Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 508 of 707

Table II. Standard Potential (E0 Electrode Reaction Na+ + e- = Na CNO- + H2O + 2e- = CN- + 2OH- Zn2+ + 2e- = Zn 2H+ + 2e-= H2 Ag/AgCl electrode, 4N KCl Calomel electrode, sat KCl (Cr2O7)2- + 14H+ + 6e- = 2Cr3+ + 7H2 O ) E0 (V) -2.714 -0.97 -0.763 0.000 +0.199 +0.244 +1.33 the sample. When the glass (pH) and reference electrodes are paired, or built togeth- er as a combination electrode, and connected to a pH meter, the voltage developed at the electrode pair is amplified and displayed on a meter or a digital readout. The meter is standardized with a known solution called a buffer. A buffer is a specially prepared solution that resists changes in pH and has a specific pH value at a specific temperature. In Table I the component of the most common buffers is shown. When highly accurate pH measurements are required, temperature, meter stan- dardization, and electrode technique should be considered. Temperature has two effects on the accuracy of pH readings. The first effect is on the electrodes, which is corrected by either automatic temperature compensation (ATC) or by manually adjusting the temperature control on the pH meter. Temperature also effects the pH value of the buffer (which is used for standardization). Because samples do not change pH with temperature in the same way, the rela- tionship between temperature and pH must be determined experimentally. RESERVE YOUR SPACE NOW FOR 201 /1 Call for all the details. Arnie Hoffman 847-559-0909 Dan Ramage 847-699-8680 David Facinelli 727-866-9647 507

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