Simple, Efficient Reduction Column for Use in the Automated Determination of Nitrate in Water M . P. Stainton Canadian Department of the Environment, Fresh water Institute, 50 1 University Crescent, Winnipeg, Manitoba, Canada R3T 2N6
A widely used procedure for determining nitrate in water employs a cadmium-copper or cadmium-mercury couple to reduce nitrate to nitrite. Nitrite is then determined colorimetrically by a diazotization reaction. The method has been described in detail by Morris and Riley ( 1 ) and Wood et al. ( 2 ) .Brewer and Riley ( 3 ) adapted the method to an AutoAnalyzer system using a cadmium-mercury in-line reductor. The automated version of this method is now in wide use although the copper-cadmium reductor seems to be preferred. The reduction column used in most systems consists of a glass column or coil packed with cadmium filings that have been pretreated with a dilute copper sulfate solution to provide a thin film of elemental copper. Because filings are used, the sample stream must be debubbled to prevent channeling. T o accommodate the filings and facilitate packing, the columns have a relatively large bore (3-5 mm) and, hence, a large dead-volume. The combination of large dead-volume and lack of segmentation broadens and lowers absorption peaks obtained, decreasing both sensitivity and peak separation. Larger than necessary sample volumes (sampling time) and wash solution volumes (wash time) are required to obtain acceptable peak separation. This decreases the rate of analysis possible. Debubbling the sample stream before the reduction column also requires one additional channel of pumping capacity. Finally, it has been our experience that cadmium filing type columns rarely achieve 100% reduction, are somewhat time-consuming to regenerate and become compacted with time, restricting liquid flow. A more suitable reduction column can be constructed from a 1-meter length of l&inch i.d. Teflon tubing threaded with a 1-meter length of 1-mm diameter cadmium wire (Koch-Light Laboratories Ltd., Colnbrook Bucks, England). The column is fitted to the tubing nipples of a 4way liquid chromatography valve (Pharmacia LV-4) a t ports A and B (Figure 1). The column is prepared for use by attaching the female luer coupling, supplied with the valve, to port C and injecting, from a syringe, 10-ml quantities of 1N HCl, distilled water, 2% copper sulfate solution, and distilled water in that sequence at about 20 ml per minute. Ports C and D are then connected to the AutoAnalyzer manifold using the tubing nipples provided with the valve.
P. Riley, Anal. Chin?.Acta 29, 272 (1963). ( 2 ) E . D. Wood, F. A . J. Armstrong, and F. A. Richards, J. Marine Bid. Ass. U.K., 47, 23 (1967). ( 3 ) P. G. Brewer and J. P. Riley, Deep-sea Res., 12, 765 ( 1 9 6 5 ) .
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COLORIMETER
540 m 15 MM FIC
PUMP TUBE
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Figure 1. Nitrate-nitrite manifold with cadmium wire reductor. Reagents used appear in Table t
Table I. Reagents Used in the Manifold of Figure 1. All Reagents Made with Distilled and Deionized Water
Reagent
Buffer
Sulfanilamide NNED
Component
Ammonium chloride Sodium tetraborate EDTA (disodium dihydrate) Sulfanilamide Hydrochloric acid N-(1-naphthyllethylene diaminedihydrochloride
Quantity per liter of reagent
100 g 20 g 1g
10 g 200 ml 1g
The manifold in Figure 1 works well with freshwater a t a rate of 30 samples per hour with a 1:2 sample to wash ratio. Pre-column debubbling is not required, saving one pumping channel. Sensitivity of the system is quite acceptable. Unit absorbance a t 540 nm with a 15 mm cuvet is given by 550 pg/ liter of nitrate nitrogen giving a detection limit of approximately 5 pg/liter of nitrate nitrogen. Reduction efficiency is essentially 100% with column life being approximately 1 month or 1000 samples. Column regeneration is simple, involving the disconnection of ports C and D from the manifold and treatment with dilute acid and copper sulfate as outlined above. By switching the column off-line, the system can be used to measure nitrite only.
(1) A . W. Morris and J .
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RECEIVEDfor review February 19, 1974. Accepted May 20, 1974.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 11, SEPTEMBER 1974
