Harold F. Walton University of Colorado Boulder, 80302
I
A Simple
Most students of chemistry perform titrations with a p H meter, plotting the p H point by point as standard acid or base is added. They get an idea of pH-titration curves and use them to find ionization constants, and if they persevere, they learn that equivalence points can be located very precisely by potentiometric titration. The operation is slow and laborious, however, and students will feel that titration with indicators is much more practical. Automatic recording of titration curves is not only less lahorious than point-by-point plotting, hut gives a much clearer view of the shapes of titration curves and the information that can he obtained from them. It can he adapted to repetitive analysis and quality control. Educationally, a session with an automatic recording titrator gives an insight into experimental errors and their control, and broadens a student's ideas of what "standardization" means. At the Pedagogical Institute of Caracas, Venezuela, we constructed the simple titrator shown in Figure 1. The reagent is contained in a 1-1 flask, A, which has a two-hole ~ b b e stopper r fitted with glass tubes, 8-mm external diameter, one slightly longer than the other, shown a t B. Air enters the flask through. the shorter tuhe as the reagent flows out of the longer tuhe. The level of liquid in the funnel rises and falls as air bubbles enter the flask, but the average level is constant and the oscillations are small. The hydrostatic head, the difference in level hetween B and the outlet F, is some 75 cm. E is a 50-cm length of capillary tubing of 1 mm internal diameter. This slows the liquid flow, but the main restriction of the flow rate occurs at the outlet F. This is a glass tube of 6 mm external diameter, drawn out into a capil-
I
STIRRER
Figure 1. The automatic titrator.
I
lary some 3 cm long. The insert, Figure la, shows how this is done. Getting the desired hydrodynamic resistance is a matter of trial and error. If the liquid flows too fast, it may be slowed somewhat by constricting the tip of F in a flame, but the tip should not be too fine or it is easily choked by dirt. Dirt particles enter the reagent in any case, and i t is essential to place a loose plug of glass wool a t C to filter them out. In our apparatus the flow rate was 6.6 ml/min. A slower rate, perhaps half this, might have been preferable. Flow is started and stopped by a pinch clamp, D. The inlet tube F may he placed at the side of the beaker opposite to the electrodes, or a t the center of the beaker; i t did not seem to make much difference where it was placed, so long as i t was not close to the electrodes. It is important to have the delivery tip high enough that it remains above the surface of the liquid throughout a series of titrations (see below), for if the liquid in the beaker rises above the delivery tip, the delivery pressure will drop and so will the flow rate. The magnetic stirrer is set to give rapid stirring and a pronounced vortex. Care must, of course, be taken that the magnetic stirrer bar does not strike the electrodes. A convenient size for the beaker is 250 ml. We used a Radiomete1 p H meter, which has recorder output terminals that provide 10 mV p e r p H unit. With it we used the new Heath Model IR-18M multi-speed recorder (price in kit form, $149.95). This has an input of 10 mV. Between the recorder and the pH meter, therefore, we placed a voltage divider, shown in Figure l h , as recommended by the manufacturers in their Assembly Manual, p. 59. The calibration control of the recorder was then adjusted to make one division of the chart paper (1in.) correspond to 2.0 p H units. Finally, a solution of potassium hydrogen phthalate, 0.05 M, was placed in the beaker as a reference buffer, p H 4.00. The p H meter was adjusted to read exactly 4.00 with the electrodes in this solution, then the zero control of the recorder was set to place the pen on a convenient line of the chart paper. Titrations were made with a solution of sodium hydroxide, approximately 0.25 M. This was standardized as follows. A solution of potassium hydrogen phthalate of accurately known concentration, about 0.1 M, was prepared in a volumetric flask. Twenty-five ml of this solution was pipetted into a 250-ml beaker, a stirrer bar was placed in the heaker, and between 25 and 50 ml of distilled water were added, so that the solution covered the electrodes of the p H meter while it was stirred. The delivery tip F was put into place and the p H meter, stirrer, and recorder were all turned on. The recorder speed was set at 1 in. per 100 sec. The operator grasped the pinch clamp D and watched the recorder pen. At the moment the recorder pen crossed one of the heavy lines on the chart paper he opened the pinch clamp, and the sodium hydroxide solution started to flow into the beaker. Usually there was a delay of a few seconds before the pen started to move, due to the delay in mixing the solutions. To compensate for this error, a second 25.00 ml portion of standard phthalate was drawn up into the pipet, and as soon as the recorder pen had traced the inflection in the p H curve this new portion of phthalate was run into the beaker. This process was repeated a second and a Volume 50, Number 11, November 1973
/
795
0
5 ID 15 20 SCALE DIVISIONS I10 D I V
.
25
30
35
200 SECONDS)
Figure 2. Titration of successive portions of potassium hydrogen phthalate.
third time, or until the beaker became too full to hold more solution; then the pinch clamp was closed and the stirrer and recorder stopped. The recorder trace, a graph of p H against time, looked like Figure 2. The intervals b, c, d were equal and very slightly less than a, indicating the initial delay in mixing. (Of course, they would he slightly greater than a if the inlet tube F were placed close to the electrodes and upstream from them.) They serve to standardize the sodium hydroxide, not in units of molarity as in conventional titration, but in milliequivalents of base per division of chart paper. This is a perfectly valid standardization so long as the flow rate of the solution and the speed of the chart paper are kept constant. Another feature of Figure 2 which is of interest is that the segments start a t successively higher p H values, due to the accumulation of doubly charged phthalate ions in the solution and their buffer action. The first seement.. a.. represents the titration curve for the biphthalate ion. After the beaker is emntied. ~ - standardization ~ ~ . . rinsed. and water is added in an amount sufficient to cover the electrodes. The titrator may now be used to analyze different brands of vinegar. A 2.00-ml sample of white vinegar. was pipetted into the beaker, the p H meter, stirrer, and recorder were turned on, and the pinch clamp D opened, as before, when the recorder pen crossed a heavy line on the chart paper. Repeated 2.00 ml portions of the same vinegar were pipetted into the beaker after each inflection. After three or four portions of white vinegar had been added and titrated, portions, again 2.00 ml, of "wine vinegar" were added, noting the change of sample on the chart paper but without interrupting the flow of sodium hydroxide or that of the chart paper. Again some four portions were titrated. It was interesting to note that the brown color of the wine vinegar becomes much more intense as the p H is raised, and it would have been difficult to see the end point with phenolphthalein indicator. With the vinegar samples the chart record looked like Figure 2, except that the starting p H was lower. The huffering action of the accumulated sodium acetate was clear, and the perceptive student could see that each successive inflection was less steep than the one before it; however, all the inflections could be located on the volume-time scale to a precision equal to the width of the pen line. A typical set of data was the following, all expressed in chart paper divisions (5 divisions = 1in.) ~
~
~
I
8
I
0 I5 20 LS JO 35 5 SCALE O I Y I S I O N S ( ( 0 Dl".. 200 SECOND51
10
Figure 3. Titration of a mixture of hydrochloric and phosphoric acids,
I 1
-
PURE
lo -
9-
pH
8
-
7-
VOLUME ADDED Figure 4. Effect of carbonate on titration curve.
~
Whitevinegar, 2.00ml: 10.8, 11.0, 10.8 Wine vinegar, 2.00ml: 10.0, 10.0.9.6, 10.2 Standardization with potassium hydrogen phthalate; 25.00 ml 0.0955Nphthalate = 15.8divisions The concentrations of acetic acid in the two vinegars were 0.815 and 0.755 M, respectively. Better precision might be obtained by using a 5-ml
796
1 0,
/ Journal of
Chemical Education
pipet, but it is worth noting,that, with good handling, a 2ml pipet will deliver volumes reproducible to 0.005 ml. The titrator was then used to analyze a solution containing hydrochloric and phosphoric acids. This was a solution that the students had already analyzed by pointby-point plotting. A graph obtained by repeated additions is shown in Figure 3. It provides an interesting exercise in stoichiometry. After the second addition the inflection corresponding to H2POn- was lost, and only the total acidity, (moles HC1) + 2 (moles HsP04). could be read from the graph. We also used the apparatus to titrate pure acids of various kinds. An interesting pair was salicylic acid and acetylsalicylic acid; the ionization constants differ by a factor of four, salicylic acid being the stronger, as expected from the hvdroeen-bonded structure of its anion. The . comparison of acetic, tartaric, and citric acids, each a t the same normalitv. is interestine. No inflections are seen for the intermediate stages of neutralization of tartaric and citric acids, but the slopes of the curves testify that these are dihasic and tribasic acids. Finally, the rharr record reveals the presence of rarbonate in the sodlum hvdroxldc. I i sodium hvdroxide sulution remains in the funnel overnight it picks up carbon dioxide, and the titration curve of acetic acid or potassium hydrogen phthalate looks like Figure 4. For quantitative titrations one must discard the solution in the funnel before starting a new day's work. I acknowledee with eratitude the hos~italitv of the Pedagogical lnstitute of"~aracasand of ~rofessorDimas Hernandez. head of the D e ~ a r t m e n tof Chemistw. Mv semester's stay was sponsored by the Creole ~oundation,to whom I also express my gratitude.
