WO1996023215A1

WO1996023215A1 - Sensor electrode for continuous measurement of hydrogen peroxide concentration

Info

Publication number: WO1996023215A1
Application number: PCT/BE1996/000006
Authority: WO
Inventors: Eduard Temmerman; Philippe Westbroek; Paul Kiekens
Original assignee: Universiteit Gent
Current assignee: Universiteit Gent
Priority date: 1995-01-26
Filing date: 1996-01-23
Publication date: 1996-08-01
Anticipated expiration: 1997-07-26
Also published as: BE1009053A5; AU4476496A

Abstract

A carbon electrode subjected successively to a mechanical and an electrochemical pretreatment, is suitable for the continuous measurement of the hydrogen peroxide concentration over a broad concentration range from 0.1 M to 3.0 M in many industrial applications.

Description

SENSOR ELECTRODE FOR CONTINUOUS MEASUREMENT OF HYDROGEN PEROXIDE CONCENTRATION

The present invention concerns a sensor elec- trode appropriate for continuous measurement of the con¬ centration of hydrogen peroxide over a concentration range from 0.1 M to 3.0 M in many applications.

The invention finds applications mainly in the textile industry for fabric bleaching and dyeing and in the paper industry. Other applications are possible, such as in the food sector.

Up to now, it has not been possible to monitor hydrogen peroxide in a continuous and automated way. To be sure, a number of techniques are available for continuous measurement of the hydrogen peroxide concen¬ tration but these do not offer satisfaction because there is too much interference and the response time of the known automatic measurement is too long.

On the one hand, it is very important in some applications (eg. bleaching) to keep the hydrogen peroxide concentration constant. On the other hand, ac- curate measurement (eg. in sewage water) is of the ut¬ most importance.

The most commonly applied technique for deter¬ mining the hydrogen peroxide concentration consists in redox titration with potassium permanganate in an acid medium. The purple permanganate ion is reduced here to manganese (II) , while hydrogen peroxide is oxidised to oxygen. This titration can be carried out by a worker who is often responsible for other tasks as well in the company. Purchasing an automatic titrator is often too expensive and the response time with this technique is also more than ten minutes. Besides this important drawback, human errors may occur such as misinterpreta¬ tion of titration data which entails faulty concentra¬ tion measurement. Also, reagents such as potassium permanganate and sulphuric acid are used. The invention aims at a sensor that can monitor the peroxide concen¬ tration in a direct way without need of sampling and sample pretreatment.

A second technique which does allow continuous monitoring of hydrogen peroxide is based on FIA (Flow Injection Analysis) techniques and colorimetry. They are described by Jola in Melliand Textilberichte 11 (1980) p. 931-936.

Solution is drawn off at constant flow rate and pumped through a by-pass. The solution is diluted with distil¬ led water via a vessel. The flow rate of this water too is constant. Dilution is important to come within the range of validity of the Law of Bouger-Lambert-Beer. Finally, a colour reagent is added which reacts with hy¬ drogen peroxide, also with constant flow rate. In the spectrophotometer, the absorption at suitable wavelength which is proportional to the hydrogen peroxide concen- tration is then measured. Although this technique al¬ lows continuous monitoring, it has a number of short¬ comings. Other particles in the solution reacting with the colour reagent may interfere. Insoluble particles such as fibre particles may affect absorption as well. Continuous measurement of the hydrogen peroxide concen¬ tration by means of the FIA-technique is based on the constant flow rate which is maintained in the pump pi¬ pes. The by-pass pipes however can narrow down because of deposition of the solution to be measured, which ir- revocably gives rise to faulty conclusions. The major drawback however is the high cost price of the tech¬ nique.

Finally, other techniques have been developed which are based on conductometry in combination with temperature and pH measurement. Sensitivity and speci¬ ficity of these techniques are not discussed and consi¬ dered inappropriate.

The sensor electrode according to the inven¬ tion is based on a specific voltammetric reaction of hy¬ drogen peroxide at a carbon electrode. This current is proportional to the hydrogen peroxide concentration.

Several types of carbon may be used such as graphite, pyrolytic graphite, glassy carbon, etc. In spite of the higher cost price, glassy carbon is prefer¬ red because of its high inertness (shows no reactions itself) , high hardness and hence good resistance to ero- sion by the measurement solution and because of its high stability at prolonged application of a potential.

This set-up allows continuous direct measure¬ ment of the hydrogen peroxide concentration, without need of sampling and sample pretreatment and without en¬ tailing extra costs for reagents.

In a special set-up suitable for industry, the carbon electrode is mounted in a by-pass. A pump then pumps solution through the by-pass. So, it comes down to the fact that the movement of the solution through the by-pass can create the same effect as the rotation of the sensor in a stagnant solution. The sensor, how¬ ever, can also be used without by-pass, even with turbu- lent liquid motion. At sufficient speed in the by-pass a so-called stationary condition can be reached. This means that the signal is independent of the time, which is an abso¬ lute condition for the technique to be applied in a con- tinuous way.

The reference electrode is a saturated calomel electrode, though the choice is variable on the condi¬ tion that the reference electrode can obtain a fixed po- tential in the solution. The cost price of the reference electrode as well will play an important part in commer¬ cialisation of the system. The counter electrode used is a graphite rod. This choice is triggered by the low cost price.

In the current application of the invention, the sensor electrode is mounted in a by-pass which is connected to the bath in which the process whose hydro¬ gen peroxide concentration is to be monitored, takes place.

The invention also concerns a procedure to pretreat a sensor electrode designed for continuous measurement of the hydrogen peroxide concentration, in which a carbon electrode is given a mechanical pretreat¬ ment followed by an electrochemical pretreatment; the first pretreatment consists in polishing a fresh surface of the carbon electrode coarse with sandpaper and subse¬ quently polishing it smooth on Al₂0₃ polishing powder with a grit size of 1, 0.3 and 0.05 μm successively.

In a specific application of the invention, the electrode surface is treated consecutively with SiC sandpaper for about 15 seconds and polishing products such as diamond paste with 1, 0.3 and 0.05 μm resp. for 10, 15 and 20 minutes resp.

These features and other features and details of the invention will become apparent from the accurate description given below, with reference to the accom¬ panying figures, which show a realisation of the inven¬ tion by way of example and not in a constraining sense.

In these figures :

Figure 1 is a principle scheme of the electrode configuration applied;

Figure 2 to figure 4 : test set-ups of the sensor electrode in a by-pass configuration;

Figure 5 is a life time test;

Figure 6 is a calibration curve.

In these figures identical reference signs re¬ fer to identical or similar elements.

As shown in figure 4, a carbon electrode of graphi¬ te, pyrolytic graphite or glassy carbon is mounted in a by-pass. By means of a pump 3, solution is pumped through the by-pass coming from a reservoir vessel 1. The carbon electrode 4 is embedded on a copper base 5 with epoxy resin 6 which joins well on the glassy carbon cylinder 7 (Fig. 2) . Various sensor electrodes can be placed in the by-pass, as shown in the top view of figu¬ re 3. The solutions used, in this case bleaching so¬ lutions (coming from TAG and Windelsbleiche) are poured in reservoir vessel 1. In an operative unit, this ves¬ sel plays the role of a- mixture vessel. By means of pump 3 the solution is sucked through by-pass 2 in which the reference and counter electrodes 8 and 9 are moun¬ ted. Via the upper pipe in figure 4, the solution comes back in vessel 1. In the operative bleaching unit, the bleaching bath 4 is situated between pump 3 and the re- servoir vessel 1.

The reaction mechanism of the oxidation of hy¬ drogen peroxide is given by the following six equa¬ tions :

HO, 0, HO, O, (1)

HO, O + OH (2)

0 + OH^" H0₂- ( 3 )

OH + OH^" o- + H₂0 ( 4 )

0^" + O^" 0₂ ^" + e^" ( 5 )

o₂- 0₂ + e^" ( 6 )

The oxygen requirement of the oxidation reac¬ tion is expressed in step 1.

Step 2 clarifies the decomposition of the hy¬ drogen peroxide. In step 3 hydrogen peroxide is formed again which shows that the hydrogen peroxide only intervenes as a catalyst.

In steps 3 and 4 unstable radicals occur which react with hydroxide. If little hydroxide is present, side reactions will become important. From this mechanism it was calculated that the reaction order of hydrogen peroxide is one which is logical given the linear rela¬ tionship between current and hydrogen peroxide concen- tration, that the reaction order of hydroxide anions is 4/3 hence requiring correction for pH-variations, that the temperature dependence is 9.5 %/°C and that the sig¬ nal is light dependent which can be explained from reac¬ tion 2 of the mechanism. Given the complete darkness in the by-pass, light effect does not influence the ap¬ plicability of the sensor.

The sensor electrode gives a simple linear re¬ lationship between signal and hydrogen peroxide concen- tration so that the latter can easily be established.

The sensor electrode is selective for hydrogen peroxide because a suitable potential has been chosen.

Measurement is done directly in the bath li¬ quor which means that neither sampling nor sample pre¬ treatment are necessary. Moreover nc additional and possibly expensive reagents are used.

Given the good stability of the sensor, the system can be relied on to be automated. Through appro¬ priate hard- and software, the whole must be capable of keeping the hydrogen peroxide concentration constant by calculating from the signal how much hydrogen peroxide must be added and by driving dosage pumps. The signal needs adjustment for temperature and pH changes.

The carbon is subjected to two subsequent pre- treatment procedures, a mechanical one followed by an electrochemical one. First, a fresh surface is obtained by polishing on SiC sandpaper for approx. fifteen se¬ conds. Next, the coarse surface has to be polished smooth by successive polishing on Al₂0₃ polishing powder with grit size 1; 0.3 and 0.05 μm for 10, 15 and 20 mi¬ nutes resp. Other polishing products such as diamond pastes can be applied as well. In order to remove the last rests of polishing powder, the electrode surface is cleaned by ultrasonic vibration.

The sensor is then subjected to an electro¬ chemical pretreatment . In order to let the electrode surface come into equilibrium with the measurement solu¬ tion, a tenfold cyclisation is carried out between the decomposition potentials of the solvent water. To this end, a triangular current can be applied or an alterna¬ ting current with appropriate amplitude and frequency. It is important that the rate of potential variation is smaller than 25 mV/s. After this cyclisation a poten- tial is applied for ten minutes situated passed the oxi¬ dation potential of water (eg. E=+1.8V) so that intense oxygen formation occurs.

Finally, the fixed potential is applied and the signal is left stabilising for 48 hours.

Oxygen formation at high potential is impor¬ tant because the voltammetric reaction mentioned above requires oxygen in its first step. Once the mechanism is in progress -._ sustains itself because the reaction product of this oxidation is oxygen as well. The sensor electrode gives a simple linear re¬ lationship between the signal and hydrogen peroxide con¬ centration so that it can quickly be established how strong the deviation is from the optimal hydrogen per- oxide concentration.

Figure 5 shows the result of a life time test carried out in the by-pass unit represented schemati¬ cally in figure 1. 25 current measurements over a pe- riod of three months are represented, with intermediate variation of temperature and pH. The rate of flow of the solution is chosen at 3 m/s and bounded by 1.5 m/s since the signal becomes time-dependent below this boun¬ dary and 5 m/s as upper boundary because at higher rates of flow turbulence occurs which has an adverse effect on the stability of the signal.

The sensor electrode according to the inven¬ tion, which is submerged in a pure alkaline hydrogen peroxide solution after the pretreatment described above, must be calibrated by taking a sample of the so¬ lution and titrating it with potassium permanganate. The corresponding parameters voltammetric signal, pH and temperature form the calibration point. Measurement points are then compared to this calibration point from which the concentration of hydrogen peroxide can be de¬ rived as unknown factor. These measurement points are shown in figure 6. During the first 20 days of the ex¬ periment, pure solution is regularly drawn off and re- placed by used solution from TAG and Windelsbleiche to check whether additives interfere. Variations in tempe¬ rature (warming up of the solution by the pump) and pH (addition of base or acid) occur as well. All signals obtained were therefore corrected via known conversion factors. Temperature and pH were measured with commer¬ cial instruments.

From this experiment one can conclude that the error on the measurement is limited to approx. 2 % all the time. This can be called very good.

Finally, it must be noted that at night the test unit was not in operation. The tests show that no new calibration is needed at restart the next morning, which is an important and positive result.

Claims

C L A I M S

1. Sensor electrode for continuous measurement of the hydrogen peroxide concentration over a concentra¬ tion range from 0.10 M to 3.0 M, characterized in that it consists of a carbon electrode based on the applica- tion of a voltammetric reaction in an alkaline medium and which is subjected to the following pretreatment procedures :

a mechanical treatment of sanding, polishing and smooth-polishing the immersed surface of the sen¬ sor electrode,

followed by an electrochemical pretreatment con¬ sisting in applying an alternating potential dif- ference between the measurement electrode and the counter electrode referred to versus the reference electrode and afterwards a constant potential, on the measurement electrode as anode, passed the oxidation potential of water so that oxygen forma- tion occurs.

2. Procedure to activate a sensor electrode made of carbon characterized in that a glassy carbon elec¬ trode is given a mechanical pretreatment followed by an electrochemical pretreatment, the first pretreatment consisting in polishing a fresh surface of the carbon electrode coarse with sandpaper and then polishing it smooth on Al₂0₃ polishing powder with grit diameter 1, 0.3 and 0.05 μm resp.

3. Procedure following conclusion 2, characte¬ rized in that part of the electrode surface is treated with SiC sandpaper for approx. 15 seconds and is subse- quently polished smooth with Al₂0₃ polishing powder or diamond pastes.

4. Procedure following conclusions 1 or 2, cha- racterized in that about ten cyclisations are carried out within the decomposition potentials of water on the carbon electrode.

5. Procedure following conclusion 4, characte- rized in that the rate of potential variation is smaller than 25 mV/S.