Acid Rain Analysis by Standard Addition Titration Charles E. Ophardt Elmhurst College, Elmhurst, IL 60126 The public debate about the causes and effects of acid rain will hate a greater meaning to students who determine the acidity of a rain or snow saml~lethat they have rollertrd. A simple standard addition titration method has heen developed for use in general and analytical chemistry labs. This method requires the use of a standnnl buret, a pH metrr, and Gran's I'lot to determine the equivalence point. Researchers usea titration method which requires the use of a micropipet since the actual titratable acidity of a rain sample isquite small (10-"M in H2SOa).Acomplew titration o f a 1M)-mlrain sampletypirally requires less than 1ml of0.01 M NaOH. In addition, thesample isrontinunllv purred with nitrogen to prevent reaction oicarbon dioxide~f~omair with the NaOH titrant (1.3).The method issomewhat tediousand requires a very careful and precise technique to get consistent resulu. In the standard addition titration method, standard sodium hydroxide is first used to standardize sulfuric acid which is diluted with lW ml of distilled water. The swond titration of the sulfuric arid is carried out after theaddition of 100 ml of rain water. The diffrrenrc in the NaOH tirrant volume for the two titrations is used to calculate the acidity of the rain sample. Experimental Procedure Collect rain or snow samples in clean, rectangular (30 X 30-cm or larger) plastic trays placed away from trees and buildings or use other collection procedures as suggested by Galloway (3).Prepare or purchase 0.0100 M standard NaOH solution and also prepare approximately 0.005 M H2S04 stock solution. A simulated rain sample is prepared by diluting 10 ml of stock HzS01 to 1I. Standardize a pH meter usine buffers in the o H ranee of 3 to 5. s&wn by pipering 10.00 ml intoa ~tandardimthestock H'?SO~ 200-ml,tail-form beaker, adding Ux) ml distilled water, and linally ritratinp lh~ssamplrwirh0.01 .W .\laOH. &cord pH readingsat 0.5-ml intervals up to a 7-ml NaOH volume, then every 0.2 ml up to pH 8. Repeat the same procedure for the standard addition titration except use a 100.0-ml rain sample in place of the distilled water. Determination of Tltration End Point The equivalence point for hoth titrations is determined from a Gran's Plot (4-6). Gran's function is given in the following equation: G = (Vi + V) l o @ H i - ~ H l where Vi is the initial sample volume, pHi is the initial pH, and
p H is the reading after the total volume, V, of base is added. Gran's function is plotted versus volume of base added. The titration equivalence point is determined by extrapolating the linear portion of the Gran plot to the base volume axis. The most linear portion of the plot is usually from about pH 3.5 to p H 5.0. Points greater than p H 5 should not be included since any weak acid including carbonic causes a marked curvature in the plot ( I ) . Results and Discussion The table gives a com~arisouof the titration results using a micropipet k t h nitrog& purging and the standard addition method without nitrogen purging. The standard deviation for all of the methods is row andindicates good precision in the results. The standard addition titration of the simulated rain sample shows the accuracy of the method since the sample was made by a dilution of the stock H2SOa. T h e calculated concentration of the simulated rain sample is 4.73 X 10-5versus an experimental value of 4.75 X 10W5M.As a further check on the aEcuracy of the method, the same sample was titrated with a m i c r .o ~. i ~and e t gives a value 6% less than the calculated value. T h e comparison of the standard addition titration and a micropipet ;itration of an actual rain sample gives ahout a 15-20% difference. Only a single trial was possible with the micropipet since there was insufficient sample. Five rainlsnow samples titrated with thestandard addition method range in concentration from 0.24.45 X 10-5M. T h e concentration of one rain sample was a negative 1.03 X 10-5M which means that less NaOH was used in the titration after the standard addition of the rain water. A rain sample with this characteristic contains some basic substances, perhaps from the dust. In summarv. the standard addition titration is a nrecise and rapid method for the determination of the acidit; in rain or snow samples and could be an interesting general laboratory procedure. Acknowledgment I would like to acknowledge the work of Judith Benescb, Anita Mokadam, and Terry Fults for developing and carrying out the titratiou procedures.
Comoarlson of Tltration Results Tiralion MelhDd Sample 100 ml
c
Standardization Stack H9SOda
Volume in ml of 0.0100 MNaOH used in libation Standard Addition Micropipet Standard Addition Simulated Simulated 3/26/83 Rain Rain RainISnow
Micropipel 3/26/82 RainlSnaw
Trial I II 111 IV
Average
9.52 9.42 9.49 9.35 9.45
D~lt.~
Standard Devialion Molarity as H&O.
0.0760 4.73 X
10.52 10.40 10.20 10.46 10.40 0.95 0.139 4.45 X 10-I
0.92 0.85 0.90
10.39 10.34 10.29
0.89
10.34 0.89 0.0500 4.45 X
0.0361 4.45 X 10-
0.73
3.65 X
10ml a m p l e *Din = Standard addnion volume - Mock H&O, volume
Volume 62 Number 3 March 1985
257
Literature Cited (1 Gallway, 3.. Likens, G., and Edgerton, E., Wolar AirSoilPoll3., 6.423 (1976). (21 Lillestrand. H., and Morgan, J..Enuiron. Sci TechnoL, 12,1271 (19781.
258
Journal of Chemical Education
(31 Galloway, J., and ~ i k . G., ~ ~WorerAir , soit P O ~ I U6,241 ~ . , (1976). (4) Gran, G..Ana?val. 17,661 (1952). (51 Asknc, C.,and B r w d C.,Afmas. Enuimn.,6,695 (1972). (6) k o t t i , F.,and h o t t i , H., J. C ~ MEDUC., . 42,375 (19651.
