Eugene D. Olren University of South Florida
Tampa
Simple Equipment for Automatic Potentiometrk pH Titrations
There are many advantages to using automated potentiometric pH titrations for student experiments. Manually performed potentiometric pH titrations are tedious and time consuming. The increasing demands on under-graduate laboratory time ofteu result in omitting such experiments, eliminating the taking of points for all hut the end point regions ( I ) , or a t the very least severely restricting the number of such titrations that are performed. Automation greatly d e creases the time that is necessary for a titration and permits complete titration curves for various types of acids and bases to he accurately determined. Recently a versatile and inexpensive pH recording electrometer became available (8). Although the recording feature of this electrometer somewhat shortens the time and effort necessary to perform a potentiometric titration with a manual buret, automatic delivery is needed to exploit its value fully. Unfortunately, commercially available constant rate burets usually cost as much as or more than the pH recording electrometer, and generally lack versatility in the flow rate and/or volume of titrant that can be delivered. Continuous action, positive displacement pumps are also generally limited to a single flow rate, and reproducibility is limited to about *1% (5). Simple Mariotte bottles with capillary delivery tubes are ofteu troublesome, and the capillary requires jacketing if a constant temperature is to be maintained for accurate work (4). This paper describes a simple, inexpensive, yet versatile apparatus employing a siphon pipet to deliver titrant solution automatically and accurately during the course of a titration. The advantages of adding titrant with a siphon pipet, in contrast to continuous addition, are enumerated, and the versatility and accuracy of the apparatus are illustrated with titrations of various acids, bases, and mixtures. Description of Apparatus
Figure 1 illustrates the apparatus used for automatic delivery of titrant solutions. An ordinary Hempel Presented to the Division of Chemical Education at the SouthemtAouthwest Regional Meeting of the ACS, Memphis, Tennessee, December, 1965.
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distilling column, modified only by bending the sidearm inlet upwards in a flame to allow the attachment of a carbon dioxide absorption trap, was used as a buret. It fed a siphon pipet through a capillary flow restrictor. A reservoir bottle kept the column filled
DRYING TUBE
[-
HLMPEL DlSTlLLiNG COLUMN
TO
c4i RECORDING ELECTRWElER
COMBINATION GLASS -CALOMEL ELECTRODES
CAPILURY FLOW RESTRICTOR IPHON PIPET
STIRRING BAR
I 0 1 MAGNETIC STIRRING MOTOR
F i g u r e 1.
Apporotvs for automatic delivery of titront rolutlan.
to a relatively constant height, and was sealed to the Hempel column to ensure that the only air inlet to the buret mas through the sidearm inlet. The 9 mm od glass feed tube from the reservoir had a '/4-'/2 in. slit in its lower end. Therefore only small air bells were admitted to the reservoir bottle as the solution in the Hempel column drained, and fluctuations in hydrostatic head were kept small (less than in.).
The pinchcock valve on the reservoir feed tube simplified the positioning of the reservoir bottle after refilling, and the pinchcock valve at the exit of the Hempel column served as the on-off control. The siphon pipets in this study were similar to the one described by Rieman and Lindenhomm (5), except that they were made smaller. The most satisfactory size was found to be about 0.5 ml.' Smaller pipets often gave too small a discharge to cause a noticeable pH pulse, thus preventing an accurate count of the number of pipet discharges. With pipets delivering more than 0.5 ml, there was less accuracy in establishinpthe equivalence point. In order to keep the volume of solution delivered by a pipet constant from increment to increment, it was necessary to keep the feed solution flow rate relatively constant. See Table 1. Data for Table 1 was obtained by running water through the apparatus and collecting and weighing double increments. Triplicate Table 1.
Volume of Solution Delivered b y a Siphon Pipet a t Various Flow Rates of Feed Solution Flow rate
(ml/mini
Volume delivered per discharge ("11)
determinations at each flow rate gave relative standard deviations of 0.2% or less. As can be seen from Table 1, the volume of solution delivered increases almost linearly with flow rate. Thus, if an ordinary buret were used to feed the siphon pipet, it would not be unusual to have the flow rate decrease as much as 30% or more as the hydrostatic head decreases. This could result in a decrease of several per cent in the volume delivered by a siphon pipet. Since an end point calculation consisted of multiplying the number of increments to the end point by the volume of titrant delivered each increment, this large fluctuation in flow rate and siphon volume could not be tolerated for high accuracy. The capillary flow restrictor proved to be the best device for regulating flow rates to a desired value. Ordinary stopcocks, including both glass and Teflon, as well as Teflon metering stopcocks, all were found to give flow rates that decreased with time, even with a constant hydrostatic head and a constant stopcock setting. This phenomenon, known to engineers (6), was demonstrated by attaching a flowmeter to the outlet of an ordinary buret, partially opening the stopcock, and monitoring the flow after repeatedly filling the buret to the same level. Even though the stopcock setting was not changed, the flow rate decreased on successive fillings. In the case of Teflon stopcocks, the decreased flow rate might have been due to a deformation of Teflon into the bore opening, and in the case of glass stopcocks, it might have been due to an extrusion of stopcock grease into the bore opening. Capillary flow rest,rictors were made. Ordinary -
-
'Siphon pipets, 0.5 ml, can now be purchased from R. H. Quincel, glassblower, Rt. 3, Box 147A, Tampa, Fkxida, at a cost of $5 each.
capillary tubing was available only down to approximately mm id, with polarographic capillaries being much too fine for significant solution flow rates. It was found that about 10 in. of the mm id capillary was necessary to decrease the flow rate from an approximately 18 in. hydrostatic head down to a flow of 1 ml/min. A simple way of restricting the flow rate with only a 4-5-in. length of capillary was to heat the '/4 mm id capillary in a flame, and pull it out 1/4-1/2 in. A dozen of these constricted capillaries gave flow rates in the range 0.8-2.0 ml/min, and it was useful to have this supply available whenever it was desirable to modify the flow rate. A flow of about 2.0 ml/min was used in most of this work, though flow rates of about 1.0 ml/min were used when rates of equilibration proved slow. If students are to prepare their own capillary constrictors, it may be expedient to pull out a mm id capillary about 1-2 in. in a flame, and after breaking the capillary in the center, using a file to break off segments of the pointed capillary until the desired flow rate is achieved. Fire polishing the end of the capillary, in an attempt to partially close or restrict the capillary, lacks control and is not recommended. Rubber tubing was used wherever a flexible connection had to be made. Tygon tubing was less reliable. When i t was used as a pinchcock valve in the presence of a strong base, the flow rate slowed after a few weelcs. This probably was caused by a partial clogging of the capillary constriction perhaps resulting from mechanical abrasion and loss of particles from the Tygon after extended use with a pinchcoclc clamp. No such difficulty was encountered with rubber tubing. Batch tests showed that neither Tygon nor rubber altered the titer of 0.1M NaOH on standing 24-30 hours in the presence of these two types of tubing. The titratiou vessel was a 150 ml beaker, set inside a 2.50 rnl beaker with a rolled up paper towel keeping them separated. This arrangement insulated the titration vessel from the heat of the magnetic stirring motor, permitting the temperature in the titration vessel to remain constant within 1 . 5 T of room temperature. A three beaker system, with water surrounding the titratiou vessel and air isolating the water from the magnetic stirring motor, served to keep the temperature constant to within 0.5'C. This system could be used if more accurate pH readings were desired (as in acid dissociation constant calculations), but 1 . 5 T variation generally could be tolerated without significantly affecting the pH readings obtained. Thomas combination glass and calomel electrodes were used in this study, giving much faster response than some other electrodes, in agreement with the findings of Malmstadt and Piepmeier (7). The electrodes were positioned about 5/1e of an in. below the surface of the solution to be titrated, and about of an inch downstream from the area of discharge of the siphon pipet. The capillary flow restrictor was touching the inside of the siphon pipet for smooth filling of the pipet. The siphon discharge had to be free of the titration vessel and solution, however, or the pH would not stabilize after each discharge because of the continuous drainage of the last drop in the siphon pipet. A Heath Model EUW301 pH recording electrometer with a chart speed of 1 in. per min was used. Voluem 43, Number 6, June 1966
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31 1
Results
Figure 2, showing the automatically recorded titration curve for the titration of 0.02 M H3P04with 0.1 M NaOH, illustrates the type of titration curve obtained using the above-described titration apparatus. Each time the siphon pipet discharges, the non-equilibrium pH rise causes an easily discernible pulse, which makes it easy to count the number of pipet increments to an end point. The height of each non-equilibrium pH
following the equivalence point increment is twice the pH change of the increment preceding the equivalence point increment, the equivalence point will fall twothirds of the way through the equivalence point volume. Thus, in general, the fraction of the equivalence point increment volume at which the equivalence point falls can he calculated by dividing the change in equilibriunl pH of the increment following the equivalence point increment by the sum of the pH changes in the iucrements immediately preceding and immediately following the equivalence point increment. Therefore, in the titration of Figure 2, the first end point occurs at 18.39 increments, and the second a t 36.73 increments. The volume delivered by a given siphon pipet varies with flow rate, as discussed earlier, and varies with the viscosity of the titrant solution. Calibration of the pipet volume is best made under the exact conditions used in a titration, and is best carried out by collecting and weighing several double or triple increments of titrant in pre-weighed weighing bottles, before or after a given titration, or both. To convert the weight per increment to volume per increment, two to three portions of a known volume of the titrant are weighed. For the sodium hydroxide titrant under the conditions used in Figure 2, the siphon volume mas found to be 0.612 ml, with a flow rate of 2.18 ml/min (constant to 1to 2%). Thesiphonvolume was found to be constant to better than *0.2% under these conditions.
Automatically recorded titration of 0.02 M HsPOl with NaOH.
Figure 2.
0.1M
pulse depends on many things, including (a) the concentration of the titrant, (b) the buffering capacity of the solution being titrated, (c) the difference in pH between the titrant and titrated solution, (d) the rate of reaction, (e) the rate of stirring, and (f) the rate of electrode reaction. More important, however, is the distance of t.he glass electrode bulb from the point of discharge of the pipet volume, and this distance serves as the most important variable for adjustment of the size of pulses. I n general, it wns found useful to slant the combination electrodes so that the glass electrode bulb is almost directly under the discharge area. But the electrodes should be far enough downstream that the discharge does not strike the electrode above the surface of the titrant solution. Otherwise drifting pH readings will result because of the titrant solution slowly running down the electrode. Determination of the number of siphon volume increments and partial increments to an equivalence point is facilitated by the fact that the volume of each increment is constant and the titration curve is approximately symmetrical in the equivalence point region. Thus, the volume increment giving the largest equilibrium pH change will be the volun~eincrement containing the equivalence point, and the fraction of that increment used to just reach the equivalence point can quickly be calculated from the equilibrium pH changes of the increment immediately preceding and the increment immediately following the equivalence point increment. For example, if the increment immediately preceding and immediately following the equivalence point increment gave identical equilibrium pH changes, the equivalence point would he half the volume of the equivalence point increment. If, as another example, the pH change in the increment 312 / Journal of Chemicol Education
Table 2.
Acid titrated HlPOl HCI HC2H302
Titration of Phosphoric, Hydrochloric, or Acetic Acids with Sodium Hydroxide
Number of trials
mXales acid taken
Average mMoles acid found
Error
(%)
Relative standard deviation (%)
10 3 3
0.783 0.958 1.011
0.785 0.957 1.016
+0.26 -0.10 +0.41)
0.30 0.16 0.16
Table 2 summarizes the results of ten phosphoric acid titrations, along with triplicate titrations of hydrochloric acid and acetic acid. The reproducibility of all titrations was excellent, with relative standard deviations in determining an end point 0.3% or better in all cases, and the accuracy was usually better than 0.5%. I n the case of the ten phosphoric acid determinations, the volume of titrant from the first to the second end points was, on the average, 0.1% more than the volume of titrant to the first end point, in general agreement with the small amount of carbonate in the reagent-grade sodium hydroxide titrant. If the constant flow rate apparatus was left open to the air, and the ascarite drying tube omitted, the volume of titrant to the second end point was consistently 2-6% more than the volume of titrant to the first end point, with the discrepancy becoming greater as the volume of sodium hydroxide in the reservior bottle became smaller. This finding is reasonable in view of normal amounts of carbon dioxide in the air which was being bubbled through the sodium hydroxide titrant during normal operation of the constant flow hydrostatic head device.
Table 3.
Base titrated Na&OsC Na.COP Na.COP NaOH
Titration of Sodium Carbonate or Sodium Hydroxide with Hydrochloric Acid
Num- mMoles her of base trials taken 4 1 1 3
0.936 0.908 1.043 0.709
Average mMoles base found
Error
0.933 0.909 1.048 0.711
-0.32 fO.11 f0.48 f0.28
(%)
Relative stsndsrd deviation (%) 0.32
..
0.'19
Aliquots of a stock solution of Na,C08 were titrated. An individual sample of solid NasCOa was weighed out and titrated.
Table 3 summarizes the titration of sodium carbonate or sodium hydroxide with standard 0.1 M hydrochloric acid. With hydrochloric acid titrant the ascarite tube was of course unnecessary. I n titrating sodium carbonate it was necessary to slow the flow rate to 1.1 ml/rnin instead of the 2.2 ml/min used in all other titrations, in order to achieve pH equilibrium in the region of the first end point (pH 7-9). When the first end point was calculated from the non-equilibrium pH's, results were consistently high by about 2-5%. Equilibrium pH's were achieved in the region of the second end point (pH 3-5), even at a flow rate of 2.2 ml/ min, and calculations of the sodium carbonate concentration based on this end point were always accurate to within 0.5%. When a flow rate of 1.1 rnl/min was used, both end points gave quantitative errors of less than 0.5'%. The slow pH equilibration in the region of pH 9 is probably due to the sodium error of the glass electrode, as evidenced by the absence of this sluggishness at pH 3-5, and by the fact that a pH 0-14 electrode responded appreciably faster than the pH 0-11 electrode in the high pH regions of the titrations. The fact that these polyequivalent acids and bases can be determined within *O.5% using either the first or the second end point means that the titration apparatus should be useful for analyzing mixtures, as well as single components, and the results in Table 4 bear this out. A mixture of phosphoric acid and hydrochloric acid (mixture I), and two different mixtures of phosphoric acid and potassium dihydrogen phosphate (mixtures 2 and 3 ) , were each titrated in triplicate, and the results show that each component can be determined with less than 1% error. Errors are usually greater when analyzing mixtures than when analyzing conlponents by themselves, because of the necessity of calculating the second conlponent by difference. However, the accuracy would probably improve with Table 4.
Mixture
mMoles HIPOI
HaPo4 mMoles HSPOI found
larger samples. The reproducibility of all titrations was excellent, with relative standard deviations of 0.4% or better. Discussion and Conclusions
The equipment described is inexpensive, simple to assemble, and allows potentiometric pH titration curves to be obtained rapidly and accurately. Repeat titrations made to check on the reproducibility of a titration, normally an onerous undertaking when done manually, are especially convenient with this apparatus, since a second sample can he prepared while the first is being titrated. By using two titrationvessels, a second titration can be started less than a minute after the first is finished, making extremely efficient use of the students' time. The technique of adding titrant in increments with the siphon pipet has several advantages over the usual method of continuously adding t,itmnt. With continuous addition of titrant, it is necessary to ascertain beforehand that both the titration reaction and the indicator electrode response are practically instantaneous before automatic recording will give satisfactory results. Very often these necessary conditions are not fulfilled, especially near the equivalence point (8). By adding titrant in increments with a siphon pipet, the recorder chart gives immediate and continuous information on the rates of equilibration. When equilibration is not achieved, the flow rate can often be decreased enough to achieve it. Another advantage of this increment method of adding titrant is that the recorder chart gives a permanent record of the flow rate throughout the titration. Thus any variations in flow rate which may have occurred during the course of a titration and which might cause error in the analysis are clearly revealed. A further advantage of adding equal volume increments throughout the titration is that the calculation and plotting of derivative titration curves is greatly facilitated. Whereas the usual first derivative titration curve involves a calculation of ApH/A ml as a function of (average) ml titrant, it is only necessary to plot ApH/increnlent as a function of average increment number (i.., increment number minus 0.5) with the data obtained here, thereby saving appreciable time and effort. The excellent reproducibility and accuracy of this equipment also allows the student to st,udy carbon dioxide errors and to determine acid and base dissociation constants (9) with greater confidence and accuracy than manual titrations usually allow. The simplicity of the apparatus makes it an ideal introduction to automation for the student, an introduction particularly welcomed if a manual potentiometric pH titration is first performed.
Titration of Acid Mixtures with Sodium Hydroxide
Error
mMoles HCI
HCI rnMoles HC1 found
Error
mMoles KH,PO.
KHIPOl mMoles KH2P04 ioond
Error
Volume 43, Number 6, June 1966
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313
Acknowledgment
The advice and encouragement of Dr. L. E. Monley are gratefully acknowledged. The work was supported in part by the National Science Foundation. Literature Cited (1) BLAEDEL,W. J.,
AND MELOCHE,V. W., "Elementary Quantitative Analysis," 2nd ed., Harper & Raw, Publishers, New York, 1963, p. 373. (2) MALMSTADT, H. V., J. CHEM.EDUC.,41,148 (1964). (3) EWINO,G. W., J. CHEM.EDUC.,42, 32 (1965). (4) PHILLIPS,J. P., "Automatic Titrators," Academic Press,
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Inc., New York, 1959, p. 14. (5) RIEMAN,W., 111, AND LINDENBOMM, S., Anal. Chem., 24, 1199 (1952). (6) SCOTT,L. A,, Dept. of Mechanical Engineering, University of South Florida, Tamps, Florida, private communication, 1965.
(7) MALMSTMT,H. Y.,
AND
PIEPMEIER,E. H., Anal. Chem.,
37,34 (1965). (8) MEITES,L..AND THOMAS, H. C., ('Advanced Anal. Chem.,' McGrew-Hill Book Company, Inc., New York, 1958, p. 66.
D. T., "Experiments for AND SAWYER, Instrumental Methods," McGraw-Hill Book Company, Inc., New York, 1961, p. 17.
(9) R E I L ~ YC. , N.,
