Geoffrey I. Gleason Oak Ridge Associated Universities Oak Ridge, Tennesee 37830
In studying the contamination of local rainfalls, we have been using a method for the determination of nitrate which was originated for use with sea water.' It is based on a conversion to nitrite by means of a cadmium amalgam reductor column. Thus, the very sensitive and specific azo dye reactions of nitrous acid can be applied. Once established, the method is reliable and rapid. It can be carried out on a variety of water samples and hence, can provide an informative trace analysis experiment in courses and projects relating to the environment. Discussion Numerous methods have been proposed for the reduction of nitrate. Most of these have some objectionable features. Indeed, a method is noted in the APHA manual on water analysis2 in which metallic zinc is used as a reductant, with due caution that conditions are critical and must be exactly controlled. I t is, of course, essential to the analysis that the reduction proceed no further than to nitrite
Nitric Acid in Rain Water We have observed improved conversion efficiency when the p H of the samples is adjusted to near 10 before applying to the column. Only a reasonable consistency of operation is demanded in carrying out the reduction step. Preparation Cadmium Amalgam Since finely granular cadmium is not commercially available, prepare a stock of filings from stick cadmium using a coarse-cut file. Because of the toxicity of the element, it is best to wear gloves and a respiration mask during this work. Place about 10 ml of filings in a 125-ml conical flask and rinse with acetone to remove grease. Rinse well with distilled water and then cover with 15 ml of 0.5% mercuric chloride solution. Swirl
However, nitrite can be further reduced to nitric oxide
Harris1 studied the reductive action of several metals in finely divided form on nitrate solutions and found that cadmium gave satisfactory yields of nitrite, particularly if i t had been amalgamated by treatment with mercuric chloride solution. Further, there was little tendency, if contact was prolonged, to reduce nitrite as was the case with most other metals tested. (It is recommended that the student look up the potentials for half-reactions (1) and (2) as well as those for metals such as zinc and cadmium and verify that the observation noted above is firmly supported by theory). It is important that the cadmium amalgam be in finely divided form and that the reductor column be sized to ensure an adequate contact time with reasonable flow-rates.
2mm CAPILLARY TUBING
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'Morris, A. W., and Riley, J. P., Anal. Chim Acta., 29, 212 11963). ,~...,.
2American Public Henlth Association, "Standard Methods lor
the Examination of Water and Wastewater," 12th ed.. The Amer. ican Puhlic Health Assoc~arion,Yew Yurk. 10019, 1969, pp. JY5 8
71 8
/ Journal of Chemical Education
Reductor column. All dimensions in millimeters
the flask occasionally for about 10 rnin. The filings lose their luster and become a dark pay. Pour off the spent mercuric chloride solution and wash the amalgam with several portions of distilled water until the washings became clear. Store under water until ready far use.
Rainwater Analyses Showing Nitrate, pH. Specific Conductance. Sulfate, and Amount of Rain Precioitated Cond. Nosin ppm (fimhorl SOtFall sample ( ~ / ~ l i pH mi inppm (in)
Fleductor Column The apparatus shown in the figure is recommended. However, if this is not readily available, a buret section or simple chromatography column of similar dimensions will suffice. The apparatus shown has the advantage of automatically providing a slow, constant flaw-rate and will not draw air into the bed. Fill the column with water and insert a tight glass-wool plug to support the bed. Transfer the amalgam to the column in small portions, tapping the column gently to produce a well-packed bed. The flow-rate should be such that 10 ml will nass the column in ahour 10 min. \
