580 Aaalyst, August, 1973, Vol. 98, pp. 580-584 A Multi-channel Dispenser - Titrator - pH-stat* BY DOUGLAS G. MITCHELL AND KENNETH M. ALDOUS (Division of Laboratories and Research, New York State Department of Health, Albany, New York 12201, U.S.A.) A multi-channel dispenser - titrator - pH-stat with switch-selection of reagents and reagent volume control and with a directly digital read-out is described. A valvc driven by a stepper motor is used to select reagents and as a coarse volume control. A drop generator dispensing - deflection mechan- ism is used as a fine volume control. The drop generator is also used to reject automatically to waste any reagent contaminated by previously dispensed reagent and, in the titration mode, to add various amounts of reagent to the reaction mixture.The instrument gives outstanding precision (relative standard deviation less than 0.2 per cent. over the range 0.5 to 7.0 ml) and linearity (correlation coefficient Y = 0.999 over the ranges 0.01 to 1.0 and 0.25 to 7.0 ml). THE dispensing and, to a lesser extent, titration of liquids are basic operations in all wet- chemical analytical procedures. The most common dispensing operations can be conveniently grouped into fixed-volume and variable-volume sample and reagent dispensing. Efficient instrumentation is available for both fixed-volume operations, and some dispenser - diluters can handle variable-volume sample dispensing, but no fully automated instrumentation is available for variable-volume reagent dispensing (Table I). Also, none of the available automatic dispensers are also capable, without modification, of being used in automatic titrations and pH-statting procedures.TABLE I INSTRUMENTATION FOR AUTOMATED DISPENSING Dispensing operation Available instrumentation Manual : Pipettes, burettes and syringes Automated : Syringe dispensers and diluters Fixed-volume sample dispensing Variable-volume sample dispensing Manual : Pipettes, burettes and syringes { Automated : Some dispenser - diluters Manual : Pre-set pipettes fitted on to reagent containers, burettes, pipettes and syringes Automated : Probably unnecessary Fixed-volume reagent dispensing Variable-volume reagent dispensing , " , a r b u , " a ; ~ r ~ ~ ~ ~ ~ d ~ ~ ~ ~ ~ e d instrumentation available In this paper, we describe a dispenser - titrator - pH-stat based on the use of a droplet generator, the liquid being dispensed into a stream of uniformly sized and uniformly spaced droplets, as a first step in the dispensing or titrating operation.Droplet generator devices have been previously used to dispense liquids. For example, Schneider and Hendricksl and Schneider, Lindblad and Hendricks2 have used a droplet generator to study the collision and coalescence of liquid aerosols; Sweet3 has described a high-frequency ink writer; Hieftje and Malmstadt4 used a droplet generator for sample introduction in flame spectrophotometry ; and Hieftj e and Manadarano5 developed a single-channel titrator. A single-channel micro- dispenser (Princeton Fluidics Corporation, Princeton, N. J., Model 390) has also been used.These instruments are not directly applicable to dispensing and titrating operations in a routine analytical laboratory because of the considerable practical difficulties in changing reagents and because of the lack of a coarse reagent on - off control to prevent wastage. The instrument described herein can dispense or titrate six reagents with dial selection of both reagent and reagent volume. It is also fitted with several devices needed for a routine analyti- cal instrument: a reagent thermostat in order to eliminate variations in volume dispensed because of viscosity changes, a coarse reagent on - off control and a flexible mode of approaching the end-point, this mode being selected by the operator. * Presented in part a t the Pittsburgh Conference, Cleveland, Ohio, March, 1972. @ SAC and the authors.MITCHELL AND ALDOUS 581 PRINCIPLE OF OPERATION- The basic dispensing system is shown schematically in Fig.1. Reagents are placed in containers, pressurised at about 11 p s i . , and connected to the entrance ports of a six-way valve driven by a stepper motor. The appropriate reagent can flow through this valve, through a thermostatically controlled coil, and then through a vibrating needle, from which it emerges as a stream of uniformly sized drops. These drops are either electrostatically charged (by applying +a00 V to the cylindrical charging electrode) or left uncharged (elec- trode at 0 V) and are then passed between two highly charged deflecting electrodes ( 5 3 kV). Charged drops are electrostatically deflected and allowed to drain to waste, while uncharged drops are allowed to proceed undeflected into the desired container.The flow-rate of the liquid is constant at constant pressure within the reagent vessel and temperature of the liquid; hence the volume dispensed can be controlled by selecting the time during which the liquid is allowed to proceed into the container. Mu Iti-port valve fi ~ To other containers reagent Needle +400V or OV To To digital pH meter Fig. 1. Basic dispensing system A dispensing operation is carried out by setting a switch to select the liquid to be dis- pensed, setting a digital switch to select the reagent volume and pressing a start button. The following events then occur in sequence: 1. The stepper motor moves the valve from “off” to the pre-selected position, thus allowing the reagent to flow.2. All liquid flowing during the first few seconds is dispensed to waste (as it is presumed t o be contaminated by previously dispensed reagent). 3. Reagent is allowed to proceed undeflected into the desired container for the time required to give the selected volume. 4. The liquid stream is deflected to waste for a few milliseconds so as to avoid switching irregularities. 5. The valve is returned to the “off” position. Thus the valve acts as a reagent selector and as a coarse on-off control, and the drop generator mechanism acts as a fine volume control and an automatic washing device to prevent cross-cont amination.582 MITCHELL AND ALDOUS : A MULTI-CHANNEL [Analyst, Vol. 98 The same basic mechanism is used for titration and pH-statting procedures, except that a digital pH meter with further control circuitry is used to control the dispensing operation.The pH meter output is fed to comparator circuitry, which senses the difference between the pre-selected pH and the pH of the reaction mixture. Titrant is dispensed at the maximum rate of approximately 8 ml min-1 until a pre-selected pH is reached. At this stage, dispensing is stopped for a selected period of between 0 and 20 s so as to allow sufficient time for the chemical reaction to proceed. (This step is necessary, for example, when titrating natural waters with a strongly acidic titrant.) Thereafter, dispensing proceeds at a selected rate of 100, 80, 60, 40, 20 or 10 per cent. of the maximum rate until the pre-selected pH is reached and the valve is turned off, Subsequent changes of pH, as in pH-statting operations, will cause the valve to open, thus allowing further reagent to be dispensed as required in order to maintain the selected pH value.The circuitry necessary to carry out these operations is shown schematically in Fig. 2. INSTRUMENT COMPONENTS- The high-voltage power supplies (400 V, less than 5 per cent. ripple; 3 kV, less than 10 per cent. ripple) and digital timing circuitry were built up from standard electronics components. Suitable components (valve, thermostatically controlled coil, reagent flasks and dispensing heads) for handling liquids were not available commercially and were constructed in our workshop. The thermostatically controlled coil comprises a 5-foot coil of 0-017 inch i.d.polypropylene tubing encased in a metal block and maintained a t 30 & 0.1 "C. A Bellingham, Model 123, air pressure regulator was used to regulate the air pressure in reagent flasks, and a Leedex 170-733-001 stepper motor was used to drive the reagent selector valve. Mode: Dispense, repeat, titrate Digital switch pH meter --------- --- Comparator Rate of approaching end-point Valve position motor switch I I - Stepper - electrode I II Mu Iti-port valve 1 driving oscillator plates Fig. 2. Schematic diagram of digital control and count- ing circuitry for multi-channel dispenser - titrator systemAugust, 19731 DISPENSER - TITRATOR - PH-STAT INSTRUMENT OPERATION- 583 The basic operating procedures are as follows.Dispense - Yepeat dis$ePzse-(i) Select reagent; (ii) dial volume; and (iii) press “Start.” Titvate - $M-stat-(i) Select titrant; (ii) fit pH meter and electrode; (iii) select pH (mV) at end-point; (iv) select pH (mV) at reaction pause; (v) select reaction pause time; ( v i ) select rate of approaching end-point; (vii) press “Start”; and (viii) read titrant volume on display. RESULTS DISPENSING MODE- Precision-The precision was evaluated by dispensing ten successive volumes of distilled water and weighing the dispensed liquid. Relative standard deviations of less than 0.2 per cent. were obtained over the range 0-5 to 7.0 ml, which is exceptionally good precision, particularly as there is no possibility of “drop at the dispenser tip” error as with conventional syringe dispensers.Most of the variation in dispensed volume is associated with the mechanical switching operation, because if the valve is locked in the “on” position, values for relative standard deviation of better than 0.05 per cent. can be obtained over this dispensing range. TABLE I1 PRECISION RESULTS FOR REPETITIVE DISPENSING The results are shown in Table 11. Volume dispensed/ml Relative standard deviation, per cent. 0.5 0.19 1.0 0-17 3.0 0.19 7.0 0.10 Linearity-Volumes dispensed over the ranges 0.01 to 1.0 and 0.25 to 7.0 ml were Correlation coefficients of 0.999 measured and plotted against instrument setting (Fig. 3). were obtained for both ranges. Volume selected/ml Fig. 3. Calibration graphs TITRATION MODE- Analytical precision in the titration mode was evaluated by connecting the instrument to a digital pH meter (Model NX, Sargent Welch Scientific Co., Skokie, Illinois 60076), by using a glass and platinum electrode as required (Model 117233; Leeds and Northrup, North Waks, Pa. 19545).Precision results were obtained for strong acid - strong base, strong acid - weak base and redox titrations. These results are shown in Table 111.584 MITCHELL AND ALDOUS TABLE I11 TITRATION RESULTS : MULTI-CHANNEL DISPENSER - TITRATOR - pH-STAT Sample End- volume/ point Sulphuric acid (0.02 N) Sodium hydroxide 10.0 pH 8.6 Sulphuric acid (0.2 N) Potable water 10.0 pH 4.5 Titrant Sample ml selected solution (approx. 0.02 N) Ammonium cerium(1V) Ammonium iron(I1) 10.0 8.6 mV sulphate solution sulphate solution (0.025 M) (0.025 M) Relative standard Mean deviation titrant of titrant volume/ volume, Titration ml per cent.time/s 9.23 0.14 150 0.47 3.3 50 10.09 0.74 150 DISCUSSION The instrument described provides an exceedingly efficient means for carrying out the numerous dispensing and titrating procedures required in most laboratories in which wet- chemical methods are used. It has the major advantage of dial selection of both reagent and reagent volume, with excellent precision, linearity and accuracy, and a directly digital read-out. The instrument is currently being used in our water analysis laboratory for preparing standard solutions €or atomic-absorption spectrophotometry and colorimetry, for neutralising reaction mixtures after acid digestion, for acid - base titrations and for various other dis- pensing operations. The instrument maintains calibration for the dilute reagents used in our laboratory but not for concentrated reagents such as 1 M sodium hydroxide solution and organic solvents, which have viscosities that are significantly different from that of distilled water; these reagents require re-calibration. Calibration can be readily checked by dispensing liquid into a 1-0-ml standard flask. As the instrument is completely linear, this check suffices for the whole calibration range. The directly digital read-out is an additional advantage. We shall shortly interface the dispenser - titrator and digital pH meter directly with a laboratory mini-computer. The authors thank Mr. Gordon Patrie for his excellent technical assistance. REFERENCES 1. 2. 3. 4. 5. Schneider, J. M., and Hendricks, C. D., Rev. Scient. Instrum., 1964, 35, 1349. Schneider, J. M., Lindblad, N. R., and Hendricks, C. D., J . Colloid Sci., 1966, 20, 610. Sweet, R. G., Rev. Scient. Instrum., 1965, 36, 131. Hieftje, G. M., and Malmstadt, H. V., Analyt. Chem., 1968, 40, 1860. Hieftje, G. M., and Rlanadarano, B. M., Ibid., 1972, 44, 1616. Received August 23rd, 1972 Amended March Znd, 1973 Accepted March 14th, 1973