JPS6287847A - Method for measuring phosphoric acid - Google Patents

Abstract

PURPOSE:To make it possible to stably, economically and automatically measure the concn. of phosphorus in a specimen, by always recirculating an acidic molybdenic acid solution through a flow passage containing an electrolytic cell to which constant potential was applied. CONSTITUTION:An acidic molybdenic acid solution is passes a three-way valve 3 from an acidic molybdenic acid solution tank 1 to be sent to an electrolytic cell 6 through a specimen injection valve 4 and a mixing pipe 5 and further passes through a three-way valve 7 to be discharged to a waste solution sump 8. By this operation, the flow passage of the valves 3, 4 is changed over to a recirculation mode and the acidic molybdenic acid solution is recirculated through a flow passage 9. Constant potential is applied to the cell 6 by a potentiostat 10 to stabilize a background current. Next, a definite amount of the specimen preliminarily converted to an orthophosphoric acid form is supplied from the valve 4 at the time of measurement and the oxidative decomposition of phosphorus is performed by an oxidizing agent. This specimen containing orthophosphoric acid and the molybdenic acid solution form phosphoric molibdate in a mixing tube 5 to undergo constant potential electrolysis in the cell 6. The concn. of phosphorus is measured from the quantity of electricity required in electrolysis at this time.

Description

[Detailed description of the invention] [Technical field to which the invention pertains] The present invention relates to a method for measuring phosphoric acid, and more specifically,
The present invention relates to a method for automatically measuring phosphoric acid concentration based on potentiostatic electrolysis coulometric measurement after converting phosphorus contained in sample water such as wastewater into phosphomolybdate.

[Prior art and its problems]

In order to prevent eutrophication of lakes and marshes, "Nitrogen and Phosphorus Effluent Standards" were established to regulate the phosphorus concentration contained in wastewater (a policy was established. As described in JISKO102, after oxidizing phosphorus contained in wastewater to orthophosphoric acid, an acidic aqueous solution of ammonium molybdate is added to the sample water, and the generated phosphomolybdate is converted to tin chloride. A common method is to reduce the phosphorus concentration with (II) or ascorbic acid and measure the absorbance of the generated molybdate blue.However, in this method, tin chloride (n) used as a reducing agent and Because ascorbic acid is unstable, it is not suitable for use in well-controlled laboratories, or as a measurement method for automatic measuring devices used at wastewater treatment sites as monitors for wastewater regulations.

In addition, as shown in Figure 3, another prior art technique involves flowing an acidic molybdic acid solution at a constant flow rate, and adding sample water that has oxidized phosphorus in the waste water to orthophosphoric acid midway through the flow. There is a method of subjecting phosphomolybdate to constant potential electrolysis in an electrolytic cell and measuring the phosphorus concentration from the amount of electricity required for the electrolysis.

In this method, a fixed amount of acidic molybdic acid solution is flowed from a tank 1 by a metering pump 2, and sample water is injected from an injection valve 4, mixed in a mixing tube 5, and then subjected to constant potential electrolysis in an electrolytic cell 6.

The amount of electricity required for this electrolysis is measured using a potentiostat 9 and a coulomb meter 10. The electrolyzed sample water flows into the waste liquid reservoir 8.

Although this method has lower measurement sensitivity than the JIS method mentioned above, it is sensitive enough to monitor the phosphorus concentration in wastewater based on wastewater standards, and its characteristics are that it is sensitive to electrochemical reactions. This method is suitable for application to automatic measuring equipment installed at wastewater treatment sites, as the structure of the measuring system can be simplified because absolute quantification can be performed based on the same method, and unstable reducing agents are not used. However, this method had the following drawbacks. First, in measuring the amount of electricity, we integrate the difference between the current flowing when reducing phosphomolybdic acid and the background current flowing when only the acidic molybdic acid solution is passed and a potential is applied.
It is necessary to keep the bank ground current constant. For this reason, it is necessary to keep the acidic molybdic acid solution flowing into the electrolytic cell at all times other than during measurement, and the amount of this solution is 1 to 5 ml per minute, which is equivalent to 1.4 ml per day.
~7.21 large amounts of chemicals were required. Furthermore, when the temperature decreases, the reaction rate at the electrode slows down, so phosphomolybdic acid passes through without being completely electrolyzed, and the quaternary
As shown by the curve of the conventional method in the figure, there were major problems in applying it to automatic measurement equipment installed on-site, such as a decrease in electrolysis efficiency and the inability to perform absolute quantification of phosphorus.

[Purpose of the invention]

The present invention was made in view of these problems, and provides a method for measuring phosphoric acid that makes it possible to automatically measure the phosphorus concentration in samples such as wastewater in a stable and economical manner. The purpose is to provide.

[Key points of the invention]

As a result of research to solve the problems of the conventional method described above, the present inventor discovered that, in a method for measuring phosphoric acid based on constant potential electrolysis coulometric measurement of phosphomolybdate, the inside of the flow path including the electrolytic cell is It was discovered that by configuring the method so that the acid solution always flows, it is possible to save the amount of acidic molybdic acid solution that was consumed in conventional methods, and also to be able to absolutely quantify phosphoric acid regardless of changes in temperature.

In summary, the present invention constantly circulates an acidic molybdic acid solution in a flow channel containing an electrolytic cell to which a constant potential is applied, and during measurement, a fixed amount of sample containing orthophosphoric acid is injected into this flow channel. The present invention relates to a method for measuring phosphoric acid, which is characterized in that the phosphomolybdate salt is reduced in an electrolytic cell, and phosphoric acid is determined from the amount of electricity reduced at that time.

[Embodiments of the invention]

Hereinafter, the present invention will be specifically explained with reference to the drawings.

First, FIG. 1 shows a block diagram of an example of an apparatus system used to carry out the method for measuring phosphoric acid according to the present invention. Acidic molybdate solution (a mixed reagent consisting of sodium molybdate, sulfuric acid, and ethanol, pH 1 or less) is passed from the acidic molybdate solution tank 1 through a three-way valve A3 by the action of a metering pump 2, to a sample injection valve 4, and a mixing tube 5. The liquid is sent to the electrolysis cell 6 through the three-way valve B7, and then discharged to the waste liquid reservoir 8. Through this operation, after the sample from the previous measurement is completely discharged from the flow path, the flow paths of the three-way valves A and B are switched to circulation mode, and the acidic molybdate solution flows through the flow path 9 containing the electrolytic cell 6. 11~5 m II/
Cycles at min. Then, in the electrolysis cell 6, a constant potential around 0.25V is applied by the potentiostat 10 to stabilize the background current.

Here, the structure of this electrolytic cell 6 is shown in FIG.

A carbon fiber 13 is packed inside the diaphragm cylinder 12 as a working electrode, and a spiral platinum wire is placed outside the diaphragm cylinder 12.
4 is placed as a target and a silver-silver chloride electrode 15 is placed as a reference electrode. The space between the diaphragm cylinder 12 and the glass outer cylinder 16 is filled with electrolyte. The acidic molybdic acid solution is subjected to constant potential electrolysis while passing through the diaphragm cylinder 12.

Next, at the time of measurement, a fixed amount of 50 to 150 .mu.l of a sample, in which phosphorus in the treated water has been previously oxidized and decomposed to form orthophosphoric acid, is supplied from the sample injection valve 4. The oxidative decomposition of phosphorus is carried out with an oxidizing agent such as potassium peroxodisulfate.

The sample containing orthophosphoric acid and the molybdate solution form phosphomolybdate in the mixing tube 5, and undergo constant potential electrolysis in the electrolytic cell 6 while circulating in the flow path.

The concentration of phosphorus can be measured from the amount of electricity required for electrolysis at this time.

This phosphorus concentration is determined by the following formula.

Phosphorus concentration (mo1/β) = Q Q F'n'V 1.93X 10'-V Here, Q [Coulomb]...Amount of electricity required for electrolysis F = 96
500 CC/mail) ・=Faraday constant number n=
-7... Number of electrons involved in electrolysis V = (1), , amount of sample Therefore, if all of the phosphomolybdic acid is electrolyzed, in other words, if the electrolysis efficiency is 100%, then the amount of phosphorus The concentration is determined from the amount of electricity required for electrolysis and the amount of sample,
Does not require a calibration curve using standard materials. Therefore, as an automatic measuring device, the structure can be significantly reduced in area.

Here, an experiment was conducted to examine the influence of temperature on electrolysis efficiency in the method according to the present invention (circulation method) and the method in which the molybdic acid solution is not circulated (conventional method). The experimental conditions were a flow rate of acidic molybdic acid solution of 2.2 m 1 / min in both methods.
and a standard phosphoric acid solution with a phosphorus concentration of 5 x lO-'M
oom 1 was injected, and its reduction electricity (■) was measured by changing the temperature. The electrolysis efficiency was calculated from the ratio of the measured value to the theoretical value. The results obtained are shown in FIG. As is clear from Figure 4, in the case of the conventional method, the electrolytic efficiency decreases little by little when the temperature goes below 15°C, and decreases significantly when the temperature goes below 10°C, but in the case of the present invention, even at 5°C. almost 10
An electrolysis efficiency of 0% is obtained. This is because, in the case of the present invention, phosphomolybdic acid that has passed through the electrolytic cell without being electrolyzed is circulated and electrolyzed again.

In addition, in the present invention, the amount of acidic molybdic acid solution required to measure a sample once is 10 to 20 mj+, and if it is assumed that measurement is performed once every 30 minutes, the consumption amount per day is 480 to 960 ml. ! Therefore, it is possible to save significantly compared to the conventional method.

[Effects of the Invention] In the present invention, the acidic molybdic acid solution is constantly circulated in the flow path including the electrolytic cell, thereby solving the drawbacks of the potentiostatic measurement method for phosphomolybdate. In addition to solving the problem of high consumption, by circulating and electrolyzing the phosphomolybdic acid produced after adding the sample, a decrease in electrolytic efficiency at low temperatures was prevented. As a result, the influence of temperature on the measurement was removed, and absolute quantification of phosphorus became possible. Therefore, by applying the method of the present invention, the structure of the automatic measuring device could be simplified.

[Brief explanation of drawings]

FIG. 1 is a block diagram schematically showing a specific example of an apparatus system used to carry out the phosphoric acid measurement method of the present invention. FIG. 2 is a sectional view showing a specific example of an electrolytic cell used in the method of the present invention. FIG. 3 is a block diagram of an apparatus system used to carry out a conventional method for measuring phosphoric acid by constant potential electrolysis. FIG. 4 is a graph showing the influence of temperature on electrolysis efficiency in the method of the present invention and the conventional method. 1. Acidic molybdate acid liquid tank, 2. Metering pump, 3. Three-way valve A, 4. Sample injection valve, 5. Mixing tube, 6. Electrolytic cell, 7.
・Three-way valve B, 8... Waste liquid reservoir, 9... Flow path, IO... Potentiostat 11... Coulomb meter

Claims (1)

[Claims] 1) Acidic molybdate solution is constantly circulated in a channel containing an electrolytic cell to which a constant potential is applied, and a fixed amount of sample containing orthophosphoric acid is injected into this channel during measurement, and the generated phosphomolybdate is A method for measuring phosphoric acid, which comprises reducing the phosphoric acid in an electrolytic cell and quantifying the amount of phosphoric acid from the amount of electricity produced at that time.