M. C. Mehro
Moncton Moncfon, N e w Brunswick, Canada Universite d e
I
I
Radiometric Analvsis 01 Ammonia in Water
Awareness of the environmental pollution problems has brought about a certain reorientation of the undergraduate curriculums and particularly so in analytical chemistry. A good deal of emphasis is now being placed on the laboratory experiments that involve some aspects of air and water pollution originating from inorganic or organic pollutants. Even in the freshman chemistry laboratory a certain amount of involvement of this sort exists, e.g., analysis of phosphate pollution caused by detergents or home vash liquids. The experiment described here involves an inorganic pollutant ammonia and its determination in water by a radiometric technique. The method is fairly simple and as such affords the possibility of analysis of other nitrogeneous compounds following their conversion to ammonia by the standard techniques (13).Another aim of the experiment was to create some research interest for the undergraduates in radioanalytical chemistry. Experimental
The principle of the experiment is based on the complexing action of ammonia on silver ion. An aqueous solution of ammonia is treated with a sparingly soluble silver compound and the release of silver concentration in the aqueous phase is taken as a measure of ammonia concentration. I n this experiment the silver concentration is determined radiometrically using silver-110 as the radiotracer. A standard solution of silver nitrate made radioactive with silver-110 (also in the same chemical form), was allowed to react slowly and with constant stirring with a solution of sodium salt of an anion forming a sparingly soluble silver compound. The precipitated silver compound was digested at room temperature for 24 hr in dark and finally i t was washed free of the extraneous material and dried to a constant weight a t 110°C. I n six or more 100-ml volumetric flasks, 150 mg of the radioactive compound was carefully transferred. Each flask was numbered and filled to mark with a prepared ammonia solution of known but increasing concentration. The ammonia solut,ion of the requircd concentration was prepared by dilution from a standard stock solution, which was initially standarized by the established procedure (1). The flasks were immediately sealed and thoroughly mixed for a couple of minutes before being placed into a constant temperature water bath. The flasks were further cquili-
brated a t 25'C for two hours before final analysis. An aqueous blank was also run simultaneously to give a comparative idea of the solubility of the compound in water and in an ammonical solution. In some preliminary experiments a 2-hr period was found adequate for the exchange of ammonia, hence it vas maintained constant in all subsequent experiments. Following the period of equilibration, a 25-ml aliquot from each flask was carefully withdrawn without disturbing the precipitate and filtered through Whatmann-42 paper. Two 10-ml samples from the filterate were transferred to the 15-ml plastic counting tubes and their radioactivities determined in a well-type NaI scintillation spectrometer under the photopeak of silver-110. Each tube was counted twice and an average of the four radioactivities was taken as a measure of ammonia concentration in solution. However, in computing the sample result, the background of the instrument and the aqueous radioactivity generated by the simple solubility of the silver c.ompou~dwere subtracted from the original radioactive data. Results and Discussion
The reaction of ammonia with silver ion produces a soluble complex of stoichiometry Ag(NHa)s+ and the reaction apparently proceeds even if the original silver compound is only sparingly soluble in water. The student may recall that in the qualitative analysis of group I elements, silver is separated from other members of this group by the same general reaction AgX
+ 2NHa(aq) = Ag(NH3)~++ X-
before the final analysis. In this experiment by tagging the silver compound n-ith its radioisotope a high degree of efficiency is achieved as far as the determination of silver ion in solution is concerned, which in turn means a lower limit for ammonia determination. Furthermore, no sample preparation is necessary as may be required in some colorimetric or spectrophotometric methods for the determination of silver in trace amounts (1, 4). In addition, the radioisotope silvcr-110 has a fairly long half-life and a convenicnt gamma photon energy (1.5 Mev) to make it an ideal indicator for a radioactive operation. Some typical results with mono, di, and trivalent silver compounds are shown in the table. The data indicate that iodate, tungstate, selenite, and phosphate of silver only fall under acceptable limits as radioanalytical reagents for ammonia. The iodate high blank value is rather unattractive, hwauw of -~ -w-hereas the other three appear to be fairly sensitive reagents. The analytical response of these three cornpounds remains linear as is seen in the graphic data
-~ ~
Presented by Paul O'Brien at Atlantic Section meeting (student chapter) of the Chemical ~ ~ ~ t of i tcaneda, ~ t i ~ ~ d ~i f X.S., ~ ~ , Canada.
Volume 49, Number 7 2, December 1972
/
837
Radiometric Response of NHa on Silver Compounds
Compound 1. AeCrO.
0 ppm NH, (C,) amm
5 2 3 ppm NHa (Cd
ascpm
=I00 P P NHa ~ (CI) wpm
Efficiency C P ~ / P P ~ ax - (a, m)lC8 - C*
308
562
10283
2.5367
1RR
1931 2799 1730 1195 1687
3216 140 1403 194 740
31127 14390 24859 6061 12646
143115 57475 113926 19729 42200
Radioactivity relea-
-
7. 8. 9. 10. 11.
A~IO, AgCN Ag,PO+ Ag2AsOl Ae.lFe(CNh1
(see figure); the phosphate slope, however, indicates a little better performance. The results shown are for a range 23-100 ppm but actually one can detect up to 5 ppm of ammonia in solution conveniently. The student can, however, further verify the limits of detection by exploring other possible combinations by setting up his own research project. In fact, characteristics of sparingly soluble silver compounds with organic ligands of varying charges can he conveniently studied by this method. I t is probable that some of these may offer a superior sensitivity as compared to inorganic compounds explored in this study. Environmental ammonia pollution in gaseous form generally occurs in an industrial or an agriculture barn atmosphere either because of direct use of ammonia compounds or due to decomposition of complex nitrogeneous compounds. In the student analytical laboratory this may he encountered both in air and laboratory effluent depending on the analysis being performed on a particular day; e.g., many gravimetric analysis sessions do involve the use of ammonia solutions. Hence, a laboratory effluent sample free of suspended impurities will suffice to familiarize the student with the radiometric analysis of ammonia in solution. Furthermore, ammonia is basic in character and is extremely soluble in water at room temperature. This property can also be utilized to transform a gaseous sample into an aqueous sample by scrubbing a known volume of air through pure vater for a similar analysis. The radiometric finish for ammonia determination may also be envisaged for the analysis of other nitrogeneous materials. The inorganic nitrates and nitrites can be quantitatively converted to ammonia by reduction with Arnd's alloy (60'%Cu40%31g), whereas the biological samples (crude or purified proteins, vitamins, amino acids, etc.) can be digested in concentrated sulfuric acid in presence of a catalyst (H202, Se, HgO or CuSOa) to transform their nitrogen to ammonium sulfatc. The latter may further he decomposed by heating in an alkaline medium to liberate ammonia as has been described in details in some earlier publications (1-5). I n this experiment the solubility of a sparingly soluble silver compound in the presence of ammonia has been determined under static conditions. This technique generally referred to as "Batch Equilibrium" procedure is attractive because of its simplicity and the easr i ~ i t hwhich reproducible results can he obtained by the st.udent. This advantage compensates for the inconvenience of a larger sample volume required 838
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-
Specific activity cpm/mg
Journal of Chemical Education
~
~
+
-
y roo z3 I' 80-
2 g
0
a-
Z >
40-
F
0
2
20-
Q
[r
Radiometric response of NH1 on insoluble silver compounds.
to have statistically significant results. One may utilize the alternate "Dynamic Equilibrium" technique by passing the sample solution over columns of radioactive silver compounds as is normally done with organic resins in the ion exchange procedures. Such a procedure will decidedly he fast, neat and may even require much less sample volume. However, it vill be somewhat involvrd for the students having little experience in radioanalytical chemistry, and to some extent would tend to obscure the significance of the experiment. In any event, the sensitivity of detection of ammonia by the static radiometric procedure for class room purpose is fairly high and is supwior to that of the known volumetric method (I), though it can not be said the same for some other micro colorimetric methods (5,5). Acknowledgment
The author is grateful to the students Miss Corinne Poirier and Paul O'Brien for their technical assistance. The information contained in this article was developed during the course of work under grants from the National Research Council of Canada and the Universitk de Moncton. Literature Cited (1) KOLTHOPP, I. M., SANDELL. E. R . , AND BRUCTENSTEIN, S.. " Q u ~ n t i b t i v e Chemioal Analysis" (4th ed.). The MaoMillsn Co., N e w York, 1969, p. 790. , L.. A N D MILLER.E. E.. A n d . Chcm.,20, 481 (1948). (2) M m ~ z n G. (3) MANN,L. T., And. Chem.. 35,2179 (1963). (4) KODAMA, K.. "Methods oi Quantitative Inorpaoio Analysis." Intersoienoe Publishers. New York, 1963, p. 1468. (5) RUCH.W. E.. "Chemioal Detection oi Gaseous Pollutants." Ann Arbor Scienoe Publishers Ino.. Ann Arbor. Michigan. 1968, p . 36.
