"Jonathan B. Britten" <jbrit...@cc.nakamura-u.ac.jp> wrote:
> Experts tell me that even dark glass jugs of distilled water, sold > specifically for scientific experiments, change in pH over time > due to leaching from the glass itself. Regardless of the label on > the bottle, researchers have to check the pH before experimenting. > If the pH has changed, there must have been something introduced > to the water; what exactly I don't know. >JBB Hi Jonathan, The ph is next to impossible to measure in ultra pure water due to the low conductance. You can only measure the conductance and say the ph lies somewhere in a range between two values. There is a triangle that describes this on the following page. Also, ordinary carbon dioxide makes dw slightly acidic so it will dissolve contaminants in the glass. The way to minimize this is to use pure crystal quartz, which you can buy, but it is very expensive. Oddly enough, there is a species of bacteria that thrives in ultra pure water. Where they get their nutrients is a mystery, but they form a film that protects them and are very difficult to remove from semiconductor fabrication equipment. This can wreak havoc on the yields. This whole topic is fascinating and can fill many books. Just for reference, I will copy this section as it comes up often enough to warrant reading: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ pH-value Pure water by definition is slightly acidic and distilled water will test out around pH 5.8. The reason is that distilled water dissolves carbon dioxide from the air. It dissolves carbon dioxide until it is in dynamic equilibrium with the atmosphere. That means that the amount being dissolved balances the amount coming out of solution. The total amount in the water is determined by the concentration in the atmosphere. The dissolved carbon dioxide reacts with the water and finally forms carbonic acid. 2H2O + CO2 --> H2O + H2CO3 (carbonic acid) --> (H30+) (charged acidified water) + (HCO3-) (charged bicarbonate ion) Only recently been produced distilled water has a pH-value of approximately 7, but affected by the presence of carbon dioxide it will reach a slightly acidic pH-value within a couple of hours. Additional, it is important to mention that the pH of ultra-pure water is difficult to measure. Not only does high-purity water rapidly pick up contaminants - such as carbon dioxide (CO2) - that affect its pH, but it also has a low conductivity that can affect the accuracy of pH meters. For instance, absorption of just a few ppm of CO2 can cause the pH of ultra-pure water to drop to 4.5, although the water is still of essentially high quality. The most accurate estimation of the pH of ultra-pure water is obtained by measuring its resistance; for a given resistance, the pH must lie between certain limits. For example, if the resistance is 10.0 MWcm, the pH must lie between 6.6 and 7.6. The relationship between the resistance and pH of high-purity water is shown in the figure below. [2] (mrm: this is the triangle I spoke of) http://www.lenntech.com/deionised-demineralised-water.htm ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Regards, Mike M. http://silversol.freewebpage.org/index.htm http://silversol.freewebpage.org/spice/xtal/clapp.htm -- The Silver List is a moderated forum for discussing Colloidal Silver. Instructions for unsubscribing are posted at: http://silverlist.org To post, address your message to: silver-list@eskimo.com Address Off-Topic messages to: silver-off-topic-l...@eskimo.com The Silver List and Off Topic List archives are currently down... List maintainer: Mike Devour <mdev...@eskimo.com>
