How to determine sulfite in a sample
Sulfite Titrimetric Method(0-100ppm)
The determination of sulfite usually is made only on boiler waters or on waters that have been treated with catalyzed sodium sulfite for corrosion prevention. Generally speaking, sulfite is not present in natural water.
In boiler feedwater conditioning sodium sulfite is fed to a boiler to remove dissolved oxygen and thus prevent pitting. For the reaction between sulfite and oxygen to proceed rapidly and completely, it is necessary to maintain an excess sulfite concentration at an elevated temperature.
Theoretically, 7.88 pounds of chemically pure sodium sulfite are required to remove one pound of oxygen. The efficiency of the oxygen removal is estimated at 75 per cent to allow for oxidation in contact with air, blowdown losses, etc. Therefore it is estimated that 10 pounds of commercial sodium sulfite are required for each pound of oxygen removed (or 10 ppm sulfite per 1 ppm dissolved oxygen.)
The use of sodium sulfite as a chemical deoxygenator is economical within certain limitations imposed by the dissolved oxygen content of the feedwater. If appreciable quantities of dissolved oxygen are permitted to enter the boiler, costs will be high if sulfite is relied on as the sole means of oxygen removal. Generally, costs are balanced by removal of as much of the oxygen as feasible by mechanical means, e.g. deaerator and by using sulfite to react with the residual oxygen.
To prevent corrosion and pitting in feed lines, closed heaters and economizers, it is desirable to feed the sulfite continuously to the boiler feedwater rather than directly to the boiler feedwater rather than directly to the boiler. Reaction between sulfite and oxygen is not instantaneous and the completion of the reaction is aided by the longer contact times provided by feeding sulfite to the feedwater.
Catalyzed sodium sulfite will, however, react almost instantaneously with dissolved oxygen even at cold water temperatures. Because of this property, catalyzed sulfite has found increased use in the treatment of cooling water, process water, distribution system, etc. for preventing oxygen corrosion.
Theory of Test
The titration for sulfite is based on the reaction of sulfite with iodine in acidic solution. The titrant used is a standard solution of iodideiodate, which generates iodine in acid solution. At the end point excess iodine combines with the indicator to form a blue color.
Apparatus Required
Buret automatic 25 ml - 1
Casserole porcelain 210 ml - 1
Cylinder graduated 50 ml - 1
Measuring dipper (plastic) - 1
Stirring rod glass - 1
Chemicals Required
Phenolphthalein Indicator
Potassium Iodide-Iodate 1 ml =0.5 mg
SO3 Sulfite Indicator
Procedure for Test
The water sample should be freshly obtained with as little contact as possible with air. Do not filter the sample, but cool it to room temperature (70 to 800F).
Measure 50 ml of the water sample with the graduated cylinder and transfer this to the casserole. Add three or four drops of phenolphthalein indicator to the sample, and a red color will develop.
Use the plastic dipper to add sulfite indicator to the sample. Add only one measure at a time and stir thoroughly between each addition of indicator. All the indicators may not dissolve and this may create a slight haze in the sample. Continue to add the indicator in this manner until the red color disappears. It is not necessary that the phenolphthalein alkalinity be exactly neutralized, only that the sample turn colorless. When the sample is colorless, add one additional measure of sulfite indicator and stir.
Titrate with potassium iodide-iodate solution until a faint permanent blue color develops in the sample. This color change is taken as the endpoint. Record the ml of potassium iodide-iodate solution used.
Calculation of Results
FORMULA:
ppm sulfite as SO3 = ml potassium iodide- iodate x 500 ml Sample
Using a 50 ml sample, sulfite, in parts per million as SO3 is equal to the ml of potassium iodide- iodate required multiplied by 10.
Limitations of Test
This method is rapid and adaptable to field determinations. It is affected by any oxidizable substances in the water such as organic matter sulfides and nitrites. The presence of these interfering substances will cause the sulfite content obtained from this titration to by shown as a higher value than actually exists.
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