Marshall wrote: > I was thinking that if you add salt to silver ions, the ions will become AgCl. > That leaves Na, which should react with the water and produce NaOH, sodium > hydroxide, a powerful base. > > So I figured I would verify this hypothesis by measuring the pH before and > after > adding salt. I feared that the NaCl would buffer the final result so that > the ph > would end up at 7 no matter what. > > I started with some CS that had been sitting around for a week or so. The pH > measured 6, which isn't surprising since it had likely absorbed some CO2 > during > that time. > > I added a small amount of salt and measured the pH again. This time it was 8 > (not a lot of accuracy when comparing colors, but easy to see the difference > between yellow and blue ). > > So that seemed to validate the theory. But I still needed a control. I found > some distilled water in the bottom of one of my jugs, so I tested it. Since > it > was a small amount of water the pH was around 5. I then added a small amount > of > salt. Well the pH went up to 8, just like the CS. > > Now I am confused. Shouldn't NaCl be neutral, and also a pretty good buffer? > Why did the pH go to 8 instead of 7? > > Any ideas?
The problem may concern the fact that measuring the pH of low ionic solutions causes erroneous readings. pH measurements depend on the presents of ions. If insufficient ions are present, then the reading just wanders around. Any measurement of DI water will cause this. In order to accurately measure low ionic solution requires that additional ions be added. The solution that does this is called an "ionic strength adjuster" or ISA. The ISA must be added uniformly (i.e. 2 mL per 100 mL of sample) to both the sample to be measured and the buffer solutions used to calibrate the pH meter. A common practice is to use buffer solutions that were created specifically for use with an ISA. Some companies sell kits that include the ISA and buffer solutions with ISA included. These kits are referred to as "pure water test" kits. Orion sells such a kit. Usually the relatively low ionic content of cs requires the use of an ISA to accurately measure pH. frank key -- The silver-list is a moderated forum for discussion of colloidal silver. To join or quit silver-list or silver-digest send an e-mail message to: silver-list-requ...@eskimo.com -or- silver-digest-requ...@eskimo.com with the word subscribe or unsubscribe in the SUBJECT line. To post, address your message to: silver-list@eskimo.com Silver-list archive: http://escribe.com/health/thesilverlist/index.html List maintainer: Mike Devour <mdev...@eskimo.com>
