ClO2 chlorine dioxide how to know if it is still active or not ? by color ? by smell ? by reaction? my wife have ClO2 tablet form. she dilutes it to be put inside mister. each time she make the dilution:
package instruction:
1 tablet to 1 liter water =&gt; 100ppm ClO2
*we name this 100ppm as &quot;original soln&quot;
she takes 100ml from the &quot;Original soln&quot; to make 1 liter of solution to do household cleaning.
may i know how much ppm is the final solution that she used to do cleaning ?
i think should be 10ppm solution , right ?  10% mix.
thx
If you start with $\pu{100 mL}$ stock solution at $\pu{100 ppm}$ concentration, and dilute this by a factor of 10 to yield $\pu{1 L}$, then you are correct, the later one has a concentration of $\pu{10 ppm}$.  Keep both in a cool place.
As to check if the solution still is active, or not, and both stock and diluted solution at the expected concentration (which will decrease already if the solution is exposed to hot summer temperatures), you may use test strips you immerse dip into the solution, dry at room temperature, and compare against a scale.  An example were the following

(source)
Depending on the manufacturer, the scale of concentration to test may vary.