Chromate Titration Method (10-100 ppm as CrO4 )
Chromates are amongst the most widely used chemicals for the control of corrosion in industrial cooling water systems. However, only the chromate or hexavalent form of chromium salts functions as a corrosion inhibitor; trivalent chromic salts do not inhibit corrosion. Chromates are anodic inhibitors and as such, can intensify pitting if used in insufficient concentrations to completely prevent corrosive attack. On the other hand, high chloride levels hinder the development of a protective film by chromate, and brines must be treated with high levels of chromate to inhibit corrosion. The combination of phosphate and chromate in the proper ratio under controlled pH conditions permits the use of relative low chromate concentrations. Inhibition of pitting as well as general corrosion is secured at relatively low treatment concentrations. For optimum chromate performance it is essential, therefore, that the chromate concentration be controlled within prescribed limits. The most commonly used chromate salts are the sodium and potassium salts of either chromate or dichromate. In basic solution, hexavalent chromium exists in the chromate form whereas in acid solution the equilibrium shifts to the dichromate ion. Control tests are designed to measure the hexavalent chromium only and do not respond to the trivalent chromium salts.
Theory of Test
This test is based on the oxidizing property of hexavalent chromium to liberate free iodine from potassium iodide in acid solution. The liberated iodine is titrated with sodium thiosulfate solution in the presence of a starch type indicator. The disappearance of the blue color is taken as the endpoint.
Apparatus Required
Buret, Automatic, 25 ml - 1
Cylinder, Graduated, 10 ml - 1
Flask Erlenmeyer, 250 ml - 1
Cylinder, Graduated, 100 ml - 1
Measuring Dipper, Brass - 1
Measuring Dipper, Plastic - 1
Chemicals Required
Iodide Crystals
Sodium Thiosulfate,
0.1N Starfamic Indicator
Sulfamic acid.
Procedure for Test
The 0.01N sodium thiosulfate, required to determine chromate in this range is not stable due to absorption of carbon dioxide from the air. Therefore, this reagent must be freshly prepared (or restandardized) at least every two weeks. To 90 ml of distilled water that has been boiled and cooled to room temperature, add 10 ml of 0.1N sodium thiosulfate. Mix thoroughly and store in a tightly stoppered bottle. Avoid unnecessary exposure of the solution to air. Measure 50 ml of the water sample into the graduate cylinder and transfer to a 250 ml. Erlenmeyer flask. Use the plastic dipper to add two dipperfuls (approximately, 2.0 grams) of dry sulfamic acid. Swirl the flask until the acid is completely dissolved. Allow the sample to stand for two minutes to eliminate any nitrite interference. Then use the brass dipper to add one dipperful (approx. 0.25 gram) of iodide crystals. Swirl until the iodide is completely dissolved. Allow the sample to stand for two minutes. Titrate the sample with 0.01N sodium thiosulfate until the yellow brown color of iodine has almost disappeared. Add one brass dipperful (approximately 0.15 gram) of dry starfamic indicator and swirl to dissolve (all of the indicator may not dissolve, creating a slight haze in the sample). Continue to titrate with 0.01N sodium thiosulfate until the blue color which developed upon the addition of the indicator first disappears. Disregard any reappearance of the blue color. Record the ml of the sodium thiosulfate used.
Calculation of Results
Using a 50 ml sample, the parts per million of chromate as CrO4 equals ml 0.01N sodium thiosulfate required multiplied by 7.74.
In some cases it may be desirable to use a sample larger than 50 ml. When this modification is required the quantities of dry reagents and the multiplication factor should be as follows
|
Sample Size
|
Sulfamic Acid
|
Iodide Crystals
|
Starfamic Indicator
|
Factor
|
|
100 ml
|
4 dippers
|
2 dippers
|
1 dippers
|
3.87
|
|
200 ml
|
8 dippers
|
4 dippers
|
1 dippers
|
1.94
|
Limitations of Test
This test is not specific for chromate, but rather is a measure of the oxidizing substances present in the water. Chlorine is a common positive interference, reading as chromate. Ferric iron and organic matter interfere. n addition for the titration reaction to be quantitative, control must be maintained over the pH the iodide concentration, and the time of reaction. Otherwise, either the reaction does not go to completion, or air oxidation of iodide occurs, giving high results. For these reasons, it is essential to allow the sample to stand for two minutes before titration. This procedure is used when chromate is present over approximately 10 ppm and where sources of appreciable interference are absent. Where chromate is to be determined in low concentration and also when interfering substances may be present, it is preferable to use the photometric diphenylcarbazide procedure.
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