WO2010132957A1

WO2010132957A1 - Water analysis

Info

Publication number: WO2010132957A1
Application number: PCT/AU2010/000621
Authority: WO
Inventors: Michael Esler
Original assignee: Aqua Diagnostic Holdings Pty Ltd
Priority date: 2009-05-22
Filing date: 2010-05-24
Publication date: 2010-11-25
Also published as: AU2010251701B2; AU2010251701A1

Abstract

A method of determining chemical oxygen demand in saline water samples containing chloride ions by a photoelectrochemical method in which the photo electrode is a titanium dioxide sensor treated by deposition of a noble metal oxide preferably silver(l) oxide or palladium, (II) oxide.

Description

WATER ANALYSIS Field of the Invention

This invention relates to a method for determining oxygen demand of water using photoelectrochemical cells. In particular, the invention relates to a photoelectrochemical method of determining chemical oxygen demand of water samples having high chloride content such as sea water.

Background to the Invention

Nearly all domestic and industrial wastewater effluents contain organic compounds, which can cause detrimental oxygen depletion (or demand) in waterways into which the effluents are released. This demand is due largely to the oxidative biodegradation of organic compounds by naturally occurring microorganisms, which utilize the organic material as a food source. In this process, organic carbon is oxidised to carbon dioxide, while oxygen is consumed and reduced to water.

Standard analytical methodologies for the determination of aggregate properties such as oxygen demand in water are biochemical oxygen demand (BOD) and chemical oxygen demand (COD). BOD involves the use of heterotrophic microorganisms to oxidise organic material and thus estimate oxygen demand. COD uses strong chemical oxidising agents, such as dichromate or permanganate, to oxidise organic material. BOD analysis is carried out over five days and oxygen demand determined by titration or with an oxygen probe. COD measures dichromate or permanganate depletion by titration or spectrophotometry. Application WO2004/088305 discloses a photoelectrochemical method of detecting chemical oxygen demand as a measure of water quality using a titanium dioxide nanoparticulate semiconductor electrode. Titanium(IV) oxide (TiO₂) has been extensively used in photooxidation of organic compounds. TiO₂ is non- photocorrosive, non-toxic, inexpensive, relatively easily synthesised in its highly active catalytic nanoparticulate form, and is highly efficient in photooxidative degradation of organic compounds. This method is satisfactory for the analysis of water and wastewater samples which contain very low levels of the chloride ion CI^' (say, [Cl^"] < ~20 mg/L). When the chloride ion concentration in the sample to be analysed exceeds this level the COD measurement may suffer increased uncertainty due to interference of the chloride ion in the electrochemical measurement process.

A problem encountered in conducting assays using this method is dealing with interference from competing oxidisable chemical species other than organic carbon. Filtration of samples reduces interference from many species but the presence of chloride still remains a significant interference that must be dealt with. The conventional dichromate COD detection method deals with chloride interference by chemically removing the chloride ions. The principle is to add a chemical that can form compounds with Cl^" that are not oxidised by the dichromate ion, as exemplified by the following reactions

Hg ⁺(aq) + 2Cr_(aq) → HgCI₂ (aq),

Addition of Ag⁺ as the ionic salt Ag₂SO₄, removes Cl^" ion from solution by precipitating out as AgCI_(S). This is not oxidised to a large extent by the dichromate ion (Cr₂O₇ ^2"). Addition of Hg²⁺ as the ionic salt HgSO₄, to a solution containing Cl^", results in the formation of mercuric chloride (HgCI₂). HgCI₂ is not an ionic salt but rather is a triatomic molecule. When dissolved in water, the Cl atoms remain complexed to the Hg atom. The HgCI₂ molecule in solution is resistant to oxidation by the dichromate ion (Cr₂Or^2"). Because of its high degree of solubility in water HgCI₂ is one of the most toxic forms of mercury known, and therefore presents a considerable toxic waste disposal problem. The conventional dichromate COD method involves the use of expensive and toxic chemicals requiring careful disposal. For online applications, the system will need a sophisticated component to achieve in situ separation of precipitated AgCI, which, on one hand will significantly undermine the accuracy and reliability of the system, and on the other hand will increase both the capital and operational costs. For both Industry and environmental management, there is considerable need to be able to measure the levels of organic content (or contamination) in water containing low to high levels of chloride (e.g. in sea water). Chloride is a problem for organic content measurement in aqueous samples as current methods of analysis can't easily distinguish between organic and chloride content, without resorting to the use of toxic mercury.

Historically, the US EPA has approved a particular method of dichromate digestion for COD measurement. The EPA approved method requires the addition of mercury and silver salts to mask the chloride interference prior to analysis. Often this is accompanied by sample dilution in order to fully prevent unwanted oxidation of the chloride ions and to provide a solution capable of being analysed spectrophotometrically or by potentiometric titration. Thus low level organic content is not easily measured with any accuracy. This method also produces toxic salts that require specific materials handling procedures for disposal. Another established method for quantifiying organic contamination of water is total organic carbon (TOC) measurement. Difficulties in the presence of chloride also exist for TOC in that for persulphate based oxidation, false positives arise due to the inability to distinguish between chloride ions and the organic compounds in the solution. For high temperature oxidation based TOC, the chloride salts calcine and can cause instrumental errors through degraded operating performance, thus leading to high maintenance requirements for the technique and an inability to conduct regular on-line measurements.

WO2007/016740 discloses an improvement in the photoelectrochemical method of detecting chemical oxygen described in WO2004/088305 which deals with the interference by chlorine. In water samples containing chloride ions above 0.5mM concentration in which the samples are diluted and a known quantity of an organic substance is added to the diluted sample which is then subjected to an assay by a photoelectrochemical method using a titanium dioxide photoactive nanoparticulate semiconductor electrode and the chemical oxygen demand is measured in the same manner as disclosed in WO2004/088305, except that a known concentration organic solution is used to obtain the blank for calculation of net charge Q_nβt . With this organic addition method, the analytical signal is generated in exactly the same way as the photoelectrochemical method disclosed in WO2004/088305. However, this addition method requires some sort of prior knowledge of the range of chloride concentration and organic contents in the samples in order to determine how much organic material to add to ensure total suppression of the chloride interference, and thus an accurate COD measurement. In many cases, this is not a serious issue, however in environments such as sea water where the chloride content is high, high levels of organic addition and sample dilution are then required and this has an impact on the sensitivity achievable. Patent application WO2009/049366 discloses an enhancement of the method disclosed in WO2004/088305 which successfully overcomes the problem presented by chloride ion interference. That enhancement was to irradiate the TiO₂ sensor surface with pulsed-UV radiation rather than with continuous-UV radiation, as is done in the method of WO2004/088305. The pulsed-UV method facilitates the measurement of COD in water and wastewater samples with chloride concentrations ranging up to at least that found in natural seawater, (i.e., [Cl^"] = 21 ,000 mg/L). One disadvantage of the pulsed-UV method is that the time required for an analysis in this mode is considerably longer (say, by a factor of approximately three) than is required for the normal continuous-UV method used for chloride-free samples. There is a significant body of literature in both the scientific and patent domains, concerning the deposition onto TiO₂ nanoparticles of noble metal nanoparticles in their zero (i.e., metallic) oxidation state (e.g., Ag⁰, Pt⁰, Pd⁰ and Au⁰) or in higher oxidation states (e.g. Ag⁺¹, Pt⁺² and Pt⁺⁴, Pd⁺² and Au⁺¹). Such treatment Of TiO₂ has frequently been observed to increase the efficiency of (both the rate of and the completeness of) its oxidation of organics in bulk water or wastewater, relative to the efficiency observed when using non-treated TiO₂. USA patent 5872072 discloses a catalytic composition useful for decomposing malodorous compounds which includes titanium dioxide and an antimicrobial metal selected from silver copper and zinc. However, the literature is silent on modifications to TiO₂ which modify its photoelectrochemical behaviour with respect to the chloride ion. It is an object of this invention to provide a simpler means of dealing with chloride interference than those currently known.

Brief Description of the Invention

To this end the present invention provides a method of determining chemical oxygen demand in water samples containing chloride ions by a photoelectrochemical method in which the photo electrode is a titanium dioxide sensor treated with a noble metal compound.

The noble metal is selected from the group of gold, palladium, platinum and preferably silver. After treatment the titanium dioxide sensor surface includes an oxide of the noble metal and this is preferably a silver or palladium oxide. The Ag₂OTiO₂ and the PdOTiO₂ density ratio of the resulting Ag₂O-TiO₂ or PdO- TiO₂ composite material is preferably controlled by manipulating the deposition parameters within the range of 0.01 to 0.4 preferably 0.05 to 0.15.

This invention is partly predicated on the discovery that interference of chloride ions in the method disclosed in WO2004/088305 (referred to as PeCOD^® analysis) for measuring COD in water and wastewater manifests in at least two distinct ways, which are (a) signal suppression; and (b) signal tailing. These terms both refer to ways in which the oxidation profile measured electrochemically at the nanoparticulate-TiO₂ working electrode, (i.e., the /_work vs. time signal recorded by the instrument) is distorted, so as to yield an error in the calculation of COD from this profile. This invention is also predicated on the discovery that a TiO₂ sensor which has been treated by the deposition of silver(l) oxide (Ag₂O), will be far less sensitive to the presence of chloride ions in the water sample than a TiO₂ sensor which has not undergone such treatment with Ag₂O. Using such an Ag₂O-deposited TiO₂ sensor facilitates the rapid analysis of chloride containing samples using continuous (rather than pulsed) UV irradiation. This advantage applies to samples in the PeCOD^® measurement cell with chloride ion concentrations ranging up to at least [CI^']=700 mg/L. This yields a significant saving in time for the analysis of chloride containing samples compared to employing the pulsed-UV method of PCT2008/001529 mentioned above. In addition to this advantageous property, Ag₂O-treated TiO₂ COD sensors are found to linearise the instrument response function (i.e., measured Q_net vs. sample COD) of the PeCOD^® analyser which, with non-Ag₂O-treated TiO₂ is significantly (and reproducibly so) nonlinear. A linear instrument function facilitates the employment of simpler calibration protocols without risking the introduction of systematic measurement errors due to nonlinearity. A further advantage is that Ag₂O-treated TiO₂ COD sensors are found to render the PeCOD^® analysis method immune to interference from dissolved carbon dioxide in the water sample. In the PeCOD^® method dissolved carbon dioxide (CO₂) in a sample of water or wastewater can yield a spuriously high COD reading when using a non-Ag₂O-treated TiO₂ COD sensor. Although not a very large effect, it can be significant particularly if a small water sample has been left open to the atmosphere for a long time and has been allowed to come to a concentration equilibrium with atmospheric CO₂. A final advantage is that combining the Ag₂O-treatment of the TiO₂ sensor with the pulsed-UV approach yields a new pulsed method for analysis of samples containing chloride ranging up to at least [Cr]=21 ,000 mg/L, which is significantly faster than that disclosed earlier. Upon absorption of light by the TiO₂ photocatalyst, electrons in the valence band are promoted to the conduction band (e_cb ^") and "photoholes" are left in the valence band (h_vt>⁺). The photohole is a very powerful oxidizing agent (+3.1 V) that will readily lead to the seizure of an electron from a species adsorbed to the solid semi-conductor TiO₂. Thermodynamically, both organic compounds and water can be oxidized by the photoholes or surface trapped photoholes but usually organic compounds are more favourably oxidized, which leads to the mineralization of a wide range of organic compounds. This is described in application WO2004/088305 the contents of which are incorporated herein by reference. Owing to the strong oxidation power of photoholes, photocatalytic oxidation of organic compounds at the TiO₂ electrode leads to stoichiometric oxidation (degradation) of organic compounds as follows:

C_nH_7nOAN₇ SiP_n + bO₂ → nC0₂ + ^{m~k~3j ~2i~3h} H₂O + /cHX + ;NH₃ + .H₂SO₄ + JiH₃PO₄ where

. , m ~k-3j— 2i-3Λ e , „ . , _ , b - n Λ - + 2i + 2h

and where X represents a halogen atom. The numbers of carbon, hydrogen, oxygen, halogen, nitrogen, sulphur, and phosphorous atoms in the organic compound are represented by n, m, e, k, j, i and h, respectively. In order to minimize the degradation time and maximize the degradation efficiency, the photoelectrochemical catalytic degradation of organic matter is preferably carried out in a thin layer photoelectrochemical cell. This process is analogous to bulk electrolysis in which all of the analytes are electrolysed and Faraday's Law can be used to quantify the concentration by measuring the charge passed if the charge/current produced is originated from photoelectrochemical degradation of organic matter. That is: Q = \idt =nFVC where n refers to the number of electrons transferred during the photoelectrocatalytic degradation, and / is the photocurrent from the oxidation of organic compounds. F is the Faraday constant, while V and C are the sample volume and the concentration of organic compound respectively. The measured charge, Q, is a direct measure of the total amount of electrons transferred that result from the complete degradation of all compounds in the sample. Since one oxygen molecule is equivalent to 4 electrons transferred, the measured Q value can be easily converted into an equivalent O₂ concentration (or oxygen demand). The equivalent COD value can therefore be represented as:

COD {mg I L of O₂ ) = — ^- x 32000

AFV

This COD equation can be used to quantify the COD value of a sample since the charge, Q, can be obtained experimentally and for a given photoelectrochemical cell, the volume, V, is a known constant.

In another aspect the present invention provides a photoelectrochemical assay apparatus for determining oxygen demand of a water sample which consists of a) a flow through measuring cell; b) a photoactive titanium dioxide working electrode which has been treated by deposition with a noble metal oxide, preferably silver(l) oxide or palladium(ll) oxide and a counter electrode disposed in said cell; c) a UV light source, adapted to illuminate the photoactive working electrode either continuously or in pulses; d) control means to control the illumination of the working electrode, the applied potential and signal measurement e) current measuring means to measure the photocurrent at the working and counter electrodes f) analysis means to derive a measure of oxygen demand from the measurements made by the photocurrent measuring means. .

Preferably a reference electrode is also located in the measuring cell and the working electrode is a nanoparticulate semiconductor electrode preferably titanium dioxide. The flow rate is adjusted to optimise the sensitivity of the measurements. This cell design is based on that disclosed in application WO2004/088305 (marketed as PeCOD^®) with means to store the organic/electrolyte solution. The sample collection device preferably includes a filter to remove any large particulates or precipitated substances that may interfere with the operation of the cell.

The method of this invention is particularly applicable to measurement of COD/organic content in industrial outflows to sea, in power plant cooling water, and shipping waste water.

Detailed Description of the Invention

Preferred embodiments will be described with reference to the drawings in which:

Figure 1 illustrates the effect of Ag₂θ-treatment on TiU2 sensors, for analysing chloride containing samples;

Figures 2 and 3 further illustrate the effect of either Ag₂O- or PdO-treatment on

TiO₂ sensors, for chloride containing samples;

Figure 4 illustrates a suite of highly linear calibration functions for chloride containing standard solutions generated for an Ag₂O-treated-TiO₂ sensor; Figures 5 and 6 illustrate the improved linearity obtained for chloride-free calibration standard solutions when using Ag₂O-treated-TiO₂ as opposed to untreated TiO₂.

Methods for introducing Ag₂OtQ the TiO₂ sensor matrix

Standard nanoparticulate TiO₂ sensors for PeCOD^® analysis were modified by deposition Of Ag₂O particles onto the TiO₂. Two simple methods of photodeposition were employed and both gave similarly successful results for the

PeCOD^® analysis of chloride containing samples.

In the first method Of Ag₂O deposition, a few drops of AgNO₃ solution were added to a NaCI solution, forming a suspension of very fine AgCI particles. Before this white coloured AgCI suspension could agglomerate and precipitate out, a small quantity was placed onto the thin film TiO₂ sensor surface. The surface was then irradiated for a few minutes with UV-radiation (or natural sunlight), causing photoreduction of the Ag⁺¹ to fine Ag⁰ metal particles, which in turn were immediately hydrolysed to Ag₂O (Ag⁺¹) particles. The Ag₂O nanoparticles are deposited onto the TiO₂ surface and become embedded in the porous nanoparticulate TiO₂ matrix. The residual fluid was washed off the now Ag₂O- treated TiO₂ sensor, which could be immediately installed into a PeCOD^® instrument for use in analysis.

In the second method Of Ag₂O deposition, a few drops Of AgNO₃ solution were added directly to the TiO₂ sensor surface, then the excess fluid sponged off, leaving the TiO₂ thin film matrix moist with AgNO₃. The sensor was then placed under a UV-lamp (or direct sunlight) for a few minutes, long enough to photoreduce the Ag⁺' to Ag⁰ metal particles, which are in turn hydrolysed to Ag₂O nanoparticles distributed evenly through the TiO₂ sensor matrix. As above, once the residual fluid was washed off the Ag₂O-treated sensor could be installed into a PeCOD^® instrument for use in analysis. Sensors prepared using either of these simple methods were used to provide the Ag₂O/TiO₂ sensor data illustrated in Figures 1-6.

It would be convenient if the Ag₂O could be introduced to the TiO₂-water colloid system subsequently used to fabricate the thin layer sensor, rather than onto the already prepared, immobilised TiO₂ thin layer. However, the high temperature (70O⁰C) required to immobilise and calcine the TiO₂ colloid to yield the optimal anatase:rutile ratio renders this impossible. Ag₂O decomposes into Ag⁰ (metal) and oxygen gas at a temperature well below 700⁰C. The resulting thin film is a mixture of TiO₂ and Ag⁰ nanoparticles. Ag metal does not provide the chloride resistance properties in the sensor that Ag₂O provides. In Figure 1 , the effect of treating a sensor with silver(l) oxide is illustrated over the chloride range O < [CP] < 100 mg/L. In Figures 2 and 3, the range of chloride concentrations over which the Ag₂O/TiO₂ sensor behaviour was tested was increased to the much broader range O < [Cl^"] < 4000 mg/L. Figures 2 and 3 differ only in that the x-axis is linear in Figure 2 and is logarithmic in Figure 3. With reference to Figures 2 and 3, a set of reference standard solutions containing a fixed 120 mg/L COD and chloride variously at [Cl^'] = O₁ 20, 50, 100, 200, 500, 1000, 1500, 2000 and 4000 mg/L was analysed with an Ag₂O-treated TiO₂ sensor. The chloride-free 120 mg/L COD solution was first used to calibrate the PeCOD^® instrument. The instrument's COD readings declined with increasing [Cl^'] at a rate of d[COD]/d[CI^"] = -0.031 (mg/L COD)/(mg/L Cl^'). For reference, the corresponding chloride sensitivity gradient (illustrated on Figures 2 and 3 with an unbroken line) observed for a non-Ag₂O-treated sensor is d[COD]/d[CI-] « -1.23 (mg/L COD)/(mg/L Cl^')). From this we conclude that the Ag₂O-treatment is capable of reducing the sensor's vulnerability to chloride signal suppression interference by a factor of approximately 40.

Ag-treatment of TiOy shifts onset of chloride tailing interference to much higher ICH

When using an untreated TiO₂ sensor we observed that the oxidation profiles are seriously distorted by "tailing" due to chloride interference even at low chloride concentrations, [Cl^'] < 100 mg/L. However, when using an Ag₂O-treated TiO₂ sensor this tailing interference effect was not seen until much higher chloride levels, [Cl^'] > 800 mg/L. We conclude that Ag₂O-treatment of a TiO₂ sensor significantly reduces the sensor's vulnerability to the chloride tailing interference effect. Extensive subsequent testing in the laboratory has confirmed this result.

Ag-treatment of TiO₂ linearises the PeCOD^® response function For normal PeCOD^® analysis of non-chloride-containing solutions using a non- Ag₂O-treated sensor, it has been found that the instrument response function, (i.e., the Qn_et vs. COD function) is significantly nonlinear. Although the degree of nonlinearity is not large, it may nevertheless result in systematic COD measurement errors of 5-10 % if a conventional straight-line, 2-point, zero and span calibration protocol is employed. This systematic COD measurement error can be effectively removed by using a more sophisticated 3-5 point calibration protocol; however this introduces a significant degree of inconvenience. It would be better still to remove the nonlinearity at its source, than to work around it using sophisticated calibration protocols and polynomial arithmetic. We have found, rather unexpectedly, that treating the TiO₂ sensors with silver(l) oxide results in a linear PeCOD^® instrument response function. This applies to PeCOD^® Ag₂O-treated TiO₂ sensor analysis of both chloride-free and chloride- containing samples, up to at least the level [Cl^"] = 700 mg/L. Figure 4 illustrates a suite of highly linear calibration functions generated for an Ag₂O-treated-TiO₂ sensor. Six sets of reference standard COD/CI^' solutions were prepared. Set (a) consisted of solutions containing [COD] = 0, 10, 20, 50 and 75 mg/L COD and a chloride content fixed at [Cl^"] = 0 mg/L. Similarly, set (b) was the same except its fixed chloride content was [Cl^"] = 50 mg/L; and so on for set (c) [Cl^'] = 100 mg/L; set (d) [Cl^"] = 300 mg/L; set (e) [Cl^"] = 400 mg/L; and set (f) [Cl^"] = 500 mg/L. The correlation coefficient, R², yielded for the straight line of best fit for each of the 5- point calibration data sets were (to the fourth decimal place): (a) 1.0000; (b) 0.9997; (c) 0.9996; (d) 0.9995; (e) 0.9996; and (f) 0.9994. In Figure 5 a set of five COD (present as glucose) calibration reference standards at 0, 20, 40, 60 and 75 mg/L were analysed with a TiO₂ sensor (open circles, dotted line) and an Ag₂O-TiO₂ sensor (filled circles, unbroken line). The normalised (at [COD] = 75 mg/L) instrument response is plotted against [COD]. The correlation coefficients for the straight line of best fit for the TiO₂ and Ag₂O- TiO₂ generated data were R² = 0.9977 and 0.9997, respectively. Figure 6 illustrates the data yielded where the COD was present as sorbitol. The correlation coefficients for the straight line of best fit for the TiO₂ and Ag₂O-TiO₂ generated data were R² = 0.9989 and 1.0000, respectively. Other test compoundsshowed much the same relative behaviour for TiO₂ and Ag₂O-TiO₂ sensors.

Ag-treatment of TiOg decreases sensor sensitivity to interference from dissolved CO2

A further advantageous property acquired by the TiO₂ sensors by treating them with Ag₂O was that they became drastically less sensitive to interference from presence of dissolved carbon dioxide in the analysed sample. Carbon dioxide (CO₂), an inorganic species, is present in ambient air at a molar mixing ratio of approximately 380 μmol/mol. A water sample held in a container open to the ambient atmosphere will, over hours and days, gradually absorb CO₂ until it reaches an equilibrium with the air CO₂ content. CO₂ dissolved in the water sample will in turn reach an equilibrium within the fluid between the several species CO_2(aq), HCO₃ ^" _(aq) and CO₃ ^2"(_aq) depending on the temperature and pH. The bicarbonate (HCO^3" _(aq)) and carbonate (CO3^2'(_aq)) ions can be readily oxidised at the UV-irradiated TiO₂ surface to give a spurious (because not derived from the oxidation of an organic species) contribution to the Q_net signal collected by the PeCOD^® instrument. This is generally not a significant problem if normal laboratory sample handling practices are followed and samples are stored in closed containers when not in use. However, the interference from CO₂ can be a significant source of measurement uncertainty when analysing relatively clean water containing low levels of COD (< 20 mg/L). It has been found that Ag₂O- treated TiO₂ sensors are quite insensitive to the presence of dissolved CO₂ in water samples, virtually eliminating this as a source of measurement uncertainty.

Ag-treatment of TiOy decreases the analysis time using oulsed-UV irradiation for highly saline samples

It has been demonstrated already that treatment of TiO₂ sensors with Ag₂O extends the range of COD analysis in chloride-containing matrices when using continuous UV-irradiation from O mg/L < [Cl^"] < 80 mg/L to at least 0 mg/L < [Cl^"] < 700 mg/L. At chloride concentrations beyond 700 mg/L it may still necessary to use pulsed-UV irradiation to circumvent chloride interference. This allows analysis for COD in water and wastewater samples containing chloride at concentrations of at least 21 ,000 mg/L (the level typically found in seawater). We have found that the Ag₂O-treatment of TiO₂ sensors also brings an improvement to this type of analysis. Pulsed-UV analysis proceeds more rapidly, by 20-30 %, when using an Ag₂O-TiO₂ sensor as opposed to a TiO₂ sensor. This yields a significant time- saving for users of this method.

Possible mechanism of the advantageous effects arising from Ag₂O treatment of the TiO₂ sensor

The mitigation of interference effects arising from dissolved Cl^* and CO₂ on a TiO₂ sensor, by treating it with silver(l) oxide, have not been reported in the scientific literature before and are not well understood. The best we can do at present is speculate on what the mechanisms of these effects may be. It is believed that the photocatalytic oxidative action Of TiO₂ occurs predominantly at the solid-liquid interface, i.e., at the TiO₂ surface, through oxidative attack by the photohole (h_vb ⁺) in the TiO₂ valence band. Species having a propensity to adsorb onto the TiO₂ surface are typically oxidised more efficiently than those species that do not adsorb. We suspect that CT₁ and dissolved CO₂ are species that do adsorb to the TiO₂ surface. In doing so they may compete with the organic species in the solution for the photoactive TiO₂ sites where oxidation may occur. This may result in interference with the oxidation of the organics, such as we have observed using untreated TiO₂ sensors.

We suspect that deposition Of Ag₂O particles onto the TiO₂ changes the nature of the TiO₂ surface in such a way that Cl^', HCO₃ ^' and CO₃ ^2* are less prone to adsorb on this surface, and thereby less able to interfere in the COD measurement using this approach. We suspect that the deposition Of Ag₂O onto the TiO₂ changes the surface charge of the combined Ag₂O-TiO₂ surface, making it more negatively charged. Plausibly, the anions in solution perceive the Ag₂O-TiO₂ surface as being more electron rich than they would a bare TiO₂ surface and are less likely to adsorb to the former because of relatively repulsive electrostatic forces. If Cl^", HCO₃ ^* and CO₃ ^2" anions are significantly less likely to adsorb to a Ag₂O-treated TiO₂ surface than to a TiO₂ surface, then we can expect the Ag₂O-TiO₂ sensor to be subject to much less interference from these anions in the process of COD analysis. This is also a plausible explanation for the Ag₂O-treatment of a TiO₂ sensor linearising the previously nonlinear response function, if we consider that this nonlinearity may be due, indirectly, to HCO₃ ^" and CO₃ ^2'. Recall that the mineralisation (i.e., exhaustive oxidation) of the organics results in CO₂ as a major product. This product CO₂ will to some extent dissolve in the sample and move towards establishing a CO₂(_aq)/HCO₃ ^"(_aq)/CO₃ ^2"(_aq) equilibrium. This product HCO₃ ^" and CO₃ ^2" may be oxidised again at the TiO₂ surface to give CO₂, and a spurious contribution to the measured COD. Thus, the higher the concentration of organics initially in the cell, the more likely there is to be established this positive feedback loop based on the CO₂, HCO₃ ^' and CO₃ ^2" oxidation-reduction cycle in solution. This will manifest as the sort of concave upward nonlinear response function we observe for bare TiO₂ sensors, as illustrated in Figures 5 and 6. To the extent that Ag₂O treatment mitigates against the adsorption of HCO₃ ^" and CO₃ ^2" anions on the Ag₂O-TiO₂ surface, the nonlinearity should diminish. 77O₂ sensor treatment using oxides of metals other than silver

Silver is an element belonging to the Group 11 metals of the Periodic Table of Elements. It also belongs to a set of metal elements, the noble metals, from the same part of the Periodic Table, that frequently exhibit similar chemical properties. This group includes another Group 11 element Gold (Au), and the two Group 10 elements Platinum (Pt) and Palladium (Pd). The scientific literature reflects that treatment with these three other metals frequently lends similar properties to nanoparticulate Tiθ₂ (and other semiconductors) as does treatment with silver. It might reasonably be expected that, analogously, treatment Of TiO₂ sensors with the oxides of Au, Pt or Pd may lend similar advantageous properties to TiO₂ sensors as does treatment with Ag₂O. With this in mind palladium(ll) oxide was deposited onto TiO₂ sensors in order to test these sensors for enhanced chloride resistance. With reference to Figures 3 and 4, the chloride resistance of the PdO- TiO₂ sensor was characterised in the same way as it was for the Ag₂O-TiO₂ and untreated TiO₂ sensors, as described above. While the profile of the measured COD vs. [Cl^"] function for the PdO-TiO₂ sensor was quite different from that of Ag₂O-TiO₂ sensors, it did show very significantly enhanced chloride interference resistance with respect to untreated TiO₂. Indeed, the PdO-TiO₂ sensor continued to function as a COD sensor at 4000 mg/L chloride, with a much smaller degree of signal suppression than that shown by Ag₂O-TiO₂ sensors. From this it follows that PdO-TiO₂ sensors will facilitate the analysis of COD in seawater (containing -21 ,000 mg/L Cl^"), requiring only a relatively modest 5* dilution of the sample prior to PeCOD analysis. Preferably, for such analysis, the instrument would be calibrated with appropriately saline standard solutions rather than chloride-free solutions. Several methods of Pd and PdO deposition are well described in the literature. These include photodeposition approaches with starting materials such as PdNO₃ or other soluble palladium salts. There are also thermal deposition approaches, using similar starting materials. In the present case, an even simpler deposition method was used. Commercially sourced PdO nanoparticles were added to the 5% TiO₂/water colloid system normally used for thin film TiO₂ sensor fabrication, and well mixed. The mass ratio PdOTiO₂ in the resulting mixed nanoparticle suspension was approximately 15%. This mixture was used to fabricate slides by the usual dip-coating method followed by calcination at 700⁰C, as disclosed in WO2009/062248.

Aq-treatment may mitigate against interference from halide ions other than chloride

Chloride is an ionic species belonging to the halide group (from Group 17 of the Periodic Table of Elements). The other halide ions Fluoride (F^"), Bromide (Br^") and Iodide (I^") frequently exhibit very similar chemical properties to those of Chloride. We claim that from this it follows that Ag₂O-treatment of Tiθ₂ sensors may mitigate against interference by these other halide ions in the same way it mitigates against interference by chloride.

Agi- O-treatment of T1O2 sensors used in amperometric measurement of COD While the present application has been described with reference to a previously disclosed coulometric method for measuring COD (WO 2004/088305), the same application also discloses an amperometric COD measurement method. Furthermore, other related applications disclose amperometric methods for COD measurement (WO2008/077192, WO2008/077191 , and AU 2010900885). These methods are also based on the principle of photocatalytic oxidation of organics on a nanoparticulate TiO₂ surface. The advantages of treating the TiO₂ sensor with Ag₂O would also apply to these methods.

Those skilled in the art will realise that the present invention provides a robust analytical tool that can provide accurate measurement of COD in a short time without interference from competing species such as chloride. It will be evident to those practiced in the art that this novel new method also has potential application for other interfering species, such as other soluble counter ions (including bromides, iodides, sulphates, phosphates etc.) Those skilled in the art will also realise that this invention may be implemented in embodiments other than those described without departing from the core teachings of the invention. PeCOD^® is a trademark of Aqua Diagnostic Holdings PTY LTD

Claims

1. A method of determining chemical oxygen demand in water samples containing fluoride, bromide or iodide and/ or chloride ions by a photoelectrochemical method in which the photo electrode is a titanium dioxide sensor which has been treated with a noble metal compound.

2. A method of determining chemical oxygen demand in water samples containing fluoride, bromide or iodide and/ or chloride ions by a photoelectrochemical method in which the photo electrode is a titanium dioxide sensor in which a noble metal oxide is incorporated into the nanoparticulate Tiθ₂ material matrix by means of photodeposition, thermal deposition or chemical synthesis.

3. A method as claimed in claim 1 or 2 in which the noble metal is selected from silver or palladium.

4. Water quality assay apparatus for determining oxygen demand of a water sample which consists of a) a flow through measuring cell b) a photoactive titanium dioxide working electrode incorporating a noble metal oxide and a counter electrode disposed in said cell, c) a UV light source, adapted to illuminate the photoactive working electrode either continuously or in pulses d) control means to control the illumination of the working electrode, the applied potential and signal measurement e) current measuring means to measure the photocurrent at the working and counter electrodes f) analysis means to derive a measure of oxygen demand from the measurements made by the photocurrent measuring means.

5. Water quality assay apparatus as claimed in claim 4 in which a noble metal oxide is incorporated into the nanoparticulate TiO₂ material matrix by means of photodeposition, thermal deposition or chemical synthesis.

6. Water quality assay apparatus as claimed in claim 5 in which the noble metal oxide is selected from silver or palladium.

7. Water quality assay apparatus as claimed in claim 6 in which the noble metal oxide is silver(l) oxide and the Ag₂OiTiO₂ density ratio of the resulting Ag₂O-TiO₂ composite material is controlled by manipulating the deposition parameters.

8. Water quality assay apparatus as claimed in claim 6 in which the noble metal oxide is palladium(ll) oxide and the PdOTiO₂ density ratio of the resulting PdO-TiO₂ composite material is controlled by manipulating the deposition parameters.