Colorimetric Titration Experiment for the Undergraduate Laboratory Edwln L6pez and Basil H. Vassos University of Puerto Rico, Rio Piedras, PR 00931 Colorimetric titrations have been known for a long time (1-3). On the other hand, optical measurements, using immersed light guides of various configurations, have received onlv a fair amount of attention (4-6). The use of such lieht g u i e s in colorimetry presents consiherahle advantages due to the comolianre of the lirht oath to mechanical motions. In addition, "mixed-bundle" fiber optics permit measurements involvine verv small volumes of s a m ~ l e(7-8). We are dewriting a colorimetrir titiat ion iktrument usable in the undermaduate laboratorv that fulfills the obiectives of ruggedness, Geedom from ambiknt light interferenee, and low cost. In the form described, a is ton buret, a mametic stirrer, and a recorder were used a* accessories. This raises the price of the instrument to about $2,000, but the basic colorimeter itself is very low-priced and, in fact, could be used manually in connection with a simple voltmeter. In this form it is easy to construct a large number of units for use in simultaneous experiments in the analytical laboratory.
-
-
Apparatus The instrument can be described as a primitive double1). Each of the two optical fibers monochromator design, (Fi. (Edmund ScientificNo. 40.640) is orovided with a fdter at the same wavelength (435.8 nm, ~ d m i n dScientific No. 30,689). This dual filtration eliminaws comolewlv the effect of ambient light. The optical fibers are coinec&d via conventional
Figure 1 (a)General view of the instrument. (b) Schematic.
plastic lI4-in.tubing connectors to an open can (a condensed milk can is just the right size). The can is painted black inside. No other precaution is needed to avoid external light. The light detector is a photovoltaic cell (Allied Electronics No. 946-0250) connected to an RC-filter. The RC-filter is needed to quiet the fluctuations due to the non-homogeneity of concentration in the cell (which is a conventional beaker). The lieht source is also obtained from Fdmund Scientifir (No. 72,181). The syringe pump was a Sage l ' r i ~ l eChannel (Fisher Scientific No. 14-831-48)with a 10 ml. syringe and long stainless steel needle extension. hocedure The titration selected for illustrating the colorimetric titration was of 0.1 M NaOH with 0.1 M HC1 using Brom Thymol Blue as indicator. The yellow form of the indicator absorbs intensively in the 435 nm region where the blue form has almost 100%transmittance. The syringe is first calibrated by running the syringe pump filled with water for a certain time and measuring the resulting volume, X. Simultaneously, the recorder is run and the distance covered, is determined.
Figure 2 Examples of colaimebic tihations. (a) 0.1 M NH3tihated with 0.1 M HCI. (b) 0.01 MNaOH wim HCi 0.01 M. Fmm this wrve, a m i t i i i t y limn ot 0.01 Mcan be inferred.
Volume 61 Number 11 November 1984
1025
The ratio ( X I Y )represents the calibration of the recorder in mL/division. The calorimeter is then adjusted to 0% transmittance (light off) with the recorder a t zero and to 100%transmittance (cell filled with water) by means of the intensity control of light source. Typical titrations are shown in Figure 2.
1026
Journal of Chemical Education
A study of errors done rather more carefully than is typical for an undergraduate student, gave a standard deviation for one measurement of 0.5270, while the average of 5 runs coincided with the volumetric determination within 0.4%. The response of the students t o this experiment was invariably enthusiastic.
