Anal. Chem. 1983, 55, 1180-1182
1180
Probe for In Situ Measurement of Alkalinity and pH in Natural Waters Elnar Hillborn, Jan Lld&,* and Sture Pettersson Department of Inorganic Chemistty, Universiw of Ume& S-901 87 Ume& Sweden
p H and alkalinity are two fundamental parameters in natural waters that have been, and most likely will be, extensively determined. This is especially true in Scandinavia where water acidification is becoming a most serious environmental problem. Considerable variations in p H and alkalinity with time and locality in freshwaters and estuaries often necessitate a large number of determinations which in turn require a fast and automated method. Although titration procedures can quite easily be automated with high precision, the time required for sampling and transportation to a laboratory will still set the speed of determinations. Further, sampling from especially anoxic environments may introduce significant errors by oxidation of Fe2+which a t neutral p H releases protons. It would also be of great value to receive the analytical information sought directly in the field since sampling sites of special interest may then be immediately studied in more detail. This paper describes a probe which enables potentiometric titrations t o be automatically performed in situ, thus avoiding both sampling and sample handling.
EXPERIMENTAL SECTION The probe works on the inverse buret principle ( 1 , Z ) and is used simultaneously as titration vessel and to incrementally introduce different reagents from two plastic syringes through a set of solenoid valves. The piston of this buret is penetrated by a combination pH electrode (the Ross electrode from Orion) used for the emf measurements. The design of the probe is shown in Figure 1. The probe can be described as consisting of three different modules each of which may be opened separately to facilitate service. The upper module contains ten 1.2-V Ni-Cd batteries of 4 A h capacity supplying the stepper motor, stirrer motor, and solenoid valves with current. It also contains a dummy buret which is in direct hydraulic contact with the water. Since the axis driving the dummy piston is common with that driving the titration buret piston, the stepper motor need not work against any overpressure. The middle module contains the four-phase linear stepper motor, from Airpax, giving a maximum force of 120 N, and electronic circuit cards for control of pulses, stirrer motor, and valves. In this the signals from the three sensors used are treated or amplified. These sensors are a temperature sensor (type AD 590, from Analog Devices), a pressure sensor for depth measurements (LX 1603 D, National Semiconductor), and the combination electrode. The glass and reference half-cells are connected to RCA CA 3140 operational amplifiers used as followers. It is important to measure the emf differentially, because there might be interference from ground currents between the electrical system and the water. If the reference half-cell is used as ground connection, it might be polarized and the measurements would be unreliable. The current to the electronics is supplied from the microcomputer. The lower module contains four solenoid valves (Model 337 New Brunswic Sientific) which are used as sample inlet, two reagent inlets, and waste outlet. These valves are mounted symmetrically in a ring to a Plexiglas device which serves as buret bottom and accommodates a magnetic centrifugal pump used for stirring. The stirrer motor from Airpax is placed directly below the Plexiglas buret end. The buret glass tube, having a diameter of 24.0 f 0.02 mm, is placed between the middle and the lower module. It is in direct contact with the ambient water to facilitate the temperature equilibration. The buret may be filled with 50 mL of solution. Beside the buret glass two plastic syringes containing the reagent solutions are mounted. They are attached to the valve inlets via Luer-Lock connections. All end walls of each module and the lower and middle modules are attached to each other by four
stainless steel rods (4 mm o.d., 8 mm). The pressure-proof sealings which are required at numerous places are all made by using O-rings (Skega, Sweden). The dimensions of the probe are as follows: length = 740 mm, maximum diameter = 130 mm, and weight = 10.5 kg (excluding the 21-wire cable). Since the batteries supply all the power required by the major components of the probe, there will be no significant potential loss in the cable. Therefore the cable length is not a limiting factor. The computer system is shown in Figure 2. The main part is a SWTPc MP-68 microcomputer based on Motorola’s processor MC6800. The MP-68 has been equipped with a spe cia1 memory board that can accommodate both PROM and RAM circuits (up to 32 kbyte), a 4 kbyte RAM-board,a serial RS-232 interface, and two parallel 1/0boards with 16 programmable input/output lines each. The MP-68 motherboard had to be modified in order to implement Digitronics PROM-BASIC, which is an 8k BASIC interpreter resident in PROM. Additional PROM contains assembler routines to control the probe. The titration program is at present in the 4 kbyte RAM but it may be permanently loaded into PROM. Programs written in BASIC are stored on ordinary audio casettes with a cheap casette recorder together with a casette interface (SWTPc, AC-30) connected to the computer. A digital voltmeter ranging from -999.9 mV to +999.9 mV is used for the measurments of the sensor signals. A Texas Instruments Silent 700 terminal is used to print out results and communicate with the user. The complete computer system shown in Figure 2 may be accommodated in a 64 dm3 box and the total weight is 6 kg. The power consumption is 100 W. Reagents. Out of the four valves of the probe, two will be permanently occupied by sample and waste, respectively. The other two available can be used for any convenient purpose. Our choice has been one strong solution of NaCl (1.69 M) which is added in a small amount to give an ionic strength of 0.2 M. This reduces the liquid junction potential of low pH values. A dilute hydrochloric acid (3-10 mM) in 0.2 M Na(C1) is used as titrant.
RESULTS AND DISCUSSION A titration procedure giving p H in situ and the alkalinity is illustrated by the flow scheme in Figure 3. A typical sample volume is 15 mL. In order to obtain fast and stable emf readings the first acid increment added is large enough to completely neutralize the sample and give a p H
