New Automated Microanalyses for Total Inorganic Fixed Nitrogen and

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Downloaded by NORTH CAROLINA STATE UNIV on December 27, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0073.ch009

New Automated Microanalyses for Total Inorganic Fixed Nitrogen and for Sulfate Ion in Water A. LAZRUS, E . L O R A N G E , and J. P. L O D G E , JR. National Center for Atmospheric Research, Boulder, Colo.

For the past several years a network of stations throughout the United States has been collecting monthly samples of atmospheric precipitation. These samples are analyzed for trace contaminants using automated methods when possible. In the course of this work, a new colorimetric test for sulfate ion, with a 0 . 5 p.p.m. limit of detection, has been developed. It is fully automated and capable of testing 30 samples per hour. A new fully automated analysis for total inorganic fixed nitrogen has also been developed. This test determines directly ammonia and total inorganic fixed nitrogen. The detection limit is 10 p.p.b. of inorganic fixed nitrogen in aqueous solution.

The

National Center for Atmospheric Research maintains a network of precipitation sampling stations throughout the United States. Cumu lative monthly samples are sent to the laboratory for chemical analysis. B y examining concentration patterns, levels, and ratios it is possible to make deductions regarding the sources, distribution, and sinks of trace chemicals i n the atmosphere. The large number of analyses required and the necessity of maintaining good reproducibility over periods of years make automation of the chemistry desirable. Total Fixed Inorganic Nitrogen Determination For the purpose of our investigation a test indicating ammonia and total oxidized forms of inorganic nitrogen is highly useful. To satisfy this need we developed a fully automated method comprised of two steps: first, an efficient reduction of nitrite and nitrate by copper-zinc alloy to 164

Baker; Trace Inorganics In Water Advances in Chemistry; American Chemical Society: Washington, DC, 1968.


9.

LAZRUS E T A L .

165

Automated Microanalyses

ammonia; and second, a sensitive indo-phenol ammonia test. The latter is an automated adaptation of the method described by Tetlow and Wilson (22). Reduction of nitrite and nitrate by C u - Z n alloy has been used since the 19th century (J, 12, 20, 23). However, reaction times varying from several hours to overnight in acidic solution are recommended in these methods. Arnd, who devoted a series of papers extending over a period of twenty years to the determination of total inorganic fixed nitrogen (2, 3, 4, 5, 6, 7, 8, 9, 10), found that concentrated electrolytes enhanced the reducing power of C u - Z n alloy. However, he d i d not report the effect of p H on the rate of reduction in the presence of concentrated electrolyte. W e have observed that C u - Z n alloy is capable of essentially instantaneous quantitative reduction of nitrite and nitrate in the presence of high concentrations of N a C l at an apparent p H of 1.8. The instrumentation used is part of the AutoAnalyzer system manu factured by Technicon, Inc. (19). The AutoAnalyzer modules required are the sampler, the proportioning pump, the colorimeter with range ex pander, and the recorder. The AutoAnalyzer sampler alternately delivers sample solution and cleansing water from a small reservoir. The quantity of sample is controlled by the sampling rate, adjusted by means of a cam on a clock mechanism, and by the diameter of the sample tube on the manifold of the pump. The AutoAnalyzer pump is capable of regulated simultaneous deliv ery of up to 15 fluids. This is accomplished by the action of steel rollers forcing the fluids through flexible tubes of selected internal diameters. The effluents of the pump tubes are mixed in the proper reaction sequence, and the final solution enters a tubular flowcell in the colorimeter. A single light source provides twin beams, one passing through the flowcell to a photocell, and the second to an identical reference photocell. A null balance system in the recorder continuously measures the ratio of the sample to reference voltage. As indicated in Figure 1, the sample stream is joined by an electrolyte solution which is 4.49N N a C l and 0.04N HC1. The liquid is then seg mented by air bubbles to maintain sharp concentration gradients along the stream. The solution passes through a vertical glass column 150 mm. length, 4 mm. i.d.) packed with Z n - C u alloy produced in the following way: Ten grams of zinc granules, 20 mesh, are etched for 1 minute with HC1 (1/5) and washed thoroughly with H 0 . To the zinc is rapidly added a solution prepared by dissolving 0.25 grams of C u C ^ ^ ^ O in 20 m l . of H 0 containing three drops of I N HC1. The reaction mixture is shaken vigorously until the blue color just disappears. Immediately, the alloy is swirled onto a filter paper in a Buchner funnel which has been kept under suction, washed once with H Q , once with methyl alcohol, 2

2

2


166

TRACE INORGANICS I N W A T E R


and then quickly air-dried. Speed is necessary in these steps for two reasons: (1) lengthy contact with moisture at this stage reduces the potency of the reducing agent, and (2) prolonged contact with volatile solvents such as acetone or alcohol used to accelerate the drying process causes flaking off of copper particles.

SHORT MIXING C O I L

,ΜΜ

*

PI

PREMIXERI I

40

FT.

(

2

SULFURIC ACID

COLORIMETER 1 5

Ε19.Μμ

Figure 1.

RECORDER

mm Tvbtilar f / c Filters

Flow diagram of the total inorganic fixed nitrogen test

The sample stream is debubbled and mixed with sodium phenoxide reagent, prepared by adding 135 ml. of 5N sodium hydroxide to 62.5 grams of phenol in a 250 ml. beaker. When the phenol is dissolved, it is trans ferred to a 500 m l . volumetric flask. F i f t y milliliters of 6% ethylenediaminetetraacetic acid ( 12.0 grams of E D T A dissolved in 65 m l . of 5N N a O H and diluted to 200 ml. with H 0 ) and 15 m l . of acetone are added. The solution is brought to volume with H 0 . After again seg menting the stream with air and mixing, a sodium hypochlorite solution containing 1 % (w/v) available chlorine (11) is introduced. A reaction period is allowed before the color intensity is measured, at 610 τημ. Bypassing the reduction column permits determination of ammonium ion only. The detection limit of the test is 10 p.p.b. nitrogen with recorder range expansion. The calibration curves deviate from the Beer-Lambert 2

2



9.

LAZRUS E T A L .

Ο

2.0

1.0

TOTAL

Figure 2.

167


4.0

3.0

5.0

I N O R G A N I C F I X E D N I T R O G E N (ppm)

Typical total inorganic fixed nitrogen calibration curve

law, being slightly convex ( Figure 2 ). The precision obtained at various concentrations is indicated i n Table I. Table I.

Nitrogen Test Reproducibility and Accuracy

Nitrogen Taken (p.p.m.) 1 5.00

β 6

4.96

2

3

5.00

5.00

Run

4

5

6

Mean Nitrogen Found (p.p.

5.07

5.03

4.93

5.00·

3.97

4.00

a

n

4.00

3.98

4.02

4.03

3.99

4.02

2.00

2.02

2.03

1.97

1.98

1.98

1.97

1.99

1.00

0.96

0.98

0.99

1.00

1.01

0.99

0.99°

0.500

0.488

0.494

0.508

0.494

0.510

0.507

0.500*

0.250

0.247

0.248

0.250

0.240

0.248

0.254

0.248

b

0.100

0.100

0.102

0.101

0.095

0.099

0.103

0.100

b

0.050

0.047

0.050

0.053

0.057

0.047

0.055

0.052

6

W i t h no recorder range expansion. With recorder range expanded X 4.

The system is calibrated once for each revolution of the sampler turn table, which accommodates 40 sample cups.


168

T R A C E

I N O R G A N I C S

I N

W A T E R

SAMPLER _ H Rate: _ 3 ° _ p # f how

. 0 5 6 * INDICATOR R G T .


.045

COLORIMETER

AIR

RECORDER

_I3_hmi Tubular f/c ^ΕΙίΛμ

Figure 3.

FilUft

Flow diagram of the colorimetric sulfate ion test

The colorimetric responses to equivalent amounts of nitrogen as ammonia, nitrite ion, nitrate ion, and hydroxylamine are identical within experimental error. Diethylamine, ethylamine, alanine, and acetamide, each present at a concentration of 0.50 p.p.m. nitrogen, yield null re sponse. Phosphate at a concentration of 0.50 p.p.m. phosphorus does not interfere in the determination of 1.0-p.p.m. nitrogen. There is no inter ference by C u , Z n , C d \ N i , F e , P b \ or C a , each at a con centration of 10 p.p.m, in the determination of 1.0 p.p.m. nitrogen. 2 +

2+

2

2 +

2+

2

2+

Sulfate Ion Determination This test is based upon formation of barium sulfate in the presence of excess barium, followed by chelation of the remaining barium with methylthymol blue, a metallochrome indicator recently developed for complexometric titrations (13, 14, 15, 16, 17). The reagent is a solution containing equimolar amounts of barium chloride and methylthymol blue at a concentration equivalent to the largest amount of sulfate ion antici pated in the test solution (18).



9.

LAZRUS E T A L .


169

The concentration range of the test is 0.5 p.p.m. to 50.0 p.p.m., but the upper limit may be extended by arranging the pump manifold to dilute the sample appropriately. The reagent routinely used in our laboratory has an upper limit of 30.0 p.p.m. sulfate, although other limits may be achieved by adjusting the amount of dye and barium in the solution. The methylthymol blue used was Eastman N o . 8068. Twenty-five millihters of solution of 1.5258 grams of barium chloride dihydrate dissolved in suf ficient water to make one liter are added to 0.1182 grams of methylthymol blue i n a 500 m l . volumetric flask. T o this is added 4 m l . of I N hydro chloric acid, which changes the color to bright orange. After adding 71 ml. of water, the solution is made up to 500 m l . with undenatured 9 5 % ethanol. The apparent p H of the solution toward a glass electrode is 2.6. The flow diagram for the sulfate determination is depicted in Figure 3. The sample stream is first passed through a glass column ( 170 mm. long, 2 mm. i.d. ) containing an indicating cationic exchange resin to re move interfering cations. A suitable resin is Dowex H C R , Hydrogen Form, D y e d Resin (Nalco Chemical C o . ) . The indicator reagent now joins the sample stream, and a time-delay coil ( 20 ft. long, 2 mm. i.d. ) is inserted in the system to allow complete formation of barium sulfate. A t this stage the solvent is 40% ethanol, the p H is acidic, and the color is light orange. Complexation of barium by methylthymol blue is not a rapidly reversible process (21); indeed sulfate ion w i l l not readily remove barium from its methylthymol blue complex in basic solution. To avoid this problem the reagent is initially acidic, allowing only negligi ble chelation of the barium until the precipitation of the barium sulfate is completed. Subsequent addition of 0.1N sodium hydroxide to an apparent p H of 12.8 causes methylthymol blue to become either blue in the presence of barium, or grey in its uncomplexed form. The maximum absorption of the uncomplexed form is at 465 m^, and the reaction stream is conducted through the colorimeter equipped with filters transmitting at this wave length. Since the indicator reagent is initially equimolar in barium ion and methylthymol blue, the amount of uncomplexed indicator remaining is directly related to the concentration of sulfate ion i n the original sample. The premixers indicated in the flow diagrams were constructed by sleeving the ends of 0.5 inch lengths of N o . 22 Teflon spaghetti with N o . 19 Teflon spaghetti. The sleeved ends were fitted into 0.5 inch lengths of 0.056 inch i.d. Solvaflex tubing. The end effect is to have 0.25 inch sections of 0.056 inch i.d. alternating with 0.5 inch lengths of 0.028 inch i.d. tubing.



170

TRACE INORGANICS I N W A T E R

20

30

40

50

Sulfate Concentration (p.p.m)

Figure 4.

Typical sulfate ion calibration curve

A typical calibration curve is shown in Figure 4. The plot does not follow the Beer-Lambert law, but is slightly concave. Typical data are shown in Table II. Table II. Sulfate Taken (p.p.m.) 1 30.0 20.0 10.0 5.00 1.00

29.5 19.3 10.0 5.10 1.00

Accuracy and Reproducibility of Sulfate Test 2

3

29.5 19.9 9.80 4.80 1.00

29.5 19.8 9.80 4.80 1.00

Run

4

5

30.4 19.7 9.85 4.95 1.10

30.4 20.4 10.1 4.95 0.95

Mean Sulfate 6 Found (p.p.m.) 30.8 20.8 10.1 5.10 1.10

30.0 20.0 9.94 4.95 1.02

Concentrations of up to 50 p.p.m. carbonate ion d i d not interfere with determination of 10 p.p.m of sulfate ion. The system is calibrated with each revolution of the sampler turn table. Because the reagent tube is gradually affected by the 80% ethanolic solvent, it is necessary to replace it every one or two days. The method


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LAZRUS E T

AL.


171

also requires a rather long warm-up time, two or three hours, with the proportioning pump operating. The indicator solution, when alkaline, oxidizes on exposure to the air. Efforts to inhibit this process have so far been unsuccessful, so that the method is limited to the automated system.


Literature Cited (1) Allport, N. L., Brocksopp, J. E., "Colorimetric Analysis," Vol. 2, p. 228, Chapman and Hall, Ltd., London, 1963. (2) Arnd, T., Chem. Ztg. 45, 537 (1921). (3) Arnd, T., Angew Chem. 30(I), 169 (1917). (4) Ibid., 33(I), 296 (1920). (5) Ibid., 45, 22 (1932). (6) Ibid., 45, 745 (1932). (7) Arnd, T., J. Chem. Soc. 112 (II), 504 (1917). (8) Arnd, T., Segerberg, H., Angew Chem. 49, 166 (1936). (9) Ibid., 50, 105 (1937). (10) Arnd, T., Segerberg, H., Bodenkunde u. Pflanzenernahrung 6, 195 (1937). (11) Furman, N . H., ed., "Scott's Standard Methods of Chemical Analysis," Vol. 1, 6th ed., p. 340, D . Van Nostrand Co., Inc., Princeton, N . J., 1962. (12) Gladstone, J., Tribe, Α., J. Chem. Soc. 33, 139 (1878). (13) Körbl, J., Přibil, R., Chem. Listy 51, 302 (1957). (14) Ibid., 51, 1061 (1957). (15) Ibid., 51, 1304 (1957). (16) Ibid., 51, 1680 (1957). (17) Körbl, J., Přibil, R., Chem. & Ind. 1957, 233. (18) Lazrus, Α., Hill, K., Lodge, J., "Automation in Analytical Chemistry, Technicon Symposia 1965," p. 291, Mediad, Inc., New York, 1965. (19) Lingemen, R., Musser, Α., "Standard Methods of Chemical Analysis," F. J. Welcher, ed., Vol. 3, Part B, 6th ed., p. 975, D . Van Nostrand Co., Inc., Princeton, N . J., 1966. (20) Navrone, R., J. Am. Water Works Assoc. 56, 781 (1964). (21) Přibil, R., Talanta 3, 91 (1959). (22) Tetlow, J., Wilson, Α., Analyst 89, 453 (1964). (23) Williams, M . , J. Chem. Soc. 39, 100 (1881). RECEIVED A p r i l 24, 1967. T h e National Center for Atmospheric Research is operated b y the University Corporation for Atmospheric Research w i t h spon sorship of the National Science Foundation.


New Automated Microanalyses for Total Inorganic Fixed Nitrogen and

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