How to determine amount of Nitrate in boiler water

Nitrate

Nitrate ion is present in natural waters in relatively small quantities. Nitrogenous compounds may be introduced by sewage with subsequent bacteriological oxidation to nitrate. High nitrate concentrations in drinking water appear to cause methemoglobinemia infants (blue babies). Federal regulations has established a nitrate limit in drinking water of 10 ppm as nitrogen. Typically, such high levels are confined to rurally dug wells subject to surface influence.

In industrial water conditioning the primary significance of nitrate regards embrittlement.  Research has shown that the maintenance of certain sodium nitrate-sodium hydroxide ratios in boiler water can inhibit intercrystaline cracking (caustic metal embrittlement). Because of the widespread use of sodium nitrate to control this condition the determination of the nitrate ion in boiler water has assumed increasing importance.

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The nitrate concentration necessary to overcome embrittling tendencies in a boiler varies  depending on the boiler water alkalinity and the boiler pressure. The optimum sodium nitrate- sodium hydroxide ratio must be determined for each plant by running tests with an embrittlement detector. Once the proper ratio has been established, the sodium nitrate feed can be controlled on this basis. Often the natural nitrate content of the raw water will provide the necessary ratio. However, since the nitrate content of natural waters is subject to seasonal variation, it may still be necessary to run nitrate control tests on the boiler water to insure adequate protection.

Two procedures for nitrate determination are commonly used. Each is a calorimetric procedure,  and both the brucine calorimetric method and the cadmium reduction calorimetric method will be discussed.

Nitrate Brucine, Colorimetric Procedure (0-30 ppm) Theory of Test

Brucine reagent combines with nitrate ions in complex. The intensity of the color is proportional to the nitrate concentration. Nitrate concentrations are obtained by comparing sample color intensity with the color developed in standard solutions.

Apparatus Required

Filter photometer complete with assorted laboratory glassware.

Chemicals Required

Brucine Reagent CAUTION: Toxic, Avoid direct contact Nitrate Standard

Sulfuric Acid, Concentrated

Procedure for Test

Pipet 5 ml of sample into a 100-ml beaker, add 10 drops of brucine reagent, and mix. Add 10 ml of concentrated sulfuric acid, with mixing, down the side of beaker. Swirl the beaker gently to mix  the sample. The solution will become warm following the addition of the acid.

Allow the solution to stand for four to six minutes. Then add 10 ml of distilled, nitrate free water. Swirl to mix, then allow the solution to cool. Read the absorbance at 410 nm using a 1-cm cell.  The reagent addition, mixing and cooling stages must consist of samples and standards because the color development depends on heat and time. A blank solution, 15 ml of distilled water and 10 ml of sulfuric acid, should be used to zero the photometer.

A standard curve is prepared by making appropriate dilutions of the nitrate standard solutions to cover the range from 0 to 30 ppm as nitrate. Standard and sample should be treated exactly as described above to permit consistent color development.

Calculation of Results

The nitrate concentration in parts per million as NO3 is obtained by reference to the nitrate.

Limitations of Test

This test is unaffected by ions in the concentrations normally found in boiler waters. Nitrate interferes with the test and shows up as additional nitrate. This interference may be avoided by the addition of sulfanilic acid to remove the nitrate present.

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