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1. (WO1979000996) METHOD AND APPARATUS FOR SIMULTANEOUSLY RECORDING VOLTAGE AND VOLTAGE DERIVATIVE IN POTENTIOMETRIC STRIPPING ANALYSIS
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METHOD AND APPARATUS FOR SIMULTANEOUSLY RECORDING VOLTAGE AND VOLTAGE DERIVATIVE IN POTENTIOMETRIC STRIPPING

ANALYSIS

The present invention relates to a method for simultaneously recording, by means of single-channel printer, both the voltage between a working electrode and a
reference electrode in a measuring instrument for potentio metric stripping analysis of a solution containing metal ions, and the derivative of this voltage, and to an apparatus for carrying out the method.
As a background of this invention, a brief description of the potentiometric stripping analysis method or, in abbreviation, the PSA-method will be given below. In Fig. 1, there is schematically shown an electrode array utilized in this analysis method and including a working electrode 1 which is also designated GCE,
meaning "glassy carbon electrode", and which is coated with a thin mercury film, a reference electrode 2, and a platinum electrode 3. The three electrodes and an agitator 4 are disposed in a container 5 and immersed in a test solution therein, which is supposed to contain small amounts of certain metals. If a voltage is applied between GCE 1 and the platinum electrode 3 in such a way that GCE 1 becomes cathodic, electrolysis will take place and metal ions will be reduced on the surface of the mercury film. Amalgam (alloy of mercury and metal) is thus obtained. The longer the electrolysis is allowed to proceed, the more metal will be obtained in the
mercury film. On account of the high hydrogen over-voltage of the mercury and the electrode surface, there is no production of hydrogen gas at GCE 1. When the electrolysis is interrupted, the oxidizing agent present in the sample, for instance oxygen or mercury ions, will oxidize one metal after the other in the mercury film. The order of oxidation of the metals is determined by their standard potentials according to the electro- chemical series and the oxidation of one metal does not start until the preceding metal has oxidized completely. The oxidation process is surveyed in that the potential difference between GCE 1 and the reference electrode is measured. It should be noted that no voltage is applied when the potential difference is measured. One might say that the electrodes are "free". The process appears from Fig. 2 illustrating a PSA-diagram where the lower curve shows how the potential difference U between GCE and the reference electrode, the so-called stripping voltage, varies with time. The upper curve shows the first derivative of the lower curve.
If the "PSA-curve" is derived with respect to time, there is obtained a curve the shape of which is shown at the top of Fig. 2. The time intervals between the peaks of this curve indicate the concentrations of the corresponding metals. These concentrations may be
evaluted with the aid of calibration curves or by the so-called standard addition method. When all the metals in the mercury film have oxidized into metal ions, the system is ready for a new analysis. Since the sample is not altered by the analysis, it may be repeated as many times as is desired.
Zinc, cadmium, lead, -copper, bismuth, tallium, gallium,indium and silver may be determined by the
PSA-method. Tin and mercury may be determined with modified technique and lower sensitivity. The metals must be dissolved in a solvent with a not too feeble ionizing capacity, such as water, methanol, ethanol, propanol, butanol, acetic acid and dimethyl sulfoxide (DMSO).
The PSA-method can be used only for the analysis of dissolved metals. If the sample is solid or the metals are in particulate form, it must be so prepared that the metals to be determined are dissolved.
In the analysis of mercury, use is made of an un plated GCE and of potassium permanganate or cerium (IV) sulfate as oxidizing agent. In this case, the mercury film itself will be oxidized.

As is evident from the above description of the PSA-method, it is advantageous to use a printer for recording both the stripping voltage and its first derivative. The recording of the voltage is the qualitative part of the analysis and provides information of which metal ions are present in the sample. The measurement of the
oxidation times for the metals is the quantitative part of the analysis and provides information of the concentrations of the metals in the sample.
In the evaluation of the metal concentrations, the incomparably best measuring accuracy is obtained by measuring the distances between the peaks of the curve corresponding to the derived voltage. Thus, in potentio metric stripping analysis it is convenient either to use a two-channel printer and let it perform a simultaneous recording of the voltage and its first derivative, or to make two successive analyses and record the two functions one after the other by means of a single-channel printer.
According to the present invention, there is proposed a method of simultaneously recording, by means of a single-channel printer, on one and the same occasion both the stripping voltage and its derivative, i.e. the entire information required in the potentiometric stripping analysis.
The major advantages of using a single-channel
printer in order to record both the stripping voltage and its derivative are:
1. A stripping voltage curve from a too short
analysis time may often prove so difficult to interpret that one cannot even see that different voltage levels exist. This means that the analysis must be repeated with a prolonged electrolysis time. According to the invention, the stripping voltage derivative is marked at the same time as the voltage curve is drawn. This means that it will become easier to interpret the
qualitative part of the analysis, thus making it possible to use shorter electrolysis times. Hence, the number of analyses per time unit can be increased.
2. In the normal use of single-channel printers, the analysis must be made twice if both functions are to be printed. The time saved by this invention will thus amount to at least 50%.
3. The difference in cost between a single-channel printer and a two-channel printer is of importance in this connection. The additional cost resulting from the utilization of one of the embodiments of the invention as described below, is relatively small.
According to the present invention, there is
obtained a method for simultaneously recording, by means of a single-channel printer, both the voltage between a working electrode and a reference electrode in a measuring instrument for potentiometric stripping analysis of a solution containing metal ions, and the derivative of this voltage, which method is characterised by simultaneously applying the output signal of the measuring instrument, i.e. the voltage between the electrodes, both to the printer to be described in a diagram as a function of time, and to a deriving amplifier to be derived, and utilizing, during said description of the voltage as a function of time, the output signal of the deriving amplifier, i.e. the deriva tive of the voltage, to mark in the diagram each point of time when the voltage is changed.
According to the invention, there is further provide an apparatus for carrying out this method, which apparatus is characterised in that the printer is adapted to describe in a diagram the output signal of the measuring instrument, i.e. the voltage between the electrodes as a function of time, that a deriving amplifier is adapted to derive this voltage, and that means are provided to receive the output signal of the deriving amplifier, i.e. the derivative of the voltage, in order, during said description of the voltage as a function of time, to mark in the diagram each point of time when the voltage is changed.

In principle, the invention can be applied in two ways:
1. A single-channel printer is provided with a so-called event marker. This attachment generally consists of an additional pen which by an external electric command can be actuated so as to make a marking in the margin of the diagram. An event marker is often included as an accessory means for a single-channel printer and is used to indicate that a process has started or has been terminated during a recording (e.g. start and stop of a machine) . A single-channel printer with an event marker is not equivalent to a two-channel printer, since the additional pen is not capable of guantifying on the y-axis. Thus, for instance, the cost of a two-channel printer is considerably higher than the cost of a
corresponding single-channel printer with an event
marker. If one wishes to record the derived signal by means of a printer with an event marker, it is advisable to perform such a connection that the marker produces a marking in the. margin of the diagram when the derivative has a maximum.
2. The stripping voltage and its derivative (either of them with reversed sign) are added up and the sum is applied to a single-channel printer.
The invention will be more fully, described herein-below with reference to the accompanying drawings, in which:
Fig. 1 schematically shows an electrode array used in potentiometric stripping analysis;
Fig. 2 is a PSA-diagram where the lower curve shows the stripping voltage as a function of time and the upper curve shows the time derivative of the lower curve;
Fig. 3 is a circuit diagram in one embodiment of the invention;
Fig. 4 is a circuit diagram in another embodiment of the invention;
Fig. 5a is an example of a stripping voltage curve which is difficult to interpret, and Fig. 5b illustrates a curve corresponding to that of Fig. 5a and in which the points of inflexion on
the curve in Fig. 5A have been marked.
In Fig. 3, there is shown a circuit diagram in one embodiment of the invention. An operational amplifier
6 serves as an impedance converter and its input is
connected to a working electrode 1 or GCE (Fig. 1) . The input signal, like the output signal, thus represents the stripping voltage U. For graphic reasons, it is
advisable to revert the sign of the signal U, which is performed in a buffer amplifier 7 delivering the output signal -C1 . U, where C, is a constant, and whose output is connected to a single-channel printer 8. The signal U is further supplied to a deriving amplifier 9 providing an output signal where C is a constant. To


the output of the deriving amplifier there is connected by a diode 10 a transistor 11 which becomes conductive when the output signal of the deriving amplifier 9
attains a certain positive value. A relay 12 is adapted, when the transistor 11 is conductive, to close an electric circuit to permit the passage of an electric current through the solenoid of an event marker 13 that is
connected to the printer 8. At this moment, the pen of the event marker 13 will make a marking in the paper of the printer 8. By suitably adjusting the electric and mechanical parameters of the apparatus, this marking can be caused to coincide with great accuracy, with the
maximum of the derivative.
In Fig. 4, there is shown a circuit diagram in another embodiment of the invention. This diagram, in certain parts, fully corresponds to' the circuit diagram of Fig.
3 and comprises an impedance converter 6, a buffer amplifier 7 with sign reversion, a single-channel printer 8, and a deriving amplifier 9. In this embodiment, the
stripping voltage and its derivative are added up
electronically (either of them suitably with reversed sign), and the sum is then supplied to the single-channel printer 8. A simple way of obtaining a sum of the stripping voltage and its first derivative in the circuit diagram is, as shown by broken lines, to interconnect points

A and B by a resistor. The output signal of the buffer amplifier 7 will then be
-C1 U + C3 .
where C is a constant.
A separate ope tional amplifier connected as a summer (inverting summer) or as a differential amplifier

(differentiator) may also be utilized. It is also easy to perform derivation and desired summation, using digi tal technique. Instead of using the first derivative, it is of course possible to utilize higher derivatives or combinations of derivatives of different orders in order to obtain a readily readable curve.
In Fig. 5a, there is shown a PSA-curve illustrating how the voltage -U varies with time. As is evident, it is difficult in this case to discern the different voltage levels. Fig. 5b relates to the same analysis sample where, however, the function
-U + C4 where C4 is a cons tant ,


has been supplied to the printer. It is here clearly seen that the points of inflexion have been marked