ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
Figure 1. Photograph of the phase transfer device
of separatory funnels. For such tube extractions, the phase transfer device provides a simple solution to a routine transfer problem, especially if the organic phase solvent is less dense than t h a t of the aqueous phase. If the extraction solvent is more dense, there can be some contamination with the aqueous material as the delivery tube is lowered through the aqueous layer. Under such conditions, it may be desirable t o discard the first fraction of t h e solvent since it would contain some of the aqueous material. Alternatively, one may eliminate contamination of the delivery tube by momentarily introducing air flow through the tube or a slight positive pressure. T h e majority of solvents used for extraction are less dense than water and thus would pose no problem. The exceptions are the halogenated solvents such as carbon tetrachloride.
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chloroform, and ethylene dichloride. Carbon tetrachloride is not recommended for laboratory use because of its potential health hazard; however, chloroform is widely used. Based on the procedures published in Sunshine’s “Methodology for Analytical Toxicology” ( I ) and Curry’s “Poison Detection in Human Organs” ( 2 ) ,chloroform, either alone or in combination with other solvents, is used in 34% of the toxicological procedures; ethyl ether, 20.1%; hexane, 9.6% ; benzene, 7 . 2 7 ~; toluene, 6.4%; ethylene dichloride and ethyl acetate, 3.2%. If these data, based on 124 methods, (werepresentative of the analytical methods as a whole, the majority (260%) of solvents used for drug analysis are less dense than water and, thus, could be transferred with the device without contamination from the aqueous layer. In instances; where it is desirable to retain chloroform as a major component of t h e extraction solvent, the transfer problem associated with the use of chloroform can be overcome simply by adding sufficient amounts of hexane (or ethyl ether) to the chloroform to effect phase inversion (3). An informative and comprehensiv’ereview covering many aspects of sample extraction has been written recently by Reid ( 4 ) . In the review, it is noted that it is still common practice to remove a portion of one phase manually with the aid of a bulb-controlled pipet. I t is also pointed out t h a t phaseseparating filter-paper can be helpful, although it may introduce contaminants such as tin. The phase transfer device described here should represent an improvement over many pipetting methods of phase transfer and should be useful not only for drug analysis but for other analytical procedures employing small scale partition systems.
LITERATURE CITED (1) I. Sunshine, “Methoddogyfor AnaMcal Toxicology”, CRC Press, Cleveland, Ohio, 1975. (2) A. Curry, “Poison Detection in Human Organs”. Charles C Thomas Publisher, Springfield, Ill. 1976. (3) H. A. Schwettner, J. E. Wallace, and K. Blum, Ch.Cbem. ( Winston-Salem, N.C.), 24, 360 (1978). (4) E. Reid, Ana/yst(Londofl),101, 1 (197611.
RECEIVEDfor review May 30, 1978. Accepted August 7, 1978.
Automatic Gravimetric Titration System Roger Guevremont’ and Byron Kratochvil’ Department of Chemistry, University of Alberta, Edmonton, Alberta T6G ZG2, Canada
Gravimetric titrations, in which solutions are prepared on a weight basis and the weight rather than the volume of titrant delivered is measured, have not attained widespread popularity owing to the slowness of the many weighing operations that have been required. Titrations in which the amount of reagent added is measured by weight rather than volume have a number of advantages, however ( I ) . T h e major disadvantage, slowness of weighing, has been alleviated with the development of top-loading balances. New electromechanical systems no longer use t h e beam and knife-edge principle; these designs, coupled with digital electronics, allow convenient interfacing of the weighing operation with microprocessor or minicomputer systems. T h e use of automated weighing to measure samples for analysis has been described by Renoe, O’Keefe, and Malmstadt ( 2 ) ,but this method has not been used t o measure Present address, Atlantic Regional Laboratory, National Research Council, H a l i f a x , Nova Scotia, Canada. 0003-2700/78/0350-1945S01 O O / O
titrants. Most commercial automatic titrators use a titrant delivery system based on a motor-driven syringe. The volume of solution delivered is obtained in terms of the movement of the syringe plunger; a precision on the order of a few tenths of a percent is possible with the best of these systems. We describe here an automatic titrator in which the weight of titrant delivered is measured and recorded automatically. Titrant delivery may be controlled manually or by minicomputer. allowing conventional titrations to a visual end point, or completely automated potentiometric titrations under minicomputer control. Precision on the order of a few hundredths of a percent is attainable, the principal uncertainty arising in the potential sensing system used to locate the end point. Two arrangements for the automatic delivery operation in a gravimetric titration were studied. In the first, the titrant is contained in a bottle, and the delivery tube is closed with a pinch clamp. Titrant is delivered by means of a mechanical arm which contacts the clamp during the delivery period only, C 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
Reser v o i r
Normally
Closed Pinch Valve
Mechanical
Top Loading B a l a n c e
Switch Figure 1. Schematic diagram of the automatic gravimetric delivery system based on a mechanical couple
Spring Loaded Syringe
1 Battery
1
V
i
I
Light Source
Top L o a d i n g B a l a n c e
Flgure 2. Schematic diagram of the automatic gravimetric delivery system based on an optical couple and not during the weighing operations. In the second, the titrant is contained in a spring-loaded syringe, and delivery is controlled by a battery-powered solenoid valve through an optically-coupled trigger. The precision and reliability of the systems were evaluated by a series of potentiometric acid-base titrations.
EXPERIMENTAL Instrumentation. The Gravimetric Delivery System. The mechanical couple design of the weight titrator is shown in Figure 1. It is composed of a titrant reservoir and delivery tubes, a normally-closed pinch valve, and a mechanical unit to open and close the valve. The reservoir is a polyethylene bottle connected to the delivery tip by Teflon tubing except in the valve, where a short section of pliable thin-walled silicone tubing was found
more satisfactory. A lever arm driven by a small electric motor opens the valve to deliver titrant. A micro switch is positioned to stop the motor when the valve reaches the open position. The motor is run in reverse to close the valve and disengage the lever arm from the delivery unit. The gravimetric delivery system is enclosed in a Plexiglas housing to prevent air currents from affecting balance readings. The optical couple delivery system is shown in Figures 2 and 3. The titrant is delivered from a springloaded 10-mL Hamilton Gas-Tight syringe with a Luer-type fitting through Teflon or polyethylene tubing to a solenoid valve. From the valve, the titrant passes to a delivery tip, the end of which is drawn to a small diameter so that the titrant is delivered under pressure as a fine jet. This eliminates the problem of evaporation from a hanging drop, which may be serious with volatile solvents and small titration volumes. The delivery tip is positioned above the solution
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
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FLOW CHART OF P R O G F A Y TTRl
and t h e e l e c t r o n i c b a l a n c e . C a l c u l a t e t h e SD a n d s l o p e on t h e e l e c t r o d e
Is any
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Add r e q c i r e d q u a n t i t i e s from a u x i l i a r y d e l i v e r y s y s t e m s
I
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I
Figure 3. Photograph of titration unit on electronic top-loading balance. polyethylene inside glass tubing, (B) solenoid valve for control of titrant delivery, (C) circuit board with phototransistor (top center of board) for switching valve on and off from external light source, (D) battery for valve operation (A) Delivery tube of
level in the titration vessel; immersion in the solution causes errors in weight on the order of 10 mg or more owing to surface tension effects. A General Valve Corporation 2-way 12-V dc, Iso-Latch miniature solenoid valve controlled the solution flow. The valve latches open with a single 12-V, 700-mA, 150-ms pulse, and close with an equivalent pulse of reversed polarity. The power is supplied from ten Eveready B225T, 1.25-V nickel-cadmium batteries. Signals for opening and closing the valve are sent through an optical couple consisting of a tungsten lamp source and a phototransistor detector. The lamp can be controlled manually or with T T L compatible signals from a computer. A Sartorius Model 3015 electronic top-loading balance (rated capacity 160 g, sensitivity 0.001g) was used. Because the titrator assembly weighed on the order of 330 g, the pan and lead tare weights were removed from the balance and replaced with a light weight aluminum support frame. The digital output of the balance was interfaced to a PDP 11/10 (Digital Equipment Corporation) minicomputer through a multiplexer. Application of t h e Grauimetric Delivery S y s t e m to Potentiometric Titrations. Potential measurements were made with a Fisher Accumet Model 520 pH Meter and a glass-silver-silver chloride electrode pair. The meter and electrodes were calibrated against buffer solutions prepared according to NBS specifications. A PDP 11/10 minicomputer was used to control the gravimetric buret and to collect the weight and indicating electrode potential data. Titrations were carried out under control of a program designed to: (a) sample the binary coded decimal (BCD) outputs of the electronic balance and the digital pH meter through a multiplexer, (b) calculate the approximate quantity of titrant to be delivered for the next data point, (c) control the delivery of titrant through the gravimetric buret, (d) calculate and store the weight of delivered solution along with the potentiometric data, and (e) calculate the titration results. The logic step (b) is based on the effect of the previous delivery on the indicating electrode potential. If the potential change is less than a predetermined minimum value, such as 5 mV, the delivery time, and thus the size of the next increment, is doubled. Within other selected ranges of potential change, the quantity of titrant delivered remains the same as for the previous increment, or is reduced or increased by factors of two to five. The limits of these ranges are selected on the basis of the sharpness of the expected end point and the number of data points desired. A flow chart of the program is shown in Figure 4. Materials. All solutions were prepared with boiled distilled water which was cooled in a glove box under nitrogen. Carbonate-free sodium hydroxide was prepared by dilution of saturated NaOH solution (AnalaR analytical reagent NaOH, BDH Chemicals, Toronto, Ontario). Potassium hydrogen phthalate
I
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w e i g h t of t i t r a n t d e l i v e r e d a n d t h e v o l u m e s of s o l u t i o n s a d d e d
1
YO
Add a new quant'i-,
Figure 4. Flow chart of program
I-.itrtrantI
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(Primary standard, J. T. Baker Chemical Co., Phillipsburg, N.J.) was dried 1 h at 120 "C before sample preparation. Nitric acid stock solutions were prepared by dilution of concentrated acid (J.T. Baker Chemical Co.). Individual samples for titration were weighed by difference in a syringe.
RESULTS AND DISCUSSION Using the mechanical couple gravimetric buret, the relative standard deviation for a set of six titrations of approximately 0.5 g KHP each with NaOH was 0.06%, or about 4 mg in the 6 to 7 g of titrant that were required. T h e relative standard deviation decreased to 0.04% or about 1 mg in 3 g of titrant for a set of 8 titrations of 5 to 8 g of nitric acid. Similar titrations performed with the optical couple gravimetric delivery system gave standard deviations (in terms of weight of titrant) of 0.06% for 9 titrations of KHP with 0.4 M NaOH, and 0.018% for 9 titrations of nitric acid with about 3 M NaOH. T h e results indicate that the two designs of titrators are equivalent in accuracy and precision. T h e limit of sensitivity of the balance used in this work, 1 mg, establishes the limit of precision for titrations in which end point detection is not a significant source of error, as in strong acid-strong base titrations. A delay was introduced between delivery of titrant and measurement of potentials to allow time for mixing and electrode equilibration. Electrode potential data were then collected for 6 s, and the average, standard deviation, and rate of electrode drift calculated. If the standard deviation or drift exceeded preset levels, the 6-s measurement period was repeated. T h e length of the delay period, and the criteria for electrode equilibria, can be varied as necessary. T h e me-
