ru 8 or os 8 catalyst the fe 3 cr 6 or ce 4 then oxidizes the mo 3 back to mo 6 the w 6 or v 5 color forming agent behaves similarly for the detection of co between 0 and 500 ppm a typical chemical system consists of in aqueous solution this composition is modified for other dynamic ranges the second part of the requirement for a co sensor is that it be specific all existent co sensors including the one defined in u s pat no 5 063 164 suffer from a variety of interferences of which hydrogen sulfide is most common for example palladium sulfide molybdenum sulfide and copper sulfide which would be formed in u s pat no 5 063 164 are all black brown which prohibits measurement of the yellow to blue color change when mo 6 is reduced to mo 3 iron sulfide which is yellow green can also cause some problems the solution therefore is to add a fourth component to the system which not only preferentially forms a sulfide but a white or colorless one which will not interfere with the co measurement to accomplish this sodium chloride 2 wt is incorporated into the system the sodium ion is the interference suppressing agent the four 4 component chemical system was extensively tested with a uv vis spectrophotometer the following table shows the results of these.