Electrochemical sensor for the specific detection of peroxyacetic acid in aqueous solutions using pulse amperometric methods

Abstract

An electrochemical sensor (A, A′) is specific for the detection of peroxyacetic acid in a solution which also contains hydrogen peroxide. A potential is applied between a reference electrode ( 120, 120′ ) and a working electrode ( 118, 118′ ). A read voltage (FIG. 7 ) is selectively pulsed across a counter electrode ( 122, 122′ ) and the working electrode. The current flowing between the working electrode and the counter electrode is dependent on the peroxyacetic acid concentration in the solution (FIG. 6 ). By careful selection of the read voltage, the contribution of hydrogen peroxide to the current flow is virtually negligible. The sensor effectively measures peroxyacetic acid concentrations in the range generally employed in sterilization and disinfection baths (100-3000 ppm.).

Having thus described the preferred embodiment, the invention is now claimed to be:  
     
         1 . A decontamination process comprising: 
 (a) circulating a decontaminant solution which includes peroxyacetic acid though a treatment vessel which contains items to be decontaminated;    (b) withdrawing a sample of the decontaminant solution into a chamber to contact a working electrode and a counter electrode;    (c) pulsing a voltage between the working electrode and the counter electrode at a selected voltage relative to a reference electrode, the voltage being selected such that an output current generated is correlated to a concentration of the peroxyacetic acid in the sample and substantially independent of a concentration of at least one other oxidizing species in the sample; and    (d) measuring the current generated.    
     
     
         2 . The process of  claim 1 , wherein the other oxidizing species is hydrogen peroxide.  
     
     
         3 . The process of  claim 1 , further including, prior to step (c): 
 applying a conditioning pulse sequence between the working electrode and the counter electrode, the conditioning pulse sequence including a negative voltage pulse and a positive voltage pulse.    
     
     
         4 . The process of  claim 1 , further including prior to step (a): 
 circulation a preconditioning solution including buffers and wetting agents;    with drawing a sample of the preconditioning solution into the chamber; and    pulsing voltages between the working and counter electrodes which electrochemically remove residues from the working electrode.    
     
     
         5 . The process of  claim 1 , further including, prior to step (c): 
 adjusting the temperature of the sample to a preselected temperature.    
     
     
         6 . The process of  claim 1 , further including, prior to step (c): 
 flushing the chamber with the decontaminant solution.    
     
     
         7 . The process of  claim 1 , wherein step (c) includes: 
 selecting the voltage in the diffusion limiting range.    
     
     
         8 . The process of  claim 1 , wherein step (c) includes: 
 pulsing a voltage of about −1.4 volts, relative to a silver/silver chloride reference electrode.    
     
     
         9 . The process of  claim 1 , wherein step (d) includes: 
 measuring the current generated at about ten to fifteen seconds after the start of the voltage pulse.    
     
     
         10 . The process of  claim 1 , wherein step c) includes: 
 pulsing the voltage at a rate of about 25 pulses/second.    
     
     
         11 . The process of  claim 1 , further including, after step (d): 
 signaling an indication of the current measured to a control system, which, in the event that the current measured is below a predetermined minimum level, conducts at least one of the following steps: 
 aborting the decontamination process;  
 extending the time of the decontamination process to compensate for the peroxyacetic acid concentration;  
 controlling the addition of additional peroxyacetic acid to the circulating decontaminant solution; and  
 providing a signal which indicates that the peroxyacetic acid concentration is below the predetermined minimum level.  
   
     
     
         12 . A method of detecting a first oxidizing species in a solution which also contains a second oxidizing species, the method comprising: 
 disposing a working electrode and a counter electrode in the solution to be tested;    pulsing a read voltage in the diffusion limiting range across the working electrode and the counter electrode; and    detecting current flowing between the working electrode and the counter electrode, the read voltage being selected such that the current flowing is substantially dependent on a concentration of the first oxidizing species and substantially independent of a concentration of the second oxidizing species in the solution.    
     
     
         13 . The method of  claim 12 , further including: 
 converting the detected current flow into an indication of the concentration of the first oxidizing species in the solution.    
     
     
         14 . The method of  claim 12 , wherein the read voltage is selected such that a contribution of the first oxidizing species to the current flowing is at least ten times that of an equivalent concentration of the second oxidizing species.  
     
     
         15 . The method of  claim 12 , further including: 
 detecting a temperature of the solution adjacent the electrodes; and    correcting the detected current flowing for a difference between the detected temperature and a preselected temperature.    
     
     
         16 . The method of  claim 12 , further including: 
 increasing the first oxidizing species concentration in the solution when the concentration is below a preselected minimum level.    
     
     
         17 . The method of  claim 12 , wherein the first oxidizing species includes peroxyacetic acid and the second oxidizing species includes hydrogen peroxide.  
     
     
         18 . The method of  claim 17 , wherein the peroxyacetic acid concentration is in the range of 100 to 3000 ppm.  
     
     
         19 . The method of  claim 17 , wherein the peroxyacetic acid concentration is determined in under one minute.  
     
     
         20 . The method of  claim 17 , wherein pulsing the read voltage, and detecting current flowing are repeated at intervals of from about fifteen to thirty seconds.  
     
     
         21 . The method of  claim 12 , further including prior to the pulsing step: 
 applying alternating 1 to 10 seconds voltage pulses of −1.5 to −2.5 volts and +2.0 to +3.5 volts between the working and counter electrodes.    
     
     
         22 . The method of  claim 21 , wherein the alternating voltage pulses are applied prior to the electrodes being disposed in the solution to be tested while the electrodes are disposed in a solution free of the oxidizing species.  
     
     
         23 . The method of  claim 22 , wherein the alternating pulses are pulses +2.5 volts for 4 seconds and −2.0 volts for 2 seconds.  
     
     
         24 . The method of  claim 11 , further including: 
 adding an electrolyte to the sample of the solution to be tested.    
     
     
         25 . A decontamination apparatus comprising: 
 a decontamination vessel which receives items to be decontaminated;    a fluid flow path which circulates a decontaminant in a solution comprising a first oxidizing species to the vessel;    a sensor system fluidly connected with the fluid flow path for specifically detecting the decontaminant in the decontaminant solution, the system comprising: 
 a chamber which receives a sample of the decontaminant solution from the fluid flow path,  
 a working electrode and a counter electrode disposed within the chamber to contact the sample of decontaminant solution; and  
 an amperometric controller electrically connected with the working and counter electrodes which: 
 selectively pulses a preselected read voltage across the working electrode and the counter electrode, and  
 detects an output current flowing in a circuit including the working electrode, the counter electrode and the solution, the read voltage being selected such that the output current is substantially dependent on a concentration of the first oxidizing species and substantially independent of a concentration of at least a second oxidizing species in the solution.  
 
   
     
     
         26 . The apparatus of  claim 25 , wherein the sensor system further includes: 
 a reference electrode disposed in the chamber, the amperometric controller maintaining a potential between the working electrode and the reference electrode.    
     
     
         27 . The apparatus of  claim 25 , further including: 
 a heater for heating the sample to a preselected temperature prior to pulsing the read voltage.    
     
     
         28 . The apparatus of  claim 25 , wherein the working electrode is formed from glassy carbon.  
     
     
         29 . The apparatus of  claim 25 , wherein the electrodes are laid down on a common substrate.  
     
     
         30 . The apparatus of  claim 25 , further including a thermistor for detecting temperatures in a portion of the solution adjacent the electrodes for correcting the measured current for temperature fluctuations.  
     
     
         31 . The apparatus of  claim 25 , further including a computer which receives current signals from the amperometric controller and determines a concentration of the first oxidizing species.  
     
     
         32 . The apparatus of  claim 25 , wherein the computer includes a look-up table which determines the concentration of the first oxidizing species from the current output at a given temperature.  
     
     
         33 . The apparatus of  claim 31 , further including a dispenser for dispensing additional amounts of the first oxidizing species into the solution, and wherein the computer directs the dispenser to dispense an additional amount of the first oxidizing species into the solution when a preselected minimum level of the first oxidizing species in the solution is detected.  
     
     
         34 . The appartus of  claim 25 , wherein the amperometric controller further applies alternating positive and negative voltage pulses across the working and counter electrodes for electrochemically cleaning the working electrode.