Potentiometric Measurements in a Freshwater Aquarium Thomas M. Harris The University of Tulsa, Tulsa, OK 74104 One of the great challenges in teaching an instrumental methods course is to create laboratories that are both instructive and interesting to the students. We have developed a laboratory exercise in which several important chemical parameters in a freshwater aquarium are monitored using direct potentiometry The lab also provides a lesson in environmental chemistry and tropical fish care. The chemistry of the water in an aquarium is critical to the well being of the fish. To avoid excessive mortality as well as susceptibility to disease, parameters such as the pH and the concentrations of several ionic species should be monitored. Typically, colorimetric analyses are utilized; test kits can be purchased in any aquarium shop. With one exception (nitrite ion), all of the important chemical species can be quantified, with much greater accuracy and precision, using ion-selective and gas-sensing electrodes.
be recognized that water purified with a single distillation or by reverse osmosis may contain detectable quantities bicarbonate and ammonium ion. The aquarium contained approximately 34 L of water, a bottom filter covered with a 1-cm laver of fine flavel. scvera1 plastic plants, one large volcanie rock and,htially, 12 fish (catfish, swordtails and assorted tetras). The tank was constantly aerated. TetraMin flake food (Tetra) was added daily in measured amounts. Following three weeks of a relatively low 7 mg/fish/day, the feeding rate was increased to 18 mgfish/day At the end of the sixth week, fluorescent lights directly over the aquarium were illuminated continuously (they had not been turned on at allup to that point). At the end of the ninth week. 25 additional fish ( ~ l a.t vand s danlos, were introduced to the tank and the feeding rate was reduced back to 7 mg/fish day.
Experimental Caution: The concentrated sulfuric acid and 50% sodium hydroxide solution used in these experiments can cause severe eye and skin irritation. Chemical workers' goggles should be warn when handling these.
Results As one of the exercises associated with this laboratory, the linearity of the electrode resDonses was determined over the raige of concentrations encountered in the aquarium water. The pH, chloride and ammonia electrodes were completely linear over these ranges; the slopes are listed in the table. The carbon dioxide electrode exhibited a slope of 43.8 mvldecade of concentration, quite different from the "Nernstian" value of 59.1. Furthermore, its response was nearly constant for concentrations less than 0.01 mM; therefore, this value was taken as the detection limit. The determination of nitrate ion using the ammonia electrode involves the conversion of the analyte to ammonium ion. To determine the efficiency of this conversion, sodium nitrate standard solutions featuring a range of concentrations were analyzed. The efficiency of the conversion was found to be 100% for concentrations ranging from 0.0100 to 5.00 mM. As noted above, several changes in the maintenance of the aquarium were made over a 1Pweek period. The resulting changes in the water chemistry are presented in the figure. During the first three weeks (which were considered to provide baseline data), the concentrations of bicarbonate ion and chloride ion remained fairly constant while the pH, ammonium ion, and nitrate ion decreased steadily. At the end of week three, the feedingrate was increased from 7-18 mg/fish/day. This produced a steady increase in the ammonium ion concentration over the next three weeks, while the concentration of chloride ion decreased dramatically. The pH exhibited a local maximum during week five. Green algae became noticeable on the glass walls of the aquarium during this period.
For the experimental results provided below, two different potential measuring devices were used. Initial measurements in the semester-long experiment were made with a Fisher Accumet pH meter; later measurements utilized a n Orion Model 901 Ionalyzer. Sample analysis was performed on 50-mL aliquots withdrawn from the aquarium. The pH and the chloride ion concentration were measured using a glass electrode (Fisher) and a solid state electrode (Orion), respectively. The concentration of bicarbonate ion in the water was determined with a carbon dioxide gas-sensing electrode (Fisher). To convert the bicarbonate to carbon dioxide, a 50-mL aliquot of the standard solution or aquarium water was treated with 0.5 mL of a pH 3.0 phosphoric acid/sodium dihydrogen phosphate buffer concentrate. The concentrations of ammonium ion and nitrate ion were determined using an ammonia gas-sensing electrode (Fisher).The ammonium ion was converted to ammonia bv adding1.0 mL of 50% sodium hydroxide solution to a 56mL aliquot of sample or standard solution. In a separate aliquot, the nitrate ions were reduced to ammonium ions by the presence of zinc metal granules (0.5 g) and sulfuric acid (20 drops):
NO,- + lOH' + 8e- + NH4++ 3H,O
(1)
To ensure complete conversion of the nitrate, the sample was allowed to react overnight. The sample was then treated with 2 mL of 50% sodium hvdroxide solution. and the concentration of ammonium ion was determined as described above. The concentration of nitrate ion was calculated fmm this result by subtractingfrom it the concentration of ammonium ion in the aauarium water determined beforehand. Water lost from the aauarium bv evawration and the Deriodic sampling was re'placed wi'th house distilled water. All solutions used to standardize the electrodes above were prepared with reagent-grade chemicals and water purified with a Milli-O \Millipore, purification syiitem. While no evidence of thishas observed in these experiments, it should
340
Journal of Chemical Education
.
Electrode Performance Characteristics Ion hydrogen chloride bicarbonate ammonium nitrate
Electrode
Slope (mvldecade)
pH
-55.0
chloride carbon dioxide ammonia ammonia
+43.8 -58.3 -58.3
-54.0
Operating range (mM) lo4to 10-I 1 to 102 10"to 10' lo-z to lo' 10-~to5
0.150
-
-
Changes in the pH correlated in part to the changes in the ammonium ion concentration. The sharp maximum observed durina E P.., - week five corresponded to an I 0.100 II increasing ammonium ion conE #\ centration. However, the pH 0 ' .... * I I N%+ failed to increase during the sec2 HCO-J- ond period of increasing ammoi nium ion concentration. sueeestn.,. ! ing that the water was at'that 0 point buffered, presumably by a weak acid present in the food or fish feces. The decrease in the bicarbona t e ion concentration t h a t occurred with a n increase in illumination of t h e aquarium correlated with the growth of E photosynthetic algae. With the 0 ---..,--- pH disappearance of the algae during weeks 10 and 11 and the in*.-. creased number of respiring fish in the tank during this time, an --+N?3increase in bicarbonate ion wncentration was expected. The fact that this was not observed suggests that the pH was low E enough to limit the conversion of carbon dioxide to bicarbonate ion (pK. = 6.5) before the former was 1 2 3 4 5 6 7 8 9 1 0 1 1 12 stripped from the water by the Week constant aeration. With the exception of the iniResults of 12-week aquarium water chemistry experiment tial measurement, the nitrate ion concentration remained fairly constant throughout the experiment. This suggests At the beginning of week seven, the fluorescent aquarium lights were illuminated. Over the next three weeks, the bacterial production of nitrate and its consumption in the production of proteins by the algae (2)were balanced both the ammonium and bicarbonate ion concentrations in this aquarium. decreased, with the latter dropping to the limit of detecSince the food being added to the tank each day contion. The pH decreased to below 4.5, resulting in the death of two swordtails and one catfish. The amount of algae on tained 3%sodium chloride, a steady increase in the chloride ion concentration was expected. The decrease in the the walls of the aquarium increased steadily At the beginchloride concentration during weeks 3-5 correlates with a ning of week 10, the aquarium was grossly overpopulated, period of increased fish activity (as indicated by an eleand the average feeding rate was reduced to 7 mglfishlday This produced an increase in the concentrations of ammovated ammonium ion concentration). Chloride is taken up nium ion, nitrate ion, and chloride ion. The bicarbonate ion by the fish in a n "active" process that requires energy (3). This correlation suggests that chloride ion measurements concentration remained a t the limit of detection. The might be useful detecting changes in the vitality of fish in platys added to the tank proved to be ill-suited to the low a freshwater aquarium. pH water, and several perished. The walls of the aquarium were free of algae by the end of the 10th week. Conclusion Discussion Overall, this laboratory exercise was well received by the students. More labs of this nature, in which scientific disDespite the fact that this experiment was limited to one coveries in a familiar system are coupled with a rigorous semester, some clear trends are evident in the data oretest of analytical methodology, are needed in the Instrusented above. The easiest behavior to explain is the change mental Methods laboratory curriculum. in the ammonium ion concentration. Freshwater fish excrete ammonia from their gills. At the pH values observed Acknowledgment in this aquarium, the ammonia was immediately converted to ammonium ion. The increases in the ammonium The author would like to thank David Iacoe for suggestion concentration observed during weeks 4 6 and weeks ing this laboratory, and the Sevens Seas Aquarium aquarium equipment donations. The data presented above were 9-12 are due to an increase in fish waste resulting from an increased feeding rate and a n increased fish population, collected by Don Zetik, John Main, Paula Morris, Holly respectively. However, the ammonium ion does not accuAutry, Christan Sack, Mike Rinehart, Dennis Schmude, mulate without limit. It is slowly converted to nitrite ion Ing Kuok Lee, Ken Parker and Brian Blackwell. by bacteria of the Nitrosomonas group, and the nitrite is in Literature Cited -~~.. turn converted to nitrate by Nitrobacter bacteria (I). The 1.Sawyer,C. N.:MeCarty,P.L ChemistvforSonifovEngimrs,2nded.;Me('rsw-Hill: decrease in the ammonium ion concentration observed New York,1967;pp 421422. during weeks 6-9 may have resulted from an increased 2. Stumrn. W.; Morgan, J. J. Aquatic Chmidry; Wllqr: New York,1981:p 562. 3. Wessells, N. R : Hopson, J.L.Biology; Random Houae: New Ymk,1988:p 845. rate of conversion by the bacteria.
E
-
.-
baseline data
increased feeding
constant increased illumination population
-
=-
..".-"
.-
.
~~
~
Volume 70 Number 4 April 1993
341
