this taken from this link
http://www.lenntech.com/processes/disinfection/chemical/disinfectants-copper-silver-ionization.htmCopper-silver affectivity depends on the pH value of the water. At a pH value of 9, only one tenth of all Legionella bacteria are removed. When dissolved solid concentrations are high, silver will precipitate. This means silver ions are no longer available for disinfection.
Silver ions easily react with chlorines and nitrates that are present in the water, causing them to no longer be effective.
Some species of microorganisms can become resistant to silver ions. They can remove metal from their systems or convert it to a less toxic product. These microorganisms can become resistant to copper-silver ionization.
Although it is suggested that Legionella bacteria can develop resistance to copper-silver ionization, this disinfectant still appears to be effective for Legionella deactivation.
To affectively kill pathogenic micro organisms, copper and silver ions should be present in the entire water system. When the system is used little and the water flow is quite slow, or when there are dead-end points in the system, this can causes problems for disinfection.
and this taken from this link I just found which is also a good read... as my questions have been asked many times before....
http://www.overclockers.com/pc-water-coolant-chemistry-part-ii/In general, it is a good idea to drain and refill your PC water-cooling system at least once a year and, in some cases, every six months. Measuring the pH and observing the condition of tubing and waterblock internal surfaces can be a good indicator for when maintenance is required. The chemical properties of the water coolant will change over time as the additives get used up and the coolant becomes contaminated with corrosion byproducts.
Most people don’t have access to a laboratory pH meter, but inexpensive pH indicator strips can be purchased at some drug stores and swimming pool supply stores. When the pH drops below 7, or the water starts looking cloudy, or internal surfaces appear to be accumulating deposits – it’s time to clean the system and replace the coolant.