A Simple Laboratory-Constructed Automatic Titrator

In the Laboratory

W

A Simple Laboratory-Constructed Automatic Titrator Kurt L. Headrick,*† Terry K. Davies, and Aaron N. Haegele Department of Chemistry, University of Victoria, Victoria, BC V8W 3V6, Canada; *[email protected]

Titrations are an integral component of chemistry laboratory curricula, but are frequently criticized as tedious and laborious (1–3). A variety of approaches have been taken toward automating titrators for the undergraduate laboratory (4–14). This paper describes the construction of a simple, versatile automatic titrator used in a senior undergraduate analytical and instrumental chemistry laboratory. Titrant is delivered at a constant rate by a metering pump. Advantages of using a metering pump are that it is a relatively inexpensive and common piece of equipment, it allows the use of a large reservoir of titrant, and the reservoir can be located remote from the apparatus, wherever convenient. With no valves and only one moving part, metering pumps are also much simpler, and are therefore inherently more reliable than the numerous valves and moving parts required for the syringe motors and motorized burets typically used for autotitrators. The titrator described in this paper is thus more convenient to use and more reliable than other autotitrators. After experimenting with directing the electrode output through an analog to digital converter to a computer (Millar, S. P.; Davies, T. K.; Headrick, K. L.; unpublished work, University of Victoria, 1998), and with using a laboratoryconstructed controller to both perform the titration and calculate and display the results (Garwood, K. L.; Akelson, G.; Headrick, K. L.; Davies, T. K.; unpublished work, University of Victoria, 1997), it was observed that directing the electrode output to a recorder via a laboratory-constructed analog differentiator provided better results than the above two approaches. Only the last approach is described in this paper. The differentiator consists of an operational amplifier (op amp) to amplify the electrode output to a range suitable for the recorder, followed by two differentiation circuits, all in series. The recorder can be connected to the differentiator output after the initial op amp, or after either of the two differentiation circuits, giving a plot of potential, first or second derivative versus time (volume), respectively. The analog differentiator is also useful pedagogically, in that it provides an introduction to electronics and instrumentation and demonstrates that useful analytical instrumentation can be constructed with only a rudimentary knowledge of electronics. The other essential component of an autotitrator is some means of actuation. This autotitrator simply has the pump and the recorder plugged in to a standard power bar; turning the power bar on and off starts and stops the titration. Apparatus A schematic diagram of the titrator is shown in Figure 1; a list of the components is given in Table 1. The metering pump used to deliver the titrant was an FMI model QSY-1 (Fluid Metering, Inc., Oyster Bay, NY), capable of deliver† Permanent address: Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada.

Table 1. Differentiator Components No. Component

No.

Component

R1

20K potentiometer (offset adjust)

R9

10 -k Ω 1⁄4-W resistor

R2

47- k Ω ⁄4-W resistor

C1

2.2- µF capacitor (nonpolar)

R3


C2


R4


C3


R5

6.8- M Ω 1⁄4-W resistor

C4


R6

47- k Ω 1⁄4-W resistor

OP1

output from amplifier; gain = 3

R7

6.8- M Ω 1⁄4-W resistor

OP2

output from 1st deriv. amp

R8


OP3

output from 2nd deriv. amp

1

Figure 1. Schematic diagram of titrator.

Figure 2. Circuit diagram of differentiator.

ing from 0 to 23.0 mL/min. Magnetic stirring was provided. Standard pH electrodes, meters, and recorders were used. The differentiator circuit diagram is given in Figure 2. Although a differentiator is a simple RC circuit (15, 16 ), in practice, additional components are required to provide a suitable output and to filter the noise to acceptable levels; the latter is particularly important for titrations in nonaqueous media. To emphasize the simplicity of the differentiator, the circuit is mounted on the outside of the differentiator case, with the essential components (the operational amplifiers, C1,

JChemEd.chem.wisc.edu • Vol. 77 No. 3 March 2000 • Journal of Chemical Education

389

In the Laboratory

R5, C3, and R7) mounted on the top of the circuit board; the additional components are mounted on the bottom side. The differentiator case is just large enough to contain the power supply, which consists of two banks of four 1.5-V batteries in series, providing ± 6 V for the operational amplifiers. The case is designed to flip up to allow easy access. An on/off switch and an LED power-on indicator are also mounted on the exterior of the case. Results Titration curves for the titration of sodium carbonate with HCl are given in Figure 3, showing the potential and the first and second derivative plotted versus time (volume) for three titrations. Using the second endpoints from the firstand second-derivative plots (Figs. 3b, 3c) with known masses of sodium carbonate gave an average HCl concentration of (0.1813 ± 0.0004) M. The relative standard deviation of 0.22% shows that this titrator is as precise as could be expected for a manual titration. It is interesting to note that the signalto-noise ratio of Figure 3 compares favorably with that of the commercial autotitrator used by Hopkins and Hamilton (1) for the titration of carbonate. These results also compare favorably to those obtained using the analog-to-digital converter (Millar, S. P.; Davies, T. K.; Headrick, K. L.; unpublished work, University of Victoria, 1998), where noise spikes were observed similar to the results of Hopkins and Hamilton. These noise spikes would render the second derivative useless, whereas the second derivative results shown in Figure 3c are excellent. Conclusions Using a metering pump for the titrator makes this titrator inherently more reliable and convenient to use in the undergraduate lab than other titrators. The analog differentiator makes the titrator more versatile and gives better results than other titrators, including commercial autotitrators. The ability of the titrator to generate data for all three output modes (potential and first or second derivative) versus time (volume) is valuable in introducing students to electronics and principles of instrument design and data handling. The titrator is relatively inexpensive and easy to build using readily available components. Although not a true autotitrator, since the rate of titrant addition is constant rather than adjusted near the end point and the titrations must be started and stopped manually via the power bar, this titrator is otherwise a good approximation of a commercial autotitrator for a fraction of the price. It has

390

(a)

(b)

(c)

Figure 3. Titration of Na2CO3 with HCl. (a) Normal; (b) first derivative; (c) second derivative.

been used successfully by students in a third-year instrumental analysis laboratory for two years and has performed flawlessly with no maintenance. W

Supplemental Material

Supplemental material for this article is available in this issue of JCE Online. Literature Cited 1. Hopkins, H. P.; Hamilton, D. D. J. Chem. Educ. 1994, 71, 965– 966. 2. Fox, J. N.; Shaner, R. A. J. Chem. Educ. 1990, 67, 163–164. 3. Lynch, J. A; Narramore, J. D. J. Chem. Educ. 1990, 67, 533– 535. 4. Hernlem, B. J. J. Chem. Educ. 1996, 73, 878–881. 5. Verbeek, A. A. J. Chem. Educ. 1985, 62, 687–688. 6. Stock, J. T. J. Chem. Educ. 1966, 43, 425–427. 7. Stangeland, L. J.; Anjo, D. M. J. Chem. Educ. 1992, 69, 296– 299. 8. Mak, W. C.; Tse, R. S. J. Chem. Educ. 1991, 68, A95. 9. Brockett, C. P. J. Chem. Educ. 1966, 43, 210–211. 10. Pinkham, C. A.; Field, B. J. Chem. Educ. 1977, 54, 577. 11. Baxter, D. N.; Huber, C. O. J. Chem. Educ. 1972, 49, 535. 12. Olsen, E. D.; Foreback, C. C. J. Chem. Educ. 1972, 49, 206– 208. 13. Knudson, G. E.; Langhus, D. J. Chem. Educ. 1971, 48, 613. 14. Olsen, E. D. J. Chem. Educ. 1966, 43, 310–314. 15. Malmstadt, H. V.; Enke, C. G.; Crouch, S. R. Electronics and Instrumentation for Scientists; Benjamin/Cumming: Menlo Park, CA, 1981; pp 122–123. 16. Skoog, D. A.; Leary, J. J. Principles of Instrumental Analysis, 4th ed.; Saunders: Orlando, FL, 1992; pp 20–22.

Journal of Chemical Education • Vol. 77 No. 3 March 2000 • JChemEd.chem.wisc.edu

A Simple Laboratory-Constructed Automatic Titrator

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