Experiment 5

Determination of Nitrate Ion in WaterGeneral Discussion

Nitrate nitrogen may be present in small amounts in fresh domestic wastewater. However, it is seldom found in influents to treatment plants because the nitrate serves as an oxygen source in the biologically unstable wastewater. On the other hand, nitrate is often found in the effluents of biological treatment plants, because it represents the final form of nitrogen from the oxidation of organic nitrogen compounds. Trickling-filter and activated sludge treatment plant effluents may contain from 0 to 50 mg/L nitrate, depending on the total nitrogen content of the influent, the degree of loading, and the temperature of the sewage.

Nitrate may also be found in river water, lake water, and most importantly in ground water. The nitrate may originate from sewage, or in rural areas, it may be produced by fertilizer or barnyard runoff. The U.S. Public Health Service designated safe limit for nitrate in water is 45 mg/L nitrate or 10 mg/L nitrate nitrogen. Nitrate in drinking water is particularly dangerous to small children, infants, and fetuses.

In this experiment, nitrate will be reduced to nitrite with zinc. The nitrite reacts with sulfanilic acid and N-1-naphthylethylenediamine to produce a red compound. The intensity of the red color is analyzed spectrophotometrically. The amount of zinc and the contact period are important.

Special Apparatus

a. Spectronic 20 - spectrophotometer set at 520 nm.

b. Filter paper and vacuum filtration apparatus.

Reagents

(All of the following are prepared by the instructor or lab technician and are available for student use in the experiment.)

a. Stock potassium nitrate solution: 0.816 g of anhydrous KNO3 is dissolved in purified water and diluted to 1 liter to produce a 500 mg/L nitrate solution. 100 mL of this solution is diluted to 1 liter to produce the stock 50 mg/L nitrate solution

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which will be used in this experiment.

b. Hydrochloric acid diluted 1:4 1 part con. HCl and 4 parts water.

c. Sulfanilic acid: Dissolve 0.60 g of sulfanilic acid in 70 mL hot purified water, cool, dilute to 100 mL with purified water, and mix thoroughly.

d. Zinc: Add 1.000 g finely powdered zinc to 200 g sodium chloride, NaCl, in a bottle and mix thoroughly by shaking for several minutes.

g. N-1-naphthylethylenediamine dihydrochloride reagent: Dissolve 0.60 g N-1-naphthylethylenediamine dihydrochloride in purified water to which 1.0 mL con. HCl has been added. Dilute to 100 mL and mix. Store in refrigerator. (Stable for about a week.)

h. Sodium acetate solution: Prepare 100 mL of 2 M NaC2H3O2.

Procedure

a. Preparation of standards:

Blank: Measure 50 mL of purified water and transfer to a 250-mL erlenmeyer flask.

2.5 mg/L standard: Add 2.5 mL of stock 50 mg/L nitrate solution to a 100-mL graduated cylinder. Add purified water and dilute to a volume of 50 mL. Transfer to a 250-mL erlenmeyer flask.

5.0 mg/L standard: Repeat the directions for the 2.5 mg/L standard using 5.0 mL of 50!mg/L nitrate solution.

10.0 mg/L standard: Repeat the directions for the 2.5 mg/L standard using 10.0 mL of 50!mg/L nitrate solution.

15.0 mg/L standard: Repeat the directions for the 2.5 mg/L standard using 15.0 mL of 50!mg/L nitrate solution.

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b. Experimental: Use the following procedure for treating standards as well as river, lake, well, or sewage water samples.

Note: Treated sewage effluent may require a 5-fold or a 10-fold dilution. A 10-fold dilution can be performed by pipetting 5.0 mL of the treated wastewater (sewage) sample into a 100-mL graduated cylinder and adding enough water to bring the volume up to 50 mL. This 50 mL sample can then be taken through the experimental and color development procedure. A 10-fold dilution results in a dilution factor of 10. Remember to multiply the concentration obtained for the diluted sample by a factor of 5 or 10.

To a 50.0-mL water sample in a 250-mL erlenmeyer flask, add 1.0 mL of dilute HCl (1:4 dilution) and 1.0 mL sulfanilic acid reagent and mix thoroughly. In a dry 10-mL graduated cylinder, measure one mL of the Zn/NaCl granular mixture and add it to the erlenmeyer flask.

Swirl the flask for seven minutes. Filter with a vacuum flask after the seven minute swirling period. Rinse the erlenmeyer flask well with purified water and pour the filtered water sample back into the flask.

Color Development: Add 1.0 mL N-1-naphthylethylenediamine dihydrochloride reagent to the filtered sample and mix. Add 1.0 mL of 2 M sodium acetate solution and mix. Allow 5 minutes (or more) for color development.

Spectrophotometric Measurement: Measure the color intensity with a spectrophotometer at a wavelength of 550 nm. Purified water may be used for a blank unless the water sample is cloudy. In that case, use a sample of cloudy water as a blank. Record the absorbance of the colored sample.

Waste Disposal

All solutions may be rinsed down the drain with water.

Calculations

From the concentration and the absorbance of the four standards, make a plot of absorbance as a function of concentration. Use the plot and the absorbance of each unknown solution to determine the concentration in mg/L nitrate ion (mg NO3-/L ) in that sample. Also, express the concentration in terms of mg/L of nitrate nitrogen (mg N/L)

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in the sample.

Discussion

Identify and discuss possible sources of error in this experiment. How does each kind of error affect the final answer in terms of making it too high, too low or either?

Also, use the concentrations of nitrate nitrogen measured in each water sample to estimate the drinking water quality (from the standpoint of nitrate content) of each of the samples tested.

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Report the following data:

Sources of Water Samples

1. 2. 3. 4.

Absorbance of Standards2.5 mg/L NO3- !!!!!!!!!!!!!5.0 mg/L NO3- !!!!!!!!!!!!!10.0 mg/L NO3- !!!!!!!!!!!!!15.0 mg/L NO3- !!!!!!!!!!!!!

Note: Prepare a standard curve and turn it in with the report sheet.

Absorbance of Water Samples Dilution Factor (if any)

1. !!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!2. !!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!3. !!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!4. !!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!

Concentration of Nitrate in Water Samples

as NO3- as N1. 2. 3. 4.

Discussion

Identify and discuss sources of error in this experiment. How would you rate the quality of the water samples tested in this experiment?

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