JPH04110763A - Electrolytic cell for measuring phosphoric acid concentration - Google Patents

Abstract

PURPOSE:To provide strong structure and easy maintenance and permit continuous measurement by inserting a counter-electrode and a reference electrode directly in a specimen flow-path. CONSTITUTION:A specimen water is pressurized and sent from an influx path 7d to an electrolytic cell 1. A working electrode 3 shall have a constant potential relative to a reference electrode 5, and the electrode reduction current flowing between the working electrode 3 and a counter-electrode 4 is measured. Therein the counter-electrode 4 and reference electrode 5 are inserted directly into a flow path for specimen water, which should eliminate provision of any diaphragm cylinder, which is easy to break and requires utmost care in handling, and any KCL solution which fills the outer space of disphragm cylinder in order to generate liquid communication between the working electrode 3, counter-electrode 4, and reference electrode 5. This will facilitate manufacture and maintenance.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a potentiostatic electrolysis cell for measuring the concentration of phosphoric acid contained in a strongly acidic liquid using flow coulometry.

[Conventional technology]

The flow coulometry method is known as a method suitable for automatic measurement as a method for measuring phosphoric acid concentration, but it is particularly difficult to measure total phosphorus in secondary treated sewage water.
A method for measuring the phosphoric acid concentration in strongly acidic sample water by flow coulometry has been filed by the same applicant as the present application (see Japanese Patent Application Laid-Open No. 2-118445). was oxidized with ozone and further made into an acidic solution with p[11.5 or less, which was then heated and decomposed into phosphoric acid to form a phosphomolybdenum complex, which was then introduced into a constant potential electrolysis cell to form a phosphomolybdenum complex. This method measures the reduction current generated when a complex is subjected to constant potential electrolysis. The constant potential electrolysis cell used in this case is
The one shown in FIG. 2 is used. In this cell, a diaphragm cylinder 22 is filled with carbon fibers as a working electrode 23, and a spiral platinum wire is arranged as a counter electrode 24 outside the diaphragm II22. The α space between the diaphragm cylinder 22 and the glass outer cylinder 26 is filled with a solution 27, and silver is added to this space.
A silver chloride reference electrode 25 is inserted. The working electrode 23, through which sample water containing the phosphomolybdenum complex flows into the cell at a constant rate from the sample inlet 29, is always set at a constant potential of +300-■ with respect to the silver-silver chloride reference electrode 25. The electrolytic reduction current flowing between the working electrode 23 and the counter electrode 24 is measured, and the measured current value is converted into phosphorus #1 degree.

[I II to be solved by the invention] Diaphragm portion IM22 of the constant potential electrolytic cell according to the above-mentioned prior art
is made of porous quartz, and measures are taken to allow sample water to enter the cell. As the sample water continues to be pressurized into the cell, the sample water gradually mixes with the α solution in the diaphragm cylinder 22 due to the high resistance of the sample water flow path of the cell, and the concentration of the 14++4 solution decreases.

If such a phenomenon occurs, the cell must be disassembled and the Ci −+=@−+ solution replaced and replenished.
There is very little movement of liquid through the liquid-liquid interface with the solution, but the connection is electrochemical. However, the diaphragm cylinder 22 made of porous quartz cannot be made very thick in view of the purpose of its installation, and therefore has the problem of being easily damaged and requiring extreme care in its handling.

In addition, as a requirement of the flow coulometry method, the sample water must flow through the cell at a constant velocity and at a relatively low velocity to ensure sufficient electrolysis of the components to be measured in the sample water. A working electrode is provided in the sample water flow path to provide high resistance. Therefore, this is not preferable when the sample water is forced to flow into the cell and the cell is inserted into a continuous measurement line.

The present invention has been made in view of the above points, and an object of the present invention is to provide a TA cell for measuring phosphoric acid concentration that is robust, easy to maintain, and suitable for use in a continuous measurement line. .

(Means for solving III!) In order to achieve the above object, in the present invention, a working electrode provided in the sample water flow path, a counter electrode paired with this working electrode, and a constant potential of the working electrode are provided. A reference electrode provided to set a potential, a counter electrode of an electrolytic cell for measuring phosphorus # concentration, and a reference electrode were directly inserted into the sample water channel.

[Effect]

Looking closely at the structure of the reference electrode 25 shown in the second diagram, we find that
As shown in the third figure, the silver-silver chloride reference electrode includes an outer tube 32 made of glass having a diaphragm section 31 made of porous ceramics at one end, an electrode section 33 provided inside the outer tube 32, and an electrode section 33.
^gc1 electrode section 3 connected to this Ag electrode section 33
A lead wire is connected to one end Ce of the ^gc1 electrode section 34 consisting of 4 and . Inside the outer tube 32 there is a H solution 36.
is fulfilled. Since the space of the electrolytic cell into which this reference electrode 25 is inserted is filled with the MCI-G-4 solution as described above, the diaphragm portion 31 is
It merely provides a liquid junction between the 2 + solution and the ÷÷ → solution. Therefore, a sample water -4+-+4 solution interface is formed in the small hole U of the diaphragm cylinder 22 in FIG.
Even if the sample water −+8+− solution interface is directly formed in the pores of the diaphragm portion 31 of the silver-chloride reference electrode instead of the above, there will be no electrochemical change.
In the figure, the counter electrode 24 is wound around the outside of the diaphragm cylinder cJ2 22 and is provided in the 4HF+ solution, and the working electrode 23 is connected to the diaphragm cylinder 22 by the diaphragm cylinder 22.
There are two different chemical potentials between the solution and the sample water.
There will be a liquid junction potential difference created by the contact of the seed electrolyte solutions. However, this is a highly concentrated electrolyte solution, and as mentioned above, the sample water has been adjusted to have a sulfuric acid acidity of pH 1.5, making it a highly concentrated electrolyte solution.
The working electrode 23 and the counter electrode 24 are electrochemically connected by a diaphragm cylinder, and the liquid potential difference at the Kα bone→→solution−sample water interface is almost negligible. There is no change in electrochemistry.

〔Example〕

FIG. 1 is a longitudinal sectional view of an embodiment of an electrolytic cell for measuring phosphoric acid concentration according to the present invention. A working electrode 3 made of grungy carbon particles 3a is provided in a lower space 1a bored inside the cell body 1, and the glassy carbon particles are covered with a mesh 8.
The cell body 1 is pressed and fixed to the constricted portion 1b of the internal space of the cell body 1 by a protruding portion 7a provided at the center of the bottom plate 7 via a tube 8.

A platinum lead wire 3b is drawn out from the working electrode 3 to the side surface of the cell body 1. Although glassy carbon particles are used as the working electrode in this example, carbon fibers may also be used as the working electrode as in the prior art.
It is sufficient that the area of the electrode-sample water interface where the phosphomolybdenum complex is electrolyzed can be made as large as possible.

The upper part of the throttle part 1a of the cell body 1 is provided with an upper space 1c having approximately the same diameter as the lower space IB, and the counter electrode 2 is inserted into this upper space 1c from the side of the cell body 1. A platinum lead wire 2b is drawn out from the side surface of the cell body 1.

The protruding portion 7a of the bottom plate 7 is tightly inserted into the cell body lower space 1a, and the bottom plate 7 is liquid-tightly fitted into the cell body 1 by suitable means such as screws 8 and 0 rings 9.

Further, the respective lead-out portions of the platinum leads ka3b and 2b are also sealed liquid-tightly.

Further, a silver-silver chloride reference electrode 5 is inserted into the upper space 1c, and the same silver-silver chloride reference electrode 25 shown in FIG. 3 can be used as the silver-silver chloride reference electrode 5. This silver-silver chloride reference electrode 5 is liquid-tightly fixed to the cell body 1 by a suitable fitting member.

A sample water inflow path 7d is bored through the center of the bottom plate 7 and the center of its protrusion 17a, and the cell body 1
Outflow R1 of sample water from the upper side to the upper space IC
d is drilled. Therefore, the inflow path 7d, the lower space 1
A, a flow path for sample water in the cell 1 is formed by the upper space IC and the outflow path 1d.

Next, a procedure for measuring the phosphoric acid concentration in sample water using this electrolytic cell 1 will be explained. First, the phosphorus compounds in the sample water are oxidized with ozone to make acidic sample water with a pH of 1.5 or less, which is then heated and decomposed into phosphoric acid.The resulting acidic sample water is converted into a phosphomolybdenum complex, and then transferred to the electrolytic cell l. The water is introduced into the electrolytic cell 1 through the inflow path 7d after being pressurized to an appropriate water pressure using a pressurizing means such as a pump. The working electrode 3 is set so that its potential is always at a constant potential of 250 mV to 350 mV DC with respect to the reference electrode 5, and at this time, the electrolytic reduction current flowing between the working electrode 3 and the counter electrode 4 is measured. The constant potential electrolysis operation is performed by a potentiostat (not shown), which is the same as in conventional electrolytic cells.The reduction reaction of the phosphomolybdenum complex at the working electrode 3 is expressed by the following equation (1).

(PMo(Vr)+tOae) '-+ 26-4(P
Mo(V)gMo(Vl) r e O41) ”-
---'------Since the reduction current due to the reaction of formula (1) + 11 is proportional to the phosphorus concentration, the phosphoric acid concentration can be determined based on the value of this reduction current, which is the same as in the conventional technology. . Fact CJ In fact, the results of measuring the total phosphorus concentration in the secondary sewage treatment water using the electrolysis cell shown in Figure 1 using reference electrodes with 4+=6=+ saturated solution and 3.3M solution are Regarding the subsequent treated water, the results showed good agreement with the measurement results using the conventional electrolytic cell shown in Figure 2, confirming the functionality of the electrolytic cell according to the present invention in which the counter electrode and reference electrode were inserted directly into the sample water flow path.

〔Effect of the invention〕

As explained above, in the present invention, a working electrode provided in the sample water flow path, a counter electrode paired with this working electrode, a reference electrode provided to set the potential of the working electrode to a constant potential, Since the counter electrode and reference electrode of the electrolytic cell for measuring phosphoric acid concentration are inserted directly into the sample water flow path, the diaphragm cylinder, which is easily damaged in conventional electrolytic cells and requires careful handling, is removed. and the air k(
The +-+-+- solution that was filled between J is no longer needed,
As a result, a tremendous effect can be obtained in that an electrolytic cell that is easy to manufacture and maintain is provided.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of an embodiment of the present invention, FIG. 2 is a sectional view of a conventional electrolytic cell for measuring phosphorus # concentration, and FIG. 3 is a sectional view for explaining a silver-silver chloride reference electrode. . 1: Cell body, 1a: Lower space, 1c: Upper space, 2:
Counter electrode, 3: working electrode, 5: reference electrode, 7: bottom plate.

Claims (1)

[Claims] 1) For measuring phosphoric acid concentration, comprising a working electrode provided in a sample water flow path, a counter electrode paired with the working electrode, and a reference electrode provided to set the potential of the working electrode to a constant potential. An electrolytic cell for measuring phosphoric acid concentration, characterized in that the counter electrode and the reference electrode are directly inserted into the sample water flow path.