|MadSci Network: Environment & Ecology|
The presence of alkalinity in a water sample may be due to many different substances. However, for the sake of simplicity, the presence of bicarbonate, carbonate, and hydroxide ions is commonly considered as alkalinity. The points of change in color of phenolphthalein and methyl orange indicators, which occur at pH 8.3 and pH 4.3, provide standard reference points which are almost universallly used to express alkalinity. From the form of your question, you realize that two samples of water with the same pH can have widely different alkalinity values. The standard calculations for pH are simple for sodium hydroxide solutions, but get more difficult for buffer solutions such as carbonate/bicarbonate solutions. The easiest way to make the kind of solutions you requested is to add dilute sodium hydroxide solution to distilled water while measuring the pH on a pH meter. This "titration" can give you a solution of pH 8.5 to 12 with very little chemical. A pH 10 NaOH solution has 10^-10 moles per liter Hydronium (H+) ion, and 10^-4 moles per liter Hydroxide ion (OH-). This would take only 10^-4 X 40,000 mg/mole or 4 mg of NaOH per liter of solution. More concentrated solutions of NaOH can be dangerous and in any case approprate laboratory safety equipment must be worn. To make a pH 10 Sodium Carbonate solution would require more calculations, and more Na2CO3. The emperical approach seems appropriate for this level of non-major chemistry laboratory, however. Secure the desired volume of distilled water and slowly add the Na2CO3 soluiton, while stirring, with pH electrodes immersed. Continue adding the carbonate solution until the pH reaches the value you achieved with the NaOH solution. NOTE: The carbonate will form some bicarbonate and the pH of this buffer will not rise much above 12 no matter how concentrated the solution. For example, a 0.1 N Na2CO3 solution has a pH of 11.6. This would requrie approximately 10.6 grams of sodium carbonate per liter of distilled water. You should be able to get the procedure for titrating the alkalinity from any analytical chemistry text or laboratory manual. You can contact me directly at LONBROUSE@AOL.COM if you have any further questions about this subject. References: BETZ Handbook of Industrial Water Conditioning 8th Edition, 1980. CRC Handbook, Chemical Rubber Company, 65th Editon, 1985.
Try the links in the MadSci Library for more information on Environment & Ecology.