Anal. Chem. 1980, 52, 1955-1957
1955
CORRESPONDENCE Reduction of Nitrate to Nitrite with Cadmium Sir: We are working on development of methods for determining nitrogen oxides (NO,) emitted from stationary sources. In these methods, the final form of the collected NO, is NO3-. Since few, if any, simple, reliable methods exist for NO3- analysis, we examined means for reducing NO3- to NO2 . With the NO, in the NOz- form, we could then use the diazotization-coupling-colorimetric method of analysis ( 1 ) . This method is specific for NO2- in the absence of oxidizing and reducing agents (2). Methods for analysis of NO3- in water specify the use of a cadmium-copper alloy to reduce NO3- to NO2- ( 3 ) . The alloy is prepared by swirling cadmium in a CuSO, solution. T h e copper reportedly acts as a catalyst in the reduction ( 4 ) . When the water sample is percolated through the alloy a t a rate of 7-10 mL/min, 100% reduction of NO3 t o NO2- can be obtained ( 3 ) . We have used the alloy method and confirmed that essentially 100% conversion of NO3- to NO2- was obtained. However, we also found that the effectiveness of the method was dependent on the technique of alloy preparation. Other workers ( 4 , 5 )have experienced the same problem. T o avoid this technique-sensitive step, we examined the use of cadmium only as a reducing agent and found it to be equally effective.
Procedure. A 50-mL aliquot was taken from the sample to be analyzed. Added to this solution were 1.0 mL of a 6.5% solution of the disodium salt of ethylenediaminetetracetic acid (EDTA) and 0.5 mL of -1 N H3B03. The pH of the sample was then adjusted to 12.0 with 0.5 N NaOH, using a pH meter. The electrodes were rinsed with distilled water and the rinsings added to the sample, giving a final volume of 75-85 mL. The prepared sample was then percolated through the column at the desired flow rate, usually 7-9 mL/min, and collected in a graduated cylinder that was also used to measure the flow rate. After the last of the sample had passed from the funnel into the buret but before air entrapment could occur, the column was rinsed with 60 mL of distilled water containing 1.0 mL of 6.5% EDTA/5O mL of water, at pH 12.0. The collected sample was diluted to volume and analyzed colorimetrically for NO2-, using the described procedure ( I ) , except that the addition of H 2 0 2was eliminated. The slope of the calbration curve was 0.53 absorbance unitsllg of NO,-/mL). NO3- to NOz- conversion efficiency was calculated as percent of theoretical.
RESULTS AND DISCUSSION The mechanism for reduction of NO< to NO, by cadmium appears t o be ( 6 )
NO3- + H,O
+ EDTA4-+ CdO
EXPERIMENTAL SECTION
Materials. The cadmium was 4 H O mesh (EM Laboratories, Elmsford, NY) and was prepared by rinsing in 2 N HC1 for a few minutes until the color was silver-gray and then rinsing with distilled water t o remove the HC1. A NO3- standard, reported to contain 6200 pg of NO,-/mI, (Orion Research Inc., Cambridge, MA) was reanalyzed by comparison to a gravimetrically prepared (from dried KN03)standard. A NO3- ion electrode, calibrated with the Orion standard, was used in the analysis. The two standards agreed within 5-6 % . The Orion standard was used for most of the work. Cadmium columns were prepared by modification of a 50-mL buret. The buret was cut off at a length 4 cm above the intended column height. The top part of a glass funnel was then attached to the end of the buret by a glass blower. The conventional stopcock was replaced by one with a micrometer-type adjustment (catalog no. 8225-t-05, Ace Glass, Inc., Louisville, KY), which allowed easy adjustment of flow rate. A plug of glass wool was placed at the bottom of the buret, and it was initially filled with distilled water. The cadmium was then added slowly t o the desired height and the buret tapped gently until no further settling occurred. All other materials were ACS reagent grade. Sample Matrix. The sample matrix was composed of either distilled water or the solution obtained from treatment of a molecular sieve (MS). The MS was used to collect NO, in the emissions from a coal-fired power plant. The MS treatment consisted of (1)boiling the MS with 0.06 M K M n 0 , a . j M NaOH for 1 h, (2) filtering to remove the MS, (3) acidifying the filtrate and adding oxalic acid t o remove MNO,-, (4)adjusting the pH t o 12.0 with NaOH, and (5) filtering. After the treatment, the matrix consists of at least the following ions: C20,'-, COB'-, SO;'-, Al", Na+. K+, and OH-. 0003-2700/80/0352-1955501 OO/O
-
NO*- -t 20H-
+ Cd(EDTA)'-
This mechanism is supported by the fact that the p H has been reported to increase during reduction (6). The column is kept alkaline during reduction to stabilize the formed NOz-, and EDTA is added to chelate Cd2+. Without EDTA, Cd(OH)z would precipitate on the column and restrict sample flow. The p H of the sample is initially adjusted to 12.0 so that it will not fall below 11 as a result of dilution. Chelation of metals by EDTA is optimum a t p H 11 and higher (7). When not in use, the column is covered with the rinse solution to minimize air oxidation of the Cd surface. Our first experiments involved studying the effect of column length and NO3- concentration on efficiency of the NO3- t o NOz- reduction. Since several publications on use of cadmium-copper columns (3,6)indicated that a new column will initially reduce some NO3- nitrogen to states lower than in NO2-, we decided t o check this point. A 20-cm column was prepared and 10 samples, each containing 11-12 pg of NO,/mL in distilled water, were passed through the column a t a flow rate of 6-7 mL/min. T h e results in Table I show t h a t 100% conversion was obtained for the first, fourth, seventh, and tenth samples. T h e experiments were then expanded t o include column heights of 10 and 39 cm and concentrations up t o 84 l g of NO;/mL. Again, the percent conversions were essentially lOOC70 (ranging from 96.1 to 101.7%). Since the upper limit of NO3- concentration that we investigated adequately covered the concentration range expected in samples from stationary sources, we decided to use ruggedness testing (8) to evaluate the procedure rather than further increasing the concentration. A ruggedness test allows identification of critical and noncritical variables by varying F 1980 Arner -an Chemical Society
1956
ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER 1980
Table I. Effect of Column Length and NO; Concentration on Conversion Efficiency column length, cm 10
39
determination no.
conversion efficiency, % 11-12 pg NO;/mL
4 6 pg
NO;/mL
84
p.rg
NO;/mL (1) 97.5 ( 2 ) 96.1
(1) 104.8 ( 4 ) 100.9 ( 7 ) 100.9 (10) 101.7
(1)96.7 ( 2 ) 97.4
(1)98.0 ( 2 ) 97.5
(1) 100.9 ( 2 ) 101.7
(1)99.4 ( 2 ) 99.0
(1) 99.5 (2)99.5u
101.7
1
variable
(1)98.2 (2) 97.8
( l ) b 101.7 (2)
20
Table 11. Experimental Design
" Flow rate was 4.6 mL/min. Numbers in parentheses refer to the order in which the samples were run.
NO; concn, pg/mL
5.0
2
3
99
99
4
99
5
6
7
8
5.0
5.0
5.0
99
39
20
20
R.T. 34
34
34
no variation
blank
column height, cm column temp,
39
39
20
39
R.T." R.T.R.T. 34
"C
20
NO; conversion, %
99.6
98.7 82.9 98.7 89.7 101.4 86.0 87.9
R.T.= room temperature, 1 9 " C . a variable at two levels, a nominal and challenging level. A blank involves no variation in the procedure. The first ruggedness test was a seven-variable (four variables and three blanks), eight-experiment test. The variables were as follows: (1) NO3- concentration, 5.0-99 pg of N03-/mL in distilled water; (2) sample flow rate, 6-7 to 9-10 mL/min; (3) column height, 39 to 10 cm; and (4) column temperature, 18 "C (room temperature) to 36 "C. The data were normalized, with respect to concentration, by reporting percent conversion efficiency. T h e results showed that one of the blanks was abnormally high suggesting that a n interaction with one of the variables had occurred; however, the experimental design precluded confirming this point. Column height and NO3- concentration showed the largest effects. Reducing the column height from 39 to 10 cm produced a 21 YO reduction in conversion efficiency. This result appears t o conflict with the results in Table I. Increasing the NO3- concentration from 5.0 to 99 pg of N03-/mL reduced the conversion efficiency by 11%. Sample flow rate and column temperature produced only a 1.4 and 3.5% reduction in conversion efficiency, respectively. From this test we concluded that sample flow rate was not a critical variable and that column height, NO3- concentration, and column temperature required further study. The selected experimental design is given in Table 11. This design is essentially a 2 X 2 X 2 factorial design with no replication. From the eight determinations, made in random order, it was possible to estimate the main effects of NO3- concentration, column height, and column temperature on percent conversion. I t was also possible to estimate all interactions. T h e third-order interaction was confounded with the error term normally used to test significance of main effects and interactions. Therefore, an error term from a previous similar experiment was used. The results of statistical analysis showed that changing the NO3- concentration from 5.0 to 99 pg/mL produced no significant effect on NO3- conversion. Additionally, changing t h e temperature of the column from a room temperature of approximately 19 "C to aproximately 34 "C did not significantly affect conversion efficiency. There was a definite effect on conversion efficiency caused by reducing the column height from 39 to 20 cm. This change lowered the conversion efficiency from an average 99.6 to 86.6%. There were no significant two-way interactions, but the three-way interaction was significant. This indicates that the interaction between column height and NO3- concentration, although not significant, does in fact change significantly when the column temperature is increased. The effect is most likely caused by the 20-cm column since it its here t h a t the most noticeable change in conversion efficiency takes place.
Table 111. Effect of Variables on Conversion Efficiency
variable NO; concn blank column height column temp
level (I
change in conversion efficiency, %
5.0-99 p g NOLlmL n o change 39-20 cm 19-34 " C
-2.1
+ 2.6 -13 + 0.8
a The first value is the nominal value, and tne second value is the challenging value.
Thus, when the column height is 39 cm, the sample concentration is 5.0-99 Kg of N03-/mL, and the column temperature is between 19 and 34 "C, 100% conversion of NO3to NOz- should take place. Other factors such as sample matrix and column lifetime could adversely affect conversion efficiency, however. T o test the effect of sample matrix on conversion efficiency, we spiked a previously analyzed NO, sample, collected a t a coal-fired power plant with a MS, with NO3- such t h a t the NO3- concentration was increased by 72 pg of N03-/mL. Analysis of the spiked and unspiked samples showed a 102.6% recovery of the spike. Thus, our particular sample matrix did not appear to affect conversion efficiency. To assess column lifetime, we ran controls (NO3- in distilled water) with most of our NO, samples. Somewhere between the 22nd and 27th runs (total of distilled water and NO, samples) the conversion efficiency a t 16 pg of N03-/mL dropped to 92.9%. The total elapsed time between sample 1and 27 was 60 days. Subsequent controls a t concentrations of 8.3 and 1.4 pg of N03-/mL, on the same column, gave conversion efficiencies of 91.8 and 100.4Y0, respectively. The use of cadmium is an effective means of reducing NO3to NO2- and obviating the technique problems associated with preparing the alloy in the cadmium-copper procedure. However, much further work is needed to establish the effect of repeated analysis of NO, samples on column lifetime. Also, the lower limit of detection, accuracy and precision of the procedure must be established on a statistical basis. We have published our findings a t an early date, in order to acquaint others working in the field with our findings and to solicit a n exchange of information.
ACKNOWLEDGMENT T h e glass blowing skill of Richard E. Berkley is greatly appreciated.
Anal. Chem. 1980, 52, 1957-1958
LITERATURE CITED (1) Fed. Regist. 1977, 4 2 , 62971. (2) Annu. Book ASTM Stand. 1975, Part 31, 362 (3) "Methods for Chemical Analysis of Water and Wastes". EPA Report No 600/4-79-020, March 1979, Method 353.3. (4) Connors, J. J.; Beland, J. J.-Am. Water Works Assoc. 1976, 68, 55-56 (5) Gales, Morris E., Jr., personal communication (Feb 7, 1980, EPA Methods Development and Quality Assurance Laboratory, Cincinnati, OH) (6) Wood, E. D.; Armstrong, F. A. J.; Richards, F. A. J . Mar. B i d . Assoc. U . K . 1967. 47. 23-31. (7) Skoog, D. A.;West, D. M. "Fundamentals of Anaitical Chemistry", 3rd ed.; Holt, Rinehart and Winston: New York, 1976; p 274.
1957
(8) Youden. W. J.; Steiner, E. H. "Statistical Manual of the Association of Official Analytical Chemists"; Association of Official Analytical Chemists: Washington, DC, 1975; pp 33-36.
J o h n H. Margeson* Jack C. Suggs M. Rodney Midgett Environmental Monitoring System Laboratory U.S. Environmental Protection Agency Research Triangle Park, North Carolina 27711 RECEIVED for review April 28, 1980. Accepted July 15, 1980.
Determination of Lead-2 10 in Environmental Samples by Gamma Spectrometry with High-Purity Germanium Detectors
Sir: Measurement of low-level environmental 'lOPb (of the order of 0.1 dps/g) is important in many fields including health physics, geochronology, and environmental science. Presently. t h e standard procedures for environmental assay involve chemical separation and electrodeposition of 210Pbfollowed by a or 6 spectrometry (1-3). Although these procedures are capable of high sensitivity with gram-size samples, many processing steps are involved, as well as time delays of days and longer for ingrowth of daughter activity. Four percent of the decays of 'l0Pb are accompanied by emission of a 47-keV photon ( 4 , 5 ) , but self-absorption in the sample and low sensitivity of standard Ge(Li) detectors to this energy photon have previously prohibited practical assay by direct photon measurement. Recently, a new generation of high-purity N-type germanium detectors has become available (6). These detectors are characterized by improved sensitivity to low-energy photons (due to thin diode blocking contacts) and increased active surface area. They offer the possibility of direct assay of 210pb in moderate-sized environmental samples (of the order of 100 g) by measurement of the 47-keV photon. Good parameters for a high-purity germanium detector might be 70 cm2 active area and energy resolution of 1 keV for 47-keV photons. Due to the short range of 47-keV photons in germanium, essentially all photons striking the germanium crystal will be stopped. Consider a representative environmental surface soil sample of 0.1 dps/g and density 1.5 g/cm3. An effective thickness for absorption of 47-keV photons might typically be 2 cm. On the assumption sufficient sample is available, 210 g can be effectively utilized adjacent to the 70-cm2surface. Assuming 25 9'0 detection efficiency ( T geometry) and the 4% photon emission, one obtains 1 detector c o u n t r a t e = - d p s / g x 10 1 0.04 p h o t o n / d e c a y x - c o u n t / p h o t o n x 210 g =
4
0.21 c o u n t / s (1) If no background were present, this result would imply only 7.9 min would be required to obtain 100 counts and 10% counting precision (Poisson counting statistics). The sensitivity for water samples would be improved further because 0003-2700/80/0352-1957$01 .OO/O
of larger effective sample size due to less self-absorption in the sample. In practice the above estimate is quite liberal and can only provide a bound on the sensitivity. '4ctual sensitivity will be much less due to the presence of radiation background, potentially poorer counting geometry. losses in the detector's dead layer, and ineffectual collisions in the germanium. Nevertheless, the potential feasibility of direct assay of moderate-sized environmental samples of 'lOPb without need of chemical processing or time delay is suggested. This technique has been experimentally tested with a surface soil sample and a high-purity germanium detector. T h e sample was surface soil from Boulder, CO, and the detector possessed a 65-cm2 active surface area. Optimum shielding for such a test was not available, so the detector was surrounded by an oversized soil sample approximately 10 cm in depth where the soil beyond the effective self-absorption layer acted as a shield from external sources. In this configuration, shielding from external radiations is adequate, but the higher energy photons in the outer layers of the soil penetrate the inner layer and add some low-energy background. Figure 1 shows the result. Figure l a is from a sealed 226Rasource which contained daughter radiations ( 4 , 5 ) including *lOPb. This source was not at equilibrium, and the ratio 214Pb/2'0Pbis enhanced over that for a source a t equilibrium. Figure 1b is the result of a 30-h count with the soil sample. The 'loPb photons are clearly present although the background is considerable. The isotope '14Pb is not visible since the sample is surface soil (top 1 cm) and the 210Pbcomes predominantly from atmospheric *l0Pb fallout (from '22Rn decay) and not decay of 226Rain the soil (7),and the emission intensity of the '14Pb 53-keV photon is less than that of the 'lOPb photon ( 5 ) . Figure ICwas taken with the soil sample removed, and the absence of the 210Pbpeak indicates that the 'loPb photon was not due to sources external to the soil. On the basis of calibration with 226Raand 241Amsources, analysis of the soil spectrum indicated an approximate concentration of 'lOPb of 0.2 dps/g. Counting error (predominantly due to the background) was *14%. Although the sensitivity is significantly worse than the bound estimate, it is still quite usable. Soil and sediment concentrations of 'lOPb typically range from 0.01 to 1.0 dps/g ( 3 , 7 ) and counting times of a C 1980 American Chemical Society
