedure

Sampling

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.

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. 

Calculation

None