We etch inner layers using Cupric Chloride Etchant. We control the 
     chemistry with ORP and Conductivity probes.  The ORP adds Sodium 
     Chlorate and the Conductivity controls HCl adds.  
     
     We have experienced gas generation (chlorine or HCl fumes) when both 
     the ORP and Conductivity are in control.  The gas/fume release occurs 
     when the copper gets as high as 33 - 35 ounces per gallon.
     
     Standard procedure is to feed copper into the system when a chemical 
     out of balance situation results in gas generation.  Clearly in this 
     case this would be the wrong thing to do.
     
     Can anyone explain the chemical reaction behind a release of gas/fumes 
     when the copper is high?

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