Analytical Procedures - Arsenic in Acid
December 19, 2002

Introduction Equipment Reagents Procedure Calculation

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Introduction

There are few methods to analyse arsenic, but the fastest and most reliable system is determination by Atomic Absorption 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.

Equipment

• Atomic Absorption equipped with the Volatile Hydride Evolution (VHE) system
• Water bath heater
• 250 ml Erlenmeyer flask
• 100 ml flask

Reagents

 

• Hydrochloric acid concentrate
• Potassium persulphate pure reagent
• Arsenic(V) standard solution
• Sodium borhydride

Procedure

Pour 50 ml of arsenic free water in a 100 ml flask. Add about 1 to 2 grams of acid to be tested (exactly weighed). Add 1 gram of potassium persulphate pure reagent and 1 ml of hydrochloric acid. Heat on the water-bath for about 90 minutes to ensure all As(III) is oxidized to AS(V).  Allow the sample to cool for about an hour and make up to the mark. The solution is ready to be analysed in Atomic Absorption by hydride system evolution.

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