William M. Allen and H. Raymond Shelden Lorna Linda Universitv La S~erraCampus R vers~de,Ca dornta 92505
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The C O N ~ U ~ Oand N Use of Constant-Rate Burets and Addition Fumnels
Among the more significant recent developments in the teaching of chemistry is the increasing use of instrumcnts for chemical separation and analysis. One of the common experiments that lends itself to instrumentation is the simple titration. The progress of a titration may be followed colorimetrically, elect,rometrically, and by a variety of other means. In many cases, such as an acid-base titration using a pH meter, it is easy to attach a recorder and automatically plot the pH as a function cf time. To complete the system, a method for adding the titrant a t a constant rate is necessary. Partly due to the high cost of a constant-rate motor-driven buret, many titrations that could conveniently use recorded output are not pcrformed. This paper describes how burets and addition funnels can be easily modified to operate in either a normal or a constant-rate mode, thus bringing recorded titrations within reach of all laboratories. At present there are three approaches to adding the titrant a t a constant rate: use of (1) a buret with an adjustable equalizing bulb, (2) a bnret that maintains a constant hydrostatic head, or (3) a buret in which the titrant is forced out a t a constant rate by a motordriven plunger. The flow from the bnret with an adjustable equalizing bulb is onlv " auuroximate a t best and would be the wealest link in the system. The cost, however, is low; therefore the system is available to anyone with a pH meter and a recorder. Burets that maintain a constant hydrostatic head, such as the IContes constant-addition funnel (R-634620), are available for under $60. They offer avariable rate by adjustment of a valve assembly, but they have the disadvantage of giving only the approximate volume added. Burets that. force out the titrant by means of a motor-driven plunger, such as the Sargent automatic constant-rate buret, are available for $145 and up. The great advantage of this buret is that volume can be determined instantaneously. The rate of addition is invariable (5 ml/rnin), but this is no problem because the concentration of the titrant can be changed to give any desired amount of titrant per unit time. Hansen, et al.,' have recently described an inexpensive motor-driven small-volume buret. The burets described in this paper are based on the principle of the constant hydrostatic head. A.
the level of the titrant decreases, the hydrostatic head above the stopcock decreases and the flow through the stopcock slows. If a stopper is placed in the top of the buret, only a small amount of t,itrant will come through the stopcock before it stops because air must enter the system to replace the outflowing titrant. It is, however, not necessary for the air to enter through the top of the buret above the titrant. If a small hole is made in the buret anywhere above the stopcock and below the top of the liquid, titrant will not $ow th~oughsuch a hole but air will entev! The hydrostatic head will be determined by the distance between this air inlet and the stopcock; thus as long as the titrant level is above the air inlet, the flow through the stopcock will be constant. It is obvious that the air inlet should be relatively close to the stopcocl~,as only the volume above the air inlet will be dispensed a t a constant rate and it is this volume that defines the useful volume of the buret. The buret is shown in Figure 1. Most burets can be readily modified for constantrate use by providing an air inlet between the stopcock and the lowest graduation. Either a short glass "T" is fused into the wall of the buret, or the side arm of a self-filling buret is shortened and capped with a serum cap. In operation the buret is filled with titrant, stoppered, and the stopcock opened until no more liquid drops. The initial volume is read and a syringe needle is thrust through the serum cap starting the flow. After the end point has passed, the needle ii withdrawn and the final volume read. Using the total volume
Constant-Rate Burets
In the normal mode of operation, a buret is filled with the titrant and the flow is controlled by a stopcock. As Based on a paper presented at the 5th Western Regional Meeting of the American Chemical Society, Anaheim, Ortoher, 1969. ' HANGEN, L. D., LITCHMAN, W. M., LEWIS,E. A., AND ALLRED,R. E., J. CHEM.EDUC., 46, 876 (1969).
Figure 1 .
Conrtmt-rate buret.
Constant addition funnel.
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delivered in a known time, the volume a t intermediate times may be determined by direct proportion. Many variations of this buret are possible. The rate of flow may be controlled by several means: (1) the stopcock may be left wide open or closed to some fractional opening or a metering stopcock may be used, (2) with the stopcock open, different sizes of syringe needles will allow air to enter the buret at different rates, (3) different lengths of capillary tubing attached to the tip of the buret will allow different rates to be delivered. These modified burets operate in the standard mode if the side tube is plugged and the top of the buret is left open. If the buret is to be used in the constant-rate mode a t all times, it is advantageous for the side tube to have an inside diameter that will permit the syringe needle to he permanently cemented in place. The beginning and the end of the titration are controlled by a stopper in the large end of the needle. Calibration
The rate of flow can be calculated by counting the time required for a set number of drops and plotting this as a function of the volume level in the buret. Such a plot is Figure 2. The plot contrasts the same buret in the standard mode with the constant-rate mode. The constant-rate buret can also be tested in an actual titration. A sample of hydrochloric acid was titrated with sodium hydroxide with the bnret in both the standard and constant-sate mode. Three 10.00-ml. samples of acid were titrated to a phenolphthalein end point, and each required 23.SO ml. The pH at the end point was found to bc S.7. Five 10.00-ml samples of acid were titrated with the bnret in the constant-rate mode. The titration was allowed to run past the end point, then from the time and total volume, the volume necessary to bring the pH to 8.7 was calculated. The average volume required for the five titrat'Ions was 23.70 ml, with an average deviation of 0.11 ml (0.5%). Unless t,he needle is cemented in place, the angle of the needle through the serum cap and the rate of flow will vary slightly from one titration to the next. The rate
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BURET VOLUME Figure 2. Comparison of the buret in the rtondord i d i d circlerl and the conrtant-rote modes (open circlerl. The buret volume is the average volume at which the drops are counted.
of flow, however, is constant for each titration; therefore the volume a t any time is equal to the total volume multiplied by the time fract,ion. Constant-Rate Addition Funnel
The same principles described above may be adapted to the construction of constaut-rate addition funnels. In many applications additions of liquid reagents to a reaction are carried out in a closed system such as an inert atmosphere. The pressure-equalizing addition funnel allows such an addition. The addition funnel sketched in Figure 1allows for the constant addition of a reagent in either a closed or open system. If it is not necessary for the addition to be constant, the addition funnel operates in the standard mode provided the stopper is removed during the addition.
