How to determine amount of Iron in boiler water

Iron

Iron exits in two states in solution the ferrous and ferric form. The iron test described here measures the total iron present; both ferrous and ferric iron are detected.

Iron is classified as an encrusting solid similar to calcium and magnesium since iron will precipitate out of solution and can form a scale. In addition to the iron content of natural waters, iron solubilizes whenever the corrosion of iron or steel surfaces occurs. Thus, interpretation of iron results must consider the factors of both scale and corrosion. For example, an iron deposit can occur in a cooling water system as a scale problem (deposition of the natural iron in the water) or as a corrosion problem (wasting away of the metal surfaces).

Thus, iron determinations can reveal information on corrosive action taking place within a system. However, the natural iron content of the water must be taken into consideration. For example, if it is found that a boiler water sample contains appreciable quantities of iron, it should not be immediately concluded that a corrosion problem exists since iron in the feedwater will concentrate in a fashion similar to other ions.

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Iron Phenanthroline Method (0-2 ppm) Theory of Test

This test is based on the reaction of ferrous iron with 1,10-phenanthroline at a pH of 3.2 to 3.3 to yield an orange red complex. The color produced is proportional to the iron content of the sample and is measured by a photometer.

Apparatus Required

Filter photometer complete with assorted laboratory glassware.

Chemicals Required

Hydrochloric Acid, 10%

Hydrogen Peroxide,

3% Iron Reagent

Iron Standard

Orthophenanthroline Reagent

Procedure for Test

This procedure uses a wavelength of 510 nm and a cell with a light path of approximately 10 mm. Prepare a calibration curve for the photometer using successive dilution of the iron standard to adequately cover the anticipated range of iron in the sample to be tested. The dilutions of the standard should be treated in exactly the same manner as that shown below for analysis of the water samples.

Using a 50 ml plastic graduate, add 25 ml of sample to a 125 ml glass flask. Next add 1.0 ml of 10 per cent hydrochloric acid and evaporate to 5.0 ml or less in the flask ( do not evaporate to dryness). After the sample is cooled until warm to the touch, add approximately 10 ml of distilled water and swirl to mix. Make volume up to 25.0 ml in the 50 ml plastic graduate, using distilled water. Pour solution back into flask, swirl to mix, and pipet 2.5 ml of iron reagent into the flask. Use a portion of the prepared solution to set the photometer at zero absorbance. Pour contents of the cuvette back into flask containing prepared solution. Pipet 2.5 ml of orthophenanthroline reagent into flask and swirl. Adjust the sample temperature to 70 to 75F, wait two minutes, and then measure absorbance on the photometer.

NOTE: If iron complexing agents are present (such as chelants, polymers, phosphates, citrates, etc.) or if it is desired to measure particulate iron in addition to soluble iron, the sample should be treated as follows to avoid low iron results: Transfer 25 ml of sample to a glass flask as above.  Add 5 ml of 10 per cent hydrochloric acid and then add 1 ml of 3 per cent hydrogen peroxide to the flask.

Place the flask on a hot plate and evaporate the solution to dryness. Add approximately 5 ml of distilled water and then follow the test procedure described above starting with “make volume up to 25.0 ml in the 50 ml plastic graduate…”

Calculation of Results

The iron concentration in parts per million as Fe is obtained by referring to the prepared iron calibration curve.

Limitations of Test

Phosphate will interfere with this test; this is particularly true of polyphosphate. Metallic interferences includes chromium, zinc (in concentrations 10 times that of iron), copper and cobalt above 5 ppm and nickel above 2 ppm. Bismuth, silver, cadmium, mercury and molybdate also interfere, but are precipitated by phenanthroline.

Interferences are also created by the strong oxidizing agents, cyanide and nitrate, but the initial boiling step with acid which converts polyphosphate to orthophosphate also removes cyanide and nitrite. The addition of excess iron reagent will eliminate errors caused  by  excessive concentrations of strong oxidizing agents.  Note also that in the presence of interfering metal ions,  a larger excess of orthophenanthroling reagent is required to replace that complexed by the interfering metals.

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