Hello NatrlHealth2013,

It all depends on how it was stored. If it was stored in a cool place out of direct light it should be fine. If it was stored at the upper end of room temperature in direct sunlight it may have lost some of its strength.

The best way to determine the remaining strength is to run a test by activating some and measuring the amount of chlorine dioxide produced. 22.4% sodium chlorite has 224000 PPM available chlorine dioxide. If you measure an amount and then drop the pH of it to below 2 you should release all of the available chlorine dioxide. Then you can dilute it and measure it and see if it has lost any strength.

For example if you put 1 ml of 22.4% sodium chlorite (fully activated) in 1 liter of water you should end up with a concentration of 224 PPM free chlorine dioxide.

citric acid should be fine as long as it is still a powder.

Tom