1590
Anal. Chem. 1986, 58, 1590-1591
Continuous Flow Reduction of Nitrate to Ammonia with Granular Zinc Robert M. Carlson
Department of Pomology, University of California, Davis, California 95616 Nitrate determination is one of the more frequently employed procedures in laboratories that process agricultural samples. An extensive discussion of various colorimetric, ion-selective electrode, and reduction-distillation methods has recently been published (1). The colorimetric and ion-selective electrode methods are troubled by interferences. Reduction-distillation procedures are time-consuming and suffer from interference from hydrolysis of labile organic nitrogen. More recent methods employing ion chromatography require pretreatment to remove humic substances (2). Mertens et al. (3)described an automated method in which nitrate is reduced to ammonia by a column of Devarda alloy in a continuous-flow apparatus. Ammonia is then quantified with a flow-through arrangement on a gas-sensing ammonia probe. The Devarda alloy is embedded in a polymer matrix, ground, and sieved to produce particle sizes that do not unduly restrict flow in the reduction column. Reduction of nitrate to ammonia is quantitative with a fresh column, but decreases slowly with column use. The maximum sampling rate is 30/h. The ammonium electrode is the rate-limiting component. I attempted to adapt the Devarda alloy-continuous flow reduction scheme for use with the continuous-flow ammonia analyzer described earlier ( 4 , 5 ) . To simplify the reduction column preparation, alloy particles retained on a 100-mesh screen were used in place of the polymer embedding technique. A reduction column prepared from approximately 1 mL of alloy quantitatively reduced nitrate to ammonia for a period in excess of 3 h. However, after 1 h the alloy developed a capacity to absorb ammonia so that increasing sampling times were required to maintain accuracy. In the course of trying to rectify the problems with Devarda alloy, it was discovered that a bed of granular zinc, pretreated with copper in a manner similar to the copper treatment of cadmium for reducing nitrate to nitrite (6), is very efficient in reducing nitrate to ammonia in continuous-flow applications.
EXPERIMENTAL SECTION Apparatus. The reduction column, shown in Figure 1,was constructed by wedging a 4.76-mm-diameter disk of 100-mesh stainless-steelscreen (convenientlycut with a 3/la-in.paper punch) into the bottom of a 1-mL polypropylene syringe barrel. The syringe barrel was loaded with 30-mesh granular zinc by adding the required zinc in three portions with 3-4 mL of 5% copper sulfate (w/v) rapidly drawn through the column after each zinc addition and a final wash with water. Polypropylene Luer to tubing connectors provided convenience for inserting and removing the reduction column from the continuous-flow apparatus. Reagents. Diethylenetriaminepentaaceticacid, 97% (DTPA), and potassium gluconate were obtained from Aldrich Chemical Co. Poly(vinylpyrrolidone), average molecular weight = 40 000 (PVP-40), was obtained from Sigma Chemical Co. All other chemicals were analytical reagent grade. Five percent CuSO, (w/v) was prepared from the pentahydrate salt. Samples were mixed with a solution containing 10 g of DTPA and 25 g of KOH/L. the DTPA/KOH combinationwas selected for its ability to prevent precipitation of Mg(OH)2. Two grams of potassium gluconate/L was added to the DTPA/KOH reagent to prevent precipitation of ferric hydroxide when testing the performance of the reduction column with samples containing 5 mM Fe3+. For testing the performance with samples containing suspended clay, 2 g of PVP-4O/L was added to the DTPA/KOH reagent to aid in maintaining the clay in suspension. Standard solutions were prepared from oven-dried KNOBand (NH4)2S04. Procedure. The instrument used for ammonium determination was a modified version of the apparatus described pre0003-2700/86/0358-1590$01.50/0
viously ( 4 , 5 ) . In this instrument sample mixed with an alkaline reagent is passed over a gas-permeable membrane through which ammonia diffuses. The ammonia is collected in a stream of water and quantified by measuring changes in electrical conductivity. The bundle of gas-permeablesilicone-polycarbonate hollow fibers in the original instrument was replaced with a microporous Teflon tube. This modification reduced response time and improved the detection limit. A freshly prepared reduction column required 2-3 min conditioning with the DTPA/KOH reagent t o reach quantitative reduction of nitrate. After completion of a set of samples, water was pumped through the reagent and sample tubes for 5 min prior to shutdown. Reduction column performance was evaluated by comparing instrument response to nitrate with the response to solutions containing equivalent concentrations of ammonium. Effects of ions commonly found in aqueous samples were evaluated by comparing responses to single salt solutions spiked with nitrate or ammonium.
RESULTS AND DISCUSSION The precipitation of Ca2+and Mg2+when a sample is mixed with alkaline reagent clogs the zinc column in the continuous-flow apparatus. The combination of NaOH and ethylenediaminetetraacetic acid (EDTA) used for ammonium determinations ( 4 , 5 )does not completely prevent precipitation of Mg(OH)2. A mixture of KOH and DTPA is superior for this purpose, because the MgDTPA chelate is more stable than MgEDTA and K has less tendency than Na to compete for the ligand. The concentrations used are designed to chelate up to 50 mM (Ca2++ Mg2+)and to neutralize up to 0.1 M acid with sufficient excess KOH and DTPA to maintain conditions necessary for nitrate reduction when the samples and reagent flow rates indicated in Figure 1are used. Reagent composition can be adjusted for samples with higher concentrations of polyvalent cations or acid or for different relative flow rates. Reduction efficiency was determined by comparing instrument response to nitrate and ammonium standards. Nitrate recovery was 99.6% for a 5 mM standard. Recoveries a t 10 mM and 20 mM were 99.1 and 98.0%. Use of 5 mM as the maximum concentration for effective reduction accommodates processing most agricultural samples without prior dilution. Samples with more than 5 mM NO3- can be diluted, or a smaller sample pump tube can be used to reduce nitrate concentration in the mixed sample-reagent stream. The useful life of a zinc column depends on the amount of NO, processed. To estimate the minimum useful life, a 5 mM NO3-standard was continuously pumped over a column with an occasional check with a 5 mM NH4+standard to ascertain quantitative reduction. Reduction was quantitative during 8.5 h the first day plus an additional 2 h on the following day. Two-thirds of the zinc had been consumed a t the time reduction efficiency decreased. In routine operation, a column is replaced when half of the zinc has been consumed. Effects of common ions on recovery of NO3- from a 5 mM standard are shown in Table I. The results are corrected for traces of NO3- and NH4+ found when the salts were run without added NO3+. Ferric ion will precipitate from samples mixed with the DTPA/KOH reagent. Addition of potassium gluconate to the reagent will prevent the precipitation of ferric hydroxide. The effect of Fe3+ on NO3-reduction was tested by continuously pumping a 5 mM NO3- standard containing 5 mM Fe3+ over a reduction column. The DTPA/KOH reagent was modified by addition of 2 g/L potassium gluco0 1986 American Chemical Society
1591
Anal. Chem. 1986. 58. 1591-1592
less for samples with appreciable Fe3*. For samples with considerable suspended materials (e.g., clays), addition of PVP-40 to the reagent aids in maintaining the reductive capacity of the zinc column. Response to a 5 mM NO; standard containing 1g/L bentonite clay (maintained in suspension by constant stirring) was constant and indistinguishable from the same standard without added clay during a continuous 6-h run. the DTPA/KOH reagent contained 2 g/L PVP-40. Samples with only slight turbidity can be processed without use of PVP-40. The complete reduction of NO; to NH, permits processing of most samples without prior removal of NH4+. Nitrate is determined by difference after running the sample with and without the reduction column. For samples in which NO3concentrations are very low compared to NH4+,prior removal of ammonium can be accomplished with procedures such as that descrihed by Mertens et al. (3). Some hydrogen gas evolves from the zinc column, but the amount is small compared to that pmduced by Devarda alloy (3). As with Devarda alloy, nitrite is reduced along with nitrate. Advantages of the procedure presented here include speed, simplicity, economy, and ruggedness. The reduction column can he prepared in minutes with off-the-shelf reagents. Very little sample pretreatment is required; colored matter and some turbidity tolerated well.
REAGENT .76 m l I min
SAMPLE .I2 mllmin AIR 76 m l l m i n
T O NH3 DETECTION
SYSTEM
1. Schematic diagram of zinc m n : (A) I d polVpopylene syringe barrel. (6) 100-mesh stalnless-stwl screen. ('4 pohlvopv%ne Luer-t-tubing connectors, (D) silicone rubber tubing. (E) three-way tubinq mnectors. ~
l5wuae . . stain-
steel, (F) 30mesh nanuiar zinc.
Registry No. DTPA, 67-43-6; PVP, 9003-39-8; CuSO,, 7758-98-7; KOH, 1310-58-3; NO3-, 14797-55-8; NH,, 7664-41-7; Zn, 7440-66-6; potassium gluconate, 299-27-4.
Table I. Recovery of Nitrate in the Presence of Common Salts'
a
salt added
concn, mM
NO, found, mM
NaOAc KeSO, KH,PO, NaHCO, MEL NaCl CaCI,
1W
5.02 5.02
100 100 100
LITERATURE C I T E D
50 100
50
0.R.; Nebon. 0.W. I n h4elk& of Soil A m W , Part 2. Agmnomy 9 : Page. A. L.: Miller. R. H.; Keeney. D. R.. Eds.: American Society of Agronomy: M a d i m . WI. 1982: pp 64b898. Marko-Varga. Gyorgy: Csiky. Istvan: Johnson. Jan Ake Anal. Chem. 1984, 56, 2066-2069. Mertens, John: Van den Winkle, Pierre: M88sari. 0.L. Anal. chem. 1975, 17, 522-526. Carlson. Robert M. Anal. Chem. 1978. 50. 1528-1531. Cariron. Robert M. US. Patent 4209299, June 24, 1980. StanbstdMsWs f a Um Exemfmtkm Of Walerand W a s t e w a t ~ 15U1 . ed.: American Public Health Asochtkm: Washington. DC, 1981.
(1) K-y.
5.00 5.00 4.99 4.97 4.97
(2) (3)
(4)
(5)
All solutions contained 5.00 mM NO;.
(6)
nate. Response was indistinguishable from the same NO3standard without Fe3+ for 3 h after which NO3- recovery gradually decreased. About two-thirds of the zinc had been consumed during the initial 3 h, so reduction column life is
RECEIVEDfor review March 4,1985. Resubmitted January 21, 1986. Accepted January 21, 1986.
Spectrophotometric Determination of Free Chlorine in Air Using a Sulfamic Acid/Triiodide Procedure Maurice R. Smith* and Harvey B. Cochran
Olin Corporation, P.O. Box 248, Charleston, Tennessee 37310 methyl orange solution (68).This method was subsequently modified to allow individual personnel monitoring (9). At the same time, OSHA personnel used an o-tolidine field kit that was also based on colorimetry (10.11). Both methods were known to have difficulties: in the case of methyl orange, the results were influenced by the rate of sampling (12);c-tolidine is suspected to cause tumors in the urinary tract (13). As a result of these problems, the OSHA laboratory developed a procedure that scrubbed chlorine into a sdfamic acid solution. Iodide reagent is added to the sulfamic acid solution to form triiodide (eq l), which is suhsequently analyzed utilizing an ion-selective electrode ( 1 1 , 14). We found ion-selective
Because chlorine is a potent respiratory tract imtant (1-3), manufacturing facilities that produce or use chlorine monitor free chlorine in the work environment. The current Occupational Safety and Health Administration (OSHA) permissible limit for free chlorine is a ceiling of 1.0 ppm (4). The National Institute suhsequently Occupational Safety and Health (NIOSH) ha4 recommended a 0.5 ppm ceiling and has specified that this concentration shall not be exceeded when averaged over a 15 min sampling period (5). Methods for quantitation have gone through several steps of refinement. The first NIOSH method recommended a colorimetric procedure based upon the bleaching of a dilute
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