Electro-Chemical Devices (ECD) SMS22 - Manual (Page 10)

1.0 GENERAL DESCRIPTION
The ECD SMS-22 Sulfide Measurement System is an all in one
analyzer for the continuous measurement of sulfide ions in aqueous
solution. Sulfide ions are present in well water, municipal waste
water and waste waters from refineries, tanneries, chemical plants
and paper and pulp facilities.
Hydrogen sulfide (H
2
S) is a gas that dissolves in water and gives it
that “rotten egg” odor. H
2
S exists as a dissolved gas in acidic water,
as bisulfide ions (HS-) in water with pH values above pH 7 and as
sulfide ions (S
-2 ) in water with very high pH values, values above pH
12.
The ECD SMS-22 uses a sulfide ion selective electrode (ISE) to measure the total amount of sulfide
present in the sample. The measurement must be made at high pH levels where S
-2 exists. Potassium
hydroxide (KOH) is added to raise the pH of the sample to around pH 13 and a pH electrode measures
the actual pH. Sulfide and bisulfide exist in a pH dependent equilibrium with the ratio dependent on the
pH. The sulfide ISE measures the sulfide present in the sample and the pH measurement infers what
percentage of the total sulfide was measured. The C22 calculates and displays the Total sulfide present.
The highly caustic sample is then neutralized with HCl and the pH is measured and displayed to verify
the neutralization. The KOH is neutralized with HCl producing potassium chloride salt (KCl) and water.
The neutralized sample can be disposed of as waste or returned to the water supply. Each cycle uses
about 1 ml of caustic and 1 ml of acid.
The analyzer is set to run 6 minute analysis cycles (10 cycles per hour) but can easily be programmed to
run 12 minute cycles (5 cycles per hour) or 30 minute cycles(2 cycles per hour). Running continuously at
10 cycles per hour, the SMS-22 uses 10 liters (2.5 gallons) of each reagent per month and at 2 cycles per
hour less than 2 liters (0.5 gallon) per month.
The 6 minute cycle is composed of 3 x 2 minute cycles.
1. Sample fill 0:00-0:30 minutes: seconds
2. Drain 1:50-1:59 minutes: seconds
3. Sample fill 2:00-2:30 minutes: seconds
4. Drain 3:50-3:59 minutes: seconds
5. Sample fill 4:00-4:30 minutes: seconds
Add KOH 4:00-4:30 minutes: seconds
6. Measure sulfide 4:50-4:59 minutes: seconds
7. Add HCl 5:00-5:30 minutes: seconds
8. Measure pH 5:40-5:49 minutes: seconds
9. Drain 5:50-5:59 minutes: seconds
SMS-22 Page 1