Conductivity Cell Apparatus for Continuous Measurement of Sulfur Dioxide Concentrations between 40 ppm and 1 Per Cent H. Richard Beard Salt Lake City Me,allurgy Research Center, Bureau of Mines, US.Department of the Interior, Salt Lake City, Utah
SULFURDIOXIDE, a t concentrations of 40 ppm to 1 in the off gas of a n SO2 removal system, must be monitored continuously in the presence of normal atmospheric gases and combustion products. An inexpensive and reliable conductivity cell using H202 as the absorbing reagent is used t o measure SO2 concentrations at less cost to the user than commercial instruments currently available ( I ) . The instrument and systems for controlling the liquid and gas flows to obtain reproducible and reliable measurements of SO, concentrations are described.
O f f gos flowmeter
50,000n resistance
6 0 m~ll~l~lers/minule
Liquid metering
METHOD AND APPARATUS
A 1 % solution of H20?having a resistance of greater than 50,000 ohms of pH equal t o 4.5 is fed with a metering pump at 6 milliliters per minute into the top of a 30-cm by 14-mm diameter glass column filled with 4-mm glass beads. The undiluted gas js fed with a tubing pump at 1 liter per minute into the bottom of the column, and the gas from the column exits through a flow meter. The liquid leaves the bottom of the flooded column and passes through a conductivity cell with a cell constant of 1.0. The resistance is measured with a conductivity bridge which has a range of 10 to 100,000 ohms. A schematic diagram of the apparatus is shown in Figure 1. DISCUSSION
The SO, analysis instrument described here is not affected by water vapor or normal concentrations of smelter or power plant gaseous contaminants. No dilution of the incoming gas is required at concentrations of SOz between 40 ppm and 1 %. The gas flow from the tubing pump varies less than 1% between tubing changes, and the lag time of the instrument is 3 t o 4 minutes. The glass beads in the column are used to reduce the liquid volume in the absorbing column and still provide enough contact area for the SOs to react with the H202solution. No SO, can be detected in the exit gas from the column by the West-Gaeke method (2) or by Mine Safety Appliances SOz detector tubes. A calibration curve of log resistance cs. log (1) H. F. Palmer. C . E. Rodes, and C. J. Nelson, J . Air Pollur. Co,~tr.ASS.,19, 778-786 (1969). ( 2 ) P. W. West and G. C. Gaeke, ANAL.CHEM., 28, 1916 (1956).
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
Conmining so2 gas
I 1.0 Ilter/mln"le
iidI
Tubing
Figure 1. Block diagram for determining low concentrations of SOz by conductivity measurement
SO, concentration gives a n almost straight line over the range of the instrument. Accuracy is + l o % without cell temperature control for calibration with sulfuric acid solutions at 25 "C and with operating conditions between 20 and 30 "C. Improved accuracy could be obtained with a constant temperature cell. An instrument of this type has been operating a t this laboratory and a t a n SOs removal pilot plant in San Manuel, Ariz., for 10 months. Normal maintenance requires a few minutes each week t o replenish reagent, t o replace the tubing in the gas pump, and t o check the standardization of the cell. Since the conductivity cell contains carbon electrodes, no cleaning or replatinizing is necessary.
RECEIVED for review June 17, 1971. Accepted November 30, 1971.
