GB2174207A

GB2174207A - Continuous determination of nitrite and/or nitrate in an aqueous medium

Info

Publication number: GB2174207A
Application number: GB08609125A
Authority: GB
Inventors: Jean-Marie Couret
Original assignee: LANGUEDOC, University of; Universite des Sciences et Techniques Du Languedoc (Montpelier I)
Current assignee: LANGUEDOC, University of; Universite des Sciences et Techniques Du Languedoc (Montpelier I)
Priority date: 1985-04-15
Filing date: 1986-04-15
Publication date: 1986-10-29
Also published as: FR2580404A1; GB8609125D0; DE3612387A1; JPS61280559A; FR2580404B1; GB2174207B

Abstract

The nitrite content of an aqueous solution is determined by mixing with a buffer solution 10 at 5, passing the buffered solution through an electro-oxidation cell 1 under conditions such at 4 nitrite ions are oxidised to nitrate and determining nitrite concentration from the cell current. Nitrate in the solution is determined by reduction to nitrite at 11 before passing the solution through cell 1. The buffer solution preferably has a pH of 6-8 and the electro-oxidation in cell 1 is preferably performed at a platinum or vitreous carbon electrode, the former being held at about 1040 mV with respect to a calomel electrode. <IMAGE>

Description

SPECIFICATION A method and apparatus for the continuous determination of nitrite and/or nitrate in an aqueous medium The present invention relates to a method and apparatus for the continuous determination of nitrite and/or nitrate in an aqueous medium.

Several methods as well as devices are already known for determining the nitrite and/or nitrate content of a liquid medium.

Thus, the quantity determination of nitrite has been carried out discontinuously up to now, using colorimetric methods or electrochemical methods.

The first are various adaptations of the same principle in which an azoic dye is formed whose concentration, determined by spectrophotometry, is proportional to the nitrite content of the sample.

Two electrochemical methods have been used: one voltamperometric and the other coulometric. Both methods are in fact only practically applicable to the laboratory, either after separation of the nitrites and the nitrates by high performance liquid chromatography, or after previous determination of the nitrate content of the sample. These electrochemical methods cannot be carried out continuously for it is necessary first to deoxygenize the sample: in fact, detection is carried out by reduction.

Moreover devices are known for continuously determining the quantity of nitrite and/or nitrate present in a fluid. For example, French Patent Specification FR-A-2 265 093 relates to a biamperometric measuring device which comprises two platinum electrodes, a micro-ammeter associated with a recorder. As an example of application the determination of nitrate content is given: the nitrate is previously reduced into nitrite using iodide ions. The device allows the iodine content to be determined which is proportional to the nitrite content. If the nitrite content is known by previous measurement, the nitrate content may therefore be deduced therefrom. This is thus an indirect measurement of the nitrate content since in fact it is the concentration of a third element i.e. iodine which is measured.

Therefore one of the aims of the present invention is to provide apparatus for the continuous amperometric quantity determination of nitrite and/or nitrate which does not require determination of the quantity of an intermediate element.

Another aim of the invention is to provide such apparatus for measuring the amount of nitrite and/or nitrate present in any aqueous medium, more particularly saline or non saline.

According generally to the present invention there is provided a method for determining the nitrate and/or nitrite content of an aqueous medium which comprises the steps of mixing the aqueous medium with a buffer solution to form a buffered aqueous solution, passing the buffered aqueous solution through an electro-oxidative cell under conditions such that nitrite ions are oxidised to nitrate ions, measuring the current flow required to effect the oxidation of the nitrite ions to nitrate ions, and expressing the current flow in terms of nitrite and/or nitrate content of the aqueous medium.

In particular the invention provides apparatus for determining the nitrate and/or nitrite content of an aqueous medium comprising a reservoir for buffer solution, means for mixing buffer solution with the aqueous medium to form a buffered aqueous solution, an electrochemical cell, means for feeding the buffered aqueous solution through the electrochemical cell, an electrode forming part of the cell, means for maintaining the electrode at a potential to promote electrooxidation of nitrite ions in the aqueous solution to nitrate ions, and means for measuring the current flowing through the electrode.

A specific apparatus for the continuous amperometric quantity determination of nitrites and/or nitrates which are present in any aqueous medium, comprises, in accordance with the invention, a circulation cell which is provided with an electrode, a microammeter connected to this electrode, and a mixture tank which is situated upstream of this cell with respect to the flow direction of the fluids and which is connected, on the one hand to a buffer solution reservoir and, on the other, to a source of liquid whose nitrite and/or nitrate content is to be determined.

When the nitrate content of the aqueous medium is to be measured, the aqueous medium or the buffered aqueous solution is subjected to a reduction process in which nitrate present is reduced to nitrite.

Advantageously, the apparatus of the present invention also comprises, disposed between the mixture tank and the circulation cell, a means for reducing the nitrate to nitrite. Preferably, this means is formed by a copper coated cadmium column or a copper coated cadmium fluidized bed reactor.

Preferably the apparatus also comprises a peristaltic two way pump disposed in the ducts connecting the mixing means to the liquid source and to the buffer solution reservoir.

The apparatus preferably includes means for automatically correcting the effects of the variation of the temperature of the circulation cell.

Preferably the apparatus furthermore comprises a means for filtering the liquid whose nitrite and/or nitrate content is to be determined prior to its being mixed with the buffer solution.

Preferably, the microammeter is a recording microammeter.

The electrode is preferably a vitreous carbon electrode or a platinum electrode. In the latter case the potential of the platinum electrode is advantageously about 1040 (+50) mV with respect to the calomel electrode.

The circulation cell is suitably an electrochemical cell comprising an indicating electrode, which is either a platinum or a vitreous carbon electrode, a reference electrode which is a calomel electrode and an auxiliary electrode. Where the indicating electrode is a platinum electrode, its potential is preferably 1040 (+50) mV with respect to the calomel electrode. If the indicating electrode is a vitreous carbon electrode its potential is selected as is usually done in amperometric determinations, by plotting the potential intensity curves of this electrode.

The buffer solution must allow a good yield in the electro-oxidation reaction of nitrite to nitrate which is used in the amperometric determination. For this purpose the buffer solution has preferably a pH in the 6-8 range, for example in the 6.5-7.5 range. Advantageously the buffer effect is very strong, so that there is a minimum pH gradient in the neighbourhood of the electrode.

If the apparatus of the present invention comprises a means for reducing nitrate into nitrite the buffer solution must allow a quantitative reduction and a long life-expectancy of this means.

When for example, the latter is a copper coated cadmium column or a copper coated cadmium fluidized bed reactor, those results require the use of a buffer solution having a proper pH, in particular in the 6-8 range and solubilizing by complexing copper and cadmium cations.

Furthermore this buffer solution must not precipitate metal hydroxides and carbonates present in the liquid whose nitrite and/or nitrate content is determined.

Advantageously, the buffer solution comprises ammonium chloride, picolinic acid (pyridine 2carboxylic acid), and monopotassium phosphate or citric acid.

In a first embodiment of the present invention, this buffer solution comprises 0.2 M ammonium chloride, 0.02 M monopotassium phosphate and 0.01 M picolinic acid and has a pH of about 7.5.

In a second embodiment, this buffer solution comprises 0.2 M ammonium chloride, 0.02 M monopotassium phosphate and 0.1 M of picolinic acid and a pH of about 6.5.

In a third embodiment of the invention, the buffer solution comprises 0.2 M of ammonium chloride, 0.05 M citric acid and 0.01 M picolinic acid and has a pH of about 7.

The invention is further illustrated by way of example with reference to the accompanying drawings in which: Figure 1 shows schematically apparatus according to the present invention; Figure 2 shows schematically apparatus according to the present invention and comprising a means for reducing nitrate to nitrite.

Thus, as can be seen in Fig. 1, apparatus for the continuous quantity determination of the nitrite ions and/or nitrate ions present in a liquid medium, comprises a circulation cell 1 which is provided with a platinum electrode 2. This electrode is connected, on the one hand, to an energy source 3 and, on the other, to a microammeter 4.

This apparatus also comprises a mixture tank 5 situated upstream of the cell 1 with respect to the flow direction F of the fluids: this tank is connected, on the one hand, to a liquid source whose nitrite and/or nitrate content is to be determined and, on the other, to a buffer solution reservoir 6. A two way peristaltic pump 7 is disposed in ducts 8 and 9 connecting respectively the reservoir 6 and the liquid source to the tank 5.

The microammeter 4 is a recording microammeter. The current measured by this microammeter is proportional to the amount of nitrite and/or nitrate contained in the aqueous medium.

As for the power source 3, it allows a potential of about 1040 mV (+50 mV) with respect to the calomel electrode to be applied to the terminals of electrode 2.

The buffer solution 10 contained in reservoir 6 comprises ammonium chloride (NH4CI) picolinic acid and monopotassium phosphate (KH2PO4), or citric acid.

In the following Table 1 are shown three compositions of buffer solution 10.

TABLE 1

pH* NH4Cl KH2PO4 citric acid picolinic acid SOLUTION NO. 1 7.5 0.2 M 172.5 g/l 0.02 M 44 g/l - - 0.01 M 20 g/l SOLUTION NO. 2 6.5 0.2 M 172.5 g/l 0.02 M 44 g/l - - 0.1 M 200 g/l SOLUTION NO. 3 7 0.2 M 172.5 g/l - - 0.05 M 168 g/l 0.01 M 20 g/l * The pH is adjusted with an ammonia solution The above Solution No. 1 is in particular intended for the determination of the quantity of nitrite and/or nitrate when the aqueous medium is fresh water, whereas Solution No. 2 is preferred when the aqueous medium is sea water.

If the aqueous medium is highly charged with magnesium and calcium, it will be necessary to increase the amount of picolinic acid to a concentration of about 0.2 M. This will consequently lead to modifying the dilution ratio and possibly the pH.

Solution No. 3 is particularly adapted for the case of hard water and sea water.

The flow rate of pump 7 is adjusted so that the ratio between the flow of the buffer solution 10 and the aqueous liquid is of the order of 6.62%.

The apparatus shown schematically in Fig. 2 is identical to that described above except that it comprises, in addition, a means 11 for reducing the nitrate into nitrite disposed between tank 5 and cell 1.

Claims

1. A method for determining the nitrate and/or nitrite content of an aqueous medium which comprises the steps of mixing the aqueous medium with a buffer solution to form a buffered aqueous solution, passing the buffered aqueous solution through an electro-oxidative cell under conditions such that nitrite ions are oxidised to nitrate ions, measuring the current flow required to effect the oxidation of the nitrite ions to nitrate ions, and expressing the current flow in terms of nitrite and/or nitrate content of the aqueous medium.

2. A method according to claim 1 wherein the nitrate content of the aqueous medium is to be measured and which includes the step of subjecting the aqueous medium or the buffered aqueous solution to a reduction process in which nitrate present is reduced to nitrite.

3. A method according to claim 1 or 2 wherein the current flow is measured using a recording microammeter.

4. A method according to any one of claims 1 to 3 wherein the buffer solution has a pH in the range of 6 to 8.

5. A method according to any one of claims 1 to 4 wherein the buffer solution has a pH in the range of 6.5-7.5.

6. A method according to claim 4 or 5 wherein the buffer solution comprises ammonium chloride, picolinic acid and monopotassium phosphate or citric acid.

7. A method according to claim 6 wherein the buffer solution comprises substantially 0.2 M of ammonium chloride, 0.02 M of monopotassium phosphate and 0.01 M of picolinic acid and has a pH of about 7.5.

8. A method according to claim 6 wherein the buffer solution comprises substantially 0.2 M of ammonium chloride, 0.02 M of monopotassium phosphate and 0.1 M of picolinic acid and has a pH of about 6.5.

9. A method according to claim 6 wherein the buffer solution comprises substantially 0.2 M of ammonium chloride, 0.05 M of citric acid and 0.01 M of picolinic acid and has a pH of about 7.

10. A method for determining the nitrate and/or nitrite content of an aqueous medium substantially as hereinbefore described with reference to the accompanying drawings.

11. Apparatus for determining the nitrate and/or nitrite content of an aqueous medium comprising a reservoir for buffer solution, means for mixing buffer solution with the aqueous medium to form a buffered aqueous solution, an electrochemical cell, means for feeding the buffered aqueous solution through the electrochemical cell, an electrode forming part of the cell, means for maintaining the electrode at a potential to promote electro-oxidation of nitrite ions in the aqueous solution to nitrate ions, and means for measuring the current flowing through the electrode.

12. Apparatus for the continuous amperometric quantity determination of the nitrites and/or nitrates which are present in any aqueous medium, the apparatus comprising a circulation cell which is provided with an electrode, a microammeter current measuring means, connected to the electrode, a mixture tank situated upstream of the cell with respect to the direction of circulation of the aqueous medium, and means for connecting the mixture tank on the one hand, to a reservoir of a buffer solution and, on the other, to a liquid source whose nitrite and/or nitrate content is to be determined.

13. Apparatus according to claim 11 or 12 and including a two way peristaltic pump set in ducts connecting the mixing means or mixture tank to the aqueous medium source and to the buffer solution reservoir.

14. Apparatus according to claim 11, 12, or 13 wherein the measuring means includes a recording microammeter.

15. Apparatus according to any one of claims 11 to 14 wherein the electrode is a vitreous carbon electrode or a platinum electrode.

16. Apparatus according to claim 15 wherein the electrode is a platinum electrode and means are provided to maintain it at a potential of about 1040 mV with respect to a calomel electrode.

17. Apparatus according to any one of claims 11 to 16 and including means for automatically correcting for the effects of variation of the temperature of the cell.

18. Apparatus according to any one of claims 11 to 17 and including, disposed between the mixing means or mixture tank and the cell, means for reducing nitrate in the aqueous medium to nitrite.

19. Apparatus according to claim 18, wherein the reducing means is a copper coated cadmium column or a copper coated cadmium fluidized bed reactor.

20. Apparatus for determining nitrite and/or nitrate concentration in an aqueous medium substantially as hereinbefore described with reference to the accompanying drawings.