Determination of trace amounts of manganese in high-purity sulfur by a combined combustion-kinetic method

Abstract

A rapid and sensitive method has been developed for the determination of manganese down to several ppb in high-purity sulfur. The method is based on the separation and concentration of manganese in sulfur by combustion in air stream followed by the kinetic determination of manganese as described in the previous paper.
The procedure can be outlined as follows.
Weigh the ground sample(containing 5 to 100 ng of manganese) into a quartz combustion boat, and carefully charge it into the combustion furnace, which is previously kept at 250±5°C. Connect up the apparatus as shown in Fig. 1. Introduce the pretreated air of room temperature into the furnace at rate of about 250 ml per min and maintain this conditions for about 30 min (for 1 to 2 g of sample).
Combustion of the sample in higher flow rate of air and higher furnace temperature is unsuitable because the lower manganese value is obtained in these conditions.
On the other hand combustion in lower flow rate and lower temperature is time-consuming.
After the completion of combustion, remove the boat and cool it to room temperature. Add 0.5 ml of 6M hydrochloric acid, then heat gently and evaporate to dryness. At this time the temperature should not be over 160°C. Dissolve the residue in 2.6 ml of an acetate buffer solution(10 M acetic acid solution: 2 M sodium acetate solution=1.6 : 1.0) and transfer the obtained solution to a test tube (25 ml) with a ground glass stopper with water, and then warm to 31°C in the thermostated water bath.
Add 0.4 ml of 2.16 × 10^-4 M Malachite Green solution and 0.5 ml of 1.7 × 10^-2 M potassium periodate solution, shake and transfer a portion of this solution into the photometer cell (1 cm), and measure the absorbance at 615 nm.
From the logarithmic plot of absorbance (a-x) against reaction time t, reaction rate constant k is obtained and manganese content is calculated from the previously prepared calibration curve (namely, relationship between manganese concentration and reaction rate constant k with standard manganese solution).
The proposed method allows determination of manganese as low as 5 ppb for 1 g sample. The time required for an analysis is about 80 min.
The estimated standard deviations were 1.9 and 3.2 ppb for the samples containing 16 and 55 ppb of manganese, respectively.
The conditions of combustion and others are also studied in detail.