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A simple apparatus for the determination of trace amounts of different ions using the hanging-mercury-drop electrode
htICROCHEMICAL
A Simple
JOURNAL
11, 54-61 (1966)
Apparatus
for
the
Amounts of Different Hanging-Mercury-Drop
Determination
of Trace
Ions Using the Electrode’
WIKTOR KEMULA Department
of Inorganic
Chemistry,
Received
University,
Warsaw,
Poland
August 30, 1965
The HMDE-employing electrolysis technique has been finding an everincreasing applicability both in electrochemical studies on inorganic and organic compound behavior and in electroanalysis. In the early research period, certain regularities (1) have been discovered, trace detection methods have been developed, involving formation of amalgams and precipitates, and agreement between the theory and experiment has been established. However, as the achievements were progressing, it became necessary to improve the repeatability of data; it is associated primarily with a perfect reproducibility of the mercury drop size and ascertaining of the effects of trace impurities such as oxygen, various ions and surfactants on the processes occurring at the electrode. Electrode and Electrolytic
Cell
The model (1) which has long been used in this Laboratory is simple and easy to make. For increasing the reproducibility of the size of the drop, which constitutes the actual electrode, the following must be taken into consideration: (a) purely mechanical reproducibility of the drop size, and (b) suppression of the effect of thermal expansion of mercury in the reservoir from which it is pushed out by a piston. Experiments showed that the recently developed model of the electrode (Fig. 1) is very useful. A thick-walled glass capillary, 1, with an internal cross-section only 1 Paper presented at the International Symposium on Microchemical Techniques -1965, held at The Pennsylvania State University, University Park, Pennsylvania, U.S.A., August 22-27, 1965. 54
TRACE
AMOUNTS
OF DIFFERENT
55
IONS
slightly higher than that of the steel piston, 3, is connected to another thick-walled capillary, 2, 0.1 to 0.2 mm in diameter. The whole of this constitutes the actual electrode. The smaller the mercury reservoir, 7, the more suppressed the detrimental effect of expansion of mercury with increase in ambient temperature.
FIG. 1.
Sew
model
of
the HMDE
electrode.
(See text
for
identification
of
numbers.)
In the model described, the steel piston is in its upper part equipped with a disc, 4, pushing the mercury out as the piston screw, 3, is screwed in. The drop size may be fixed with precision and reproduced by the use of a movable disc, 5, with marks engraved on it to designate onefourth and one-eighth of the revolution. Synthetic materials, e.g. perspex, Teflon, polyethylene, etc., are suitable for the seal, 6. A vertical dash should be engraved on the seal to enable the required disc revolution, 4, angle to be read with precision. As the piston screw is turned in, the indicating disc, 5, is put back so that the S-to-6 distance is suitable for precision reading. It is important that the piston screw, 3, is set tight in the plug, 6. For extreme tightness, it is occasionally useful to lubricate the screw, 3,
56
WIKTOR
KEMULA
with a small amount of a vacuum grease. This is effective only if the screw is fitted fairly tight. Choice of the internal diameter of the lower part (exit) of the capillary, 2, is related to the type of investigations to be carried out with the electrode. For investigations on organic compound solutions, the opening may be wider, up to 0.25 mm; for investigations on the forming amalgams or for analysis of metal trace impurities, the opening must be as narrow as possible (about 0.1 mm) to prevent diffusion of metals from the mercury drop to the further portions of the mercury in the capillary. In the lastmentioned case, the capillary must be treated with silicones before use, to render its surface hydrophobic. In numerous cases, atmospheric oxygen and mercury salts occurring in the solution, owing to dissolution of metallic mercury in the presence of air, are detrimental. Therefore, the solution must be thoroughly deaerated. Recently, the author tested a cell (Fig. 1) equipped exclusively with glass ground joints, 8, which carried the HMDE and the bridge, 9, joining it with the standard electrode. Fused-in ceramic plugs used instead of filter paper separating the solution from the bridge, proved useful. Hydrogen (or any other inert gas used for deoxygenation purposes) is admitted through a three-way stopcock, 11, and escapes in bubbles from the lower opening, 13. If necessary, the gas may be passed so as to escape from the pipe opening, 14, above the solution. The liquid from the bridge, 9, can penetrate, if slowly, to the electrolytic cell. For this reason, to avoid penetration of the oxygen and of the solution from the bridge (in aqueous solutions oxygen dissolves in amounts of 1 mmole, depending upon the composition of the solution) the following modification was recently introduced: the hydrogen escaping from the opening 15 is introduced into the inlet, 16, to de-aerate also the bridge solution. Then it passes through the opening, 17, and a tube immersed in water. The depth of immersion of the tube connected to the opening 17 and the difference of levels in 16 determine the overpressure of the gas in the electrolytic cell, preventing ingress of air through the ground joints. A three-way pipe may be interconnected between the exits 15 and 16, with its exit being in water. According to the depth of immersion, the gas escapes either through the opening, 17, or by-passes the bridge, 9, to leave through the tube, 19. The typical procedure is as follows: the three-way pipe exit, 19, is deeply immersed for removal of oxygen from the cell and from the bridge, and then it is raised. The gas continues to flow only through the cell, in which overpressure is thus maintained.
TRACE
AMOUNTS
OF
DIFFERENT
IONS
57
It was found very useful to make a magnetic glass stirrer, 18, with an oval-shaped iron bar and thus to minimize the friction of the glass against the cell bottom and to improve the rotation of the stirrer even in a weak magnetic field, 12. Gases for De-aeration of Solutions In numerous cases, the oxygen present--even in small quantitiesin the solutions investigated can greatly affect the electrochemical processes or the resulting products. For this reason, the gases used for de-aeration of the solutions must be additionally purified from traces of oxygen, e.g., by being passed over hot copper. Good results were obtained by using palladinized asbestos for purification of electrolytic hydrogen from oxygen traces (2). The hydrogen thus obtained contained less than 0.25 p.p.m. oxygen. The standard electrode, 10, should have a large surface area so that its potential is constant over long periods of time, also during the electrolysis. The above-described efforts are foredoomed to failure if the purified gases are admitted through rubber or plastic tubing. It is absolutely necessary that the gases are admitted through glass tubing, because oxygen diffuses with ease through rubber and plastics and spoils the results of purification. Preparation and Filling
of the Electrode
After the piston has been carefully purified and the internal surface of the capillary-the crucial part of the HMDE-coated with a silicone, the whole equipment is assembled and the space between the material in which the piston screw, 6, is rotated and the preheated glass capillary, 1, walls are sealed with picein or like substances. Then the electrode is filled with mercury. For production of drops identical in size, it is required that the space, 7, be free from even the slightest amounts of air. A special vessel (Fig. 2) is therefore used which makes faultless filling practicable. The vessel should be equipped with a strictly similar ground joint, 1, corresponding to the joint, 8, in Fig. 1, and be as high as the electrolytic cell so that the capillary fits well and reaches up to 0.5 cm from the bottom. The vessel is filled with thoroughly purified redistilled mercury and placed in the horizontal position so that the mercury in part A is at bottom. Then the three-way stopcock, K, is connected by means of a vacuum tubing with a reliable vacuum oil pump and the cell and the electrode capillary are pumped down to less than 0.01 mm. Then the vessel is turned to the vertical position (as in Fig. 2). If the piston screw
58
WIKTOR
KEMULA
is leaky, air bubbles pass through the capillary and mercury electrode may not be filled. Therefore, it should be tightened scribed above). When everything is in order, the stopcock, K, to disconnect the vessel from the pump and then turned further air to the vessel. The pressure of air forces mercury inside and
FIG. 2. containing
A special vessel used for making mercury; K I three-way stopcock.
faultless
filling
practicable.
and the (as deis turned to admit makes it
A = part
fill completely the space, 7, of the electrode (Fig. 1). If the operations are carried out correctly, the mercury in the electrode capillary fails, after a drop is pushed out and removed, to regress towards the reservoir, 7 (Fig. 1). It is only then that the electrode is serviceable. If not, the filling operation must be repeated, because the vacuum must have been poor. If the laboratory room temperature is variable, the apparatus must be thermostated. Variation of temperature may result in two types of error: as the temperature rises, the volume of the hanging drop can increase during the experiment, despite the small volume of mercury, 7, in the electrode cell (Fig. 1)-and particularly when the electrode is freshly filled-the accuracy of measurement being thereby deteriorated. Temperature controls also the rate of the electrochemical processes, and its variation may affect the results. The temperature effects may be avoided by either placing the apparatus inside a thermostat or by providing it with double walls and then connecting to an ultrathermostat. Constancy of temperature as well as magnetic stirrer revolutions determines the repeatability of the results obtained with a given apparatus
TRACE
AMOUNTS
OF
DIFFERENT
IONS
59
in a given laboratory; however, the data obtained continue to be relative rather than absolute. For example, in determinations of minute amounts of metal ions by the chronovoltamperometric method for ascertaining the calibration curves, the HMDE double-layer capacity (3) should be taken into account. Adsorption of ions on the electrolytic cell walls, cementation of ions, etc. may also be of consequence. Therefore, to avoid tiresome determination of calibration curves which are required for obtention of quantitative data, it was necessary to develop an equipment enabling trace impurities to be determined by the method involving addition of known amounts of the ions examined. Such a method (increment method) has long been successfully used in polarography. With HMDE, precautions must be taken to eliminate the presence of atmospheric oxygen dissolved in the solution and in the increment added. For this reason, the vessel had to be redesigned to enable known amounts of the standard solution-thoroughly de-aerated-to be added to the solution examined, preferably with no dilution correction required. This proved practicable by using a special “sluice” (Fig. 3). The action of the sluice is evident from the diagram (Fig. 3) of the apparatus part attached to the vessel shown in Fig. 1. The sluice is attached to the wall at the point where pure solution-de-aerating gas is admitted. The sluice is a sort of stopcock which in the position shown in Fig. 3 admits the de-aerating gas and in the position marked by a dotted line may be filled with the standard solution from the reservoir (Fig. 3, C). The operation is as follows: The electrolytic cell is first filled with the solution examined, e.g., 20 cc, and the standard solution with a suitable concentration is added to vessel C (Fig. 3). The volume of the solution examined is preferably a multiple of the volume of the liquid added from the sluice. The sluice is set in the position shown in Fig. 3, and the solution is de-aerated by the gas, passed in succession through the electrolytic cell, bridge, and vessel that contains the standard solution (Fig. 3). Then the sluice is turned so that the liquid from vessel C fills it completely. The excess liquid escapes through an overflow, 0. The sluice is then reset to the original position. The de-aerating gas pressure is used to force the standard solution to the vessel containing the solution examined. The gas pressure is varied by suitably immersing the outlet pipes in a waterfilled cylinder, 20 (Fig. 1) so that the solution examined washes the
60
WIKTOR
KEMULA
sluice thoroughly. The sluice must be individually ing the mercury with which it has been filled. General Analytical
calibrated by weigh-
Procedure
The electrolytical cell and the sluice are filled with the solution investigated used preferably in quantities equal to multiple sluice volumes. The vessel C is filled with the standard solution and the two solutions are carefully de-aerated. The time of de-aeration is related to the de-aerating gas flow rate.
\Q&
C
-0 -----------2 - - _ /*- ----I\ ---\ .----0 -----_ - - -- -- -- --- ---- -w \ :
0
,
cl , ‘Iyf
0,
g--------ti ---
FIG. 3. Diagram of a special sluice that is attached to the wall at the point where pure solution-de-aeration gas is admitted. C = reservoir vessel; 0 = overflow for excess liquid.
TRACE
AMOUNTS
OF DIFFERENT
IONS
61
Then stripping curves are recorded for the pure solution, the preelectrolysis time being the more prolonged the less is the concentration of the ion determined. The standard solution is then added from the sluice and the metal is accumulated on mercury in the same conditions, the stripping curve of the resulting amalgam is recorded once or, if necessary, twice. The standard solution is chosen so that the concentration of the ion added is approximately equal to that in the solution examined. The content of the ion determined in the analyzed unknown is calculated by extrapolation. SUMMARY -4 new model of the electrolytic cell and a sluice technique, a refined model of the HMDE, and the with mercury are described. The sluice-equipped cell enables quantitative repeated tion to the solution examined, with neither dilution of aeration of the solution required.
attached to it, the operation technique of faultless filling adding of the standard soluthe sample nor additional de-
REFERENCES 1.
KEMULA, W., Voltammetry with the hanging mercury drop electrode. Advan. Polavography 1, 105 (1960). Kemula, W., and Kublik, Z., Application of hanging mercury drop electrode in analytical chemistry. Advan. Anal. Chew Instrument. 2, 123. (1963). -7. KEMULA, W., AND SIODA, R. Apparatus for purification of hydrogen from traces of oxygen. Chem. Anal. (Warsaw) 8, 629 (1963). 3. KEMULA, W., AND STROJEK, J. Chem. Anal. (Warsaw), in press.
A Simple
JOURNAL
11, 54-61 (1966)
Apparatus
for
the
Amounts of Different Hanging-Mercury-Drop
Determination
of Trace
Ions Using the Electrode’
WIKTOR KEMULA Department
of Inorganic
Chemistry,
Received
University,
Warsaw,
Poland
August 30, 1965
The HMDE-employing electrolysis technique has been finding an everincreasing applicability both in electrochemical studies on inorganic and organic compound behavior and in electroanalysis. In the early research period, certain regularities (1) have been discovered, trace detection methods have been developed, involving formation of amalgams and precipitates, and agreement between the theory and experiment has been established. However, as the achievements were progressing, it became necessary to improve the repeatability of data; it is associated primarily with a perfect reproducibility of the mercury drop size and ascertaining of the effects of trace impurities such as oxygen, various ions and surfactants on the processes occurring at the electrode. Electrode and Electrolytic
Cell
The model (1) which has long been used in this Laboratory is simple and easy to make. For increasing the reproducibility of the size of the drop, which constitutes the actual electrode, the following must be taken into consideration: (a) purely mechanical reproducibility of the drop size, and (b) suppression of the effect of thermal expansion of mercury in the reservoir from which it is pushed out by a piston. Experiments showed that the recently developed model of the electrode (Fig. 1) is very useful. A thick-walled glass capillary, 1, with an internal cross-section only 1 Paper presented at the International Symposium on Microchemical Techniques -1965, held at The Pennsylvania State University, University Park, Pennsylvania, U.S.A., August 22-27, 1965. 54
TRACE
AMOUNTS
OF DIFFERENT
55
IONS
slightly higher than that of the steel piston, 3, is connected to another thick-walled capillary, 2, 0.1 to 0.2 mm in diameter. The whole of this constitutes the actual electrode. The smaller the mercury reservoir, 7, the more suppressed the detrimental effect of expansion of mercury with increase in ambient temperature.
FIG. 1.
Sew
model
of
the HMDE
electrode.
(See text
for
identification
of
numbers.)
In the model described, the steel piston is in its upper part equipped with a disc, 4, pushing the mercury out as the piston screw, 3, is screwed in. The drop size may be fixed with precision and reproduced by the use of a movable disc, 5, with marks engraved on it to designate onefourth and one-eighth of the revolution. Synthetic materials, e.g. perspex, Teflon, polyethylene, etc., are suitable for the seal, 6. A vertical dash should be engraved on the seal to enable the required disc revolution, 4, angle to be read with precision. As the piston screw is turned in, the indicating disc, 5, is put back so that the S-to-6 distance is suitable for precision reading. It is important that the piston screw, 3, is set tight in the plug, 6. For extreme tightness, it is occasionally useful to lubricate the screw, 3,
56
WIKTOR
KEMULA
with a small amount of a vacuum grease. This is effective only if the screw is fitted fairly tight. Choice of the internal diameter of the lower part (exit) of the capillary, 2, is related to the type of investigations to be carried out with the electrode. For investigations on organic compound solutions, the opening may be wider, up to 0.25 mm; for investigations on the forming amalgams or for analysis of metal trace impurities, the opening must be as narrow as possible (about 0.1 mm) to prevent diffusion of metals from the mercury drop to the further portions of the mercury in the capillary. In the lastmentioned case, the capillary must be treated with silicones before use, to render its surface hydrophobic. In numerous cases, atmospheric oxygen and mercury salts occurring in the solution, owing to dissolution of metallic mercury in the presence of air, are detrimental. Therefore, the solution must be thoroughly deaerated. Recently, the author tested a cell (Fig. 1) equipped exclusively with glass ground joints, 8, which carried the HMDE and the bridge, 9, joining it with the standard electrode. Fused-in ceramic plugs used instead of filter paper separating the solution from the bridge, proved useful. Hydrogen (or any other inert gas used for deoxygenation purposes) is admitted through a three-way stopcock, 11, and escapes in bubbles from the lower opening, 13. If necessary, the gas may be passed so as to escape from the pipe opening, 14, above the solution. The liquid from the bridge, 9, can penetrate, if slowly, to the electrolytic cell. For this reason, to avoid penetration of the oxygen and of the solution from the bridge (in aqueous solutions oxygen dissolves in amounts of 1 mmole, depending upon the composition of the solution) the following modification was recently introduced: the hydrogen escaping from the opening 15 is introduced into the inlet, 16, to de-aerate also the bridge solution. Then it passes through the opening, 17, and a tube immersed in water. The depth of immersion of the tube connected to the opening 17 and the difference of levels in 16 determine the overpressure of the gas in the electrolytic cell, preventing ingress of air through the ground joints. A three-way pipe may be interconnected between the exits 15 and 16, with its exit being in water. According to the depth of immersion, the gas escapes either through the opening, 17, or by-passes the bridge, 9, to leave through the tube, 19. The typical procedure is as follows: the three-way pipe exit, 19, is deeply immersed for removal of oxygen from the cell and from the bridge, and then it is raised. The gas continues to flow only through the cell, in which overpressure is thus maintained.
TRACE
AMOUNTS
OF
DIFFERENT
IONS
57
It was found very useful to make a magnetic glass stirrer, 18, with an oval-shaped iron bar and thus to minimize the friction of the glass against the cell bottom and to improve the rotation of the stirrer even in a weak magnetic field, 12. Gases for De-aeration of Solutions In numerous cases, the oxygen present--even in small quantitiesin the solutions investigated can greatly affect the electrochemical processes or the resulting products. For this reason, the gases used for de-aeration of the solutions must be additionally purified from traces of oxygen, e.g., by being passed over hot copper. Good results were obtained by using palladinized asbestos for purification of electrolytic hydrogen from oxygen traces (2). The hydrogen thus obtained contained less than 0.25 p.p.m. oxygen. The standard electrode, 10, should have a large surface area so that its potential is constant over long periods of time, also during the electrolysis. The above-described efforts are foredoomed to failure if the purified gases are admitted through rubber or plastic tubing. It is absolutely necessary that the gases are admitted through glass tubing, because oxygen diffuses with ease through rubber and plastics and spoils the results of purification. Preparation and Filling
of the Electrode
After the piston has been carefully purified and the internal surface of the capillary-the crucial part of the HMDE-coated with a silicone, the whole equipment is assembled and the space between the material in which the piston screw, 6, is rotated and the preheated glass capillary, 1, walls are sealed with picein or like substances. Then the electrode is filled with mercury. For production of drops identical in size, it is required that the space, 7, be free from even the slightest amounts of air. A special vessel (Fig. 2) is therefore used which makes faultless filling practicable. The vessel should be equipped with a strictly similar ground joint, 1, corresponding to the joint, 8, in Fig. 1, and be as high as the electrolytic cell so that the capillary fits well and reaches up to 0.5 cm from the bottom. The vessel is filled with thoroughly purified redistilled mercury and placed in the horizontal position so that the mercury in part A is at bottom. Then the three-way stopcock, K, is connected by means of a vacuum tubing with a reliable vacuum oil pump and the cell and the electrode capillary are pumped down to less than 0.01 mm. Then the vessel is turned to the vertical position (as in Fig. 2). If the piston screw
58
WIKTOR
KEMULA
is leaky, air bubbles pass through the capillary and mercury electrode may not be filled. Therefore, it should be tightened scribed above). When everything is in order, the stopcock, K, to disconnect the vessel from the pump and then turned further air to the vessel. The pressure of air forces mercury inside and
FIG. 2. containing
A special vessel used for making mercury; K I three-way stopcock.
faultless
filling
practicable.
and the (as deis turned to admit makes it
A = part
fill completely the space, 7, of the electrode (Fig. 1). If the operations are carried out correctly, the mercury in the electrode capillary fails, after a drop is pushed out and removed, to regress towards the reservoir, 7 (Fig. 1). It is only then that the electrode is serviceable. If not, the filling operation must be repeated, because the vacuum must have been poor. If the laboratory room temperature is variable, the apparatus must be thermostated. Variation of temperature may result in two types of error: as the temperature rises, the volume of the hanging drop can increase during the experiment, despite the small volume of mercury, 7, in the electrode cell (Fig. 1)-and particularly when the electrode is freshly filled-the accuracy of measurement being thereby deteriorated. Temperature controls also the rate of the electrochemical processes, and its variation may affect the results. The temperature effects may be avoided by either placing the apparatus inside a thermostat or by providing it with double walls and then connecting to an ultrathermostat. Constancy of temperature as well as magnetic stirrer revolutions determines the repeatability of the results obtained with a given apparatus
TRACE
AMOUNTS
OF
DIFFERENT
IONS
59
in a given laboratory; however, the data obtained continue to be relative rather than absolute. For example, in determinations of minute amounts of metal ions by the chronovoltamperometric method for ascertaining the calibration curves, the HMDE double-layer capacity (3) should be taken into account. Adsorption of ions on the electrolytic cell walls, cementation of ions, etc. may also be of consequence. Therefore, to avoid tiresome determination of calibration curves which are required for obtention of quantitative data, it was necessary to develop an equipment enabling trace impurities to be determined by the method involving addition of known amounts of the ions examined. Such a method (increment method) has long been successfully used in polarography. With HMDE, precautions must be taken to eliminate the presence of atmospheric oxygen dissolved in the solution and in the increment added. For this reason, the vessel had to be redesigned to enable known amounts of the standard solution-thoroughly de-aerated-to be added to the solution examined, preferably with no dilution correction required. This proved practicable by using a special “sluice” (Fig. 3). The action of the sluice is evident from the diagram (Fig. 3) of the apparatus part attached to the vessel shown in Fig. 1. The sluice is attached to the wall at the point where pure solution-de-aerating gas is admitted. The sluice is a sort of stopcock which in the position shown in Fig. 3 admits the de-aerating gas and in the position marked by a dotted line may be filled with the standard solution from the reservoir (Fig. 3, C). The operation is as follows: The electrolytic cell is first filled with the solution examined, e.g., 20 cc, and the standard solution with a suitable concentration is added to vessel C (Fig. 3). The volume of the solution examined is preferably a multiple of the volume of the liquid added from the sluice. The sluice is set in the position shown in Fig. 3, and the solution is de-aerated by the gas, passed in succession through the electrolytic cell, bridge, and vessel that contains the standard solution (Fig. 3). Then the sluice is turned so that the liquid from vessel C fills it completely. The excess liquid escapes through an overflow, 0. The sluice is then reset to the original position. The de-aerating gas pressure is used to force the standard solution to the vessel containing the solution examined. The gas pressure is varied by suitably immersing the outlet pipes in a waterfilled cylinder, 20 (Fig. 1) so that the solution examined washes the
60
WIKTOR
KEMULA
sluice thoroughly. The sluice must be individually ing the mercury with which it has been filled. General Analytical
calibrated by weigh-
Procedure
The electrolytical cell and the sluice are filled with the solution investigated used preferably in quantities equal to multiple sluice volumes. The vessel C is filled with the standard solution and the two solutions are carefully de-aerated. The time of de-aeration is related to the de-aerating gas flow rate.
\Q&
C
-0 -----------2 - - _ /*- ----I\ ---\ .----0 -----_ - - -- -- -- --- ---- -w \ :
0
,
cl , ‘Iyf
0,
g--------ti ---
FIG. 3. Diagram of a special sluice that is attached to the wall at the point where pure solution-de-aeration gas is admitted. C = reservoir vessel; 0 = overflow for excess liquid.
TRACE
AMOUNTS
OF DIFFERENT
IONS
61
Then stripping curves are recorded for the pure solution, the preelectrolysis time being the more prolonged the less is the concentration of the ion determined. The standard solution is then added from the sluice and the metal is accumulated on mercury in the same conditions, the stripping curve of the resulting amalgam is recorded once or, if necessary, twice. The standard solution is chosen so that the concentration of the ion added is approximately equal to that in the solution examined. The content of the ion determined in the analyzed unknown is calculated by extrapolation. SUMMARY -4 new model of the electrolytic cell and a sluice technique, a refined model of the HMDE, and the with mercury are described. The sluice-equipped cell enables quantitative repeated tion to the solution examined, with neither dilution of aeration of the solution required.
attached to it, the operation technique of faultless filling adding of the standard soluthe sample nor additional de-
REFERENCES 1.
KEMULA, W., Voltammetry with the hanging mercury drop electrode. Advan. Polavography 1, 105 (1960). Kemula, W., and Kublik, Z., Application of hanging mercury drop electrode in analytical chemistry. Advan. Anal. Chew Instrument. 2, 123. (1963). -7. KEMULA, W., AND SIODA, R. Apparatus for purification of hydrogen from traces of oxygen. Chem. Anal. (Warsaw) 8, 629 (1963). 3. KEMULA, W., AND STROJEK, J. Chem. Anal. (Warsaw), in press.