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D6423_10

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Make sure sample is at 22 degrees Celsius, plus or minus 2 degrees Celsius. Fill the electrode as needed with 3 molar aqueous potassium chloride solution supplied with the electrode. Standardize the assembly with two reference buffer solutions as described in 9.3 and 9.4 of the written standard. Start with a new aliquot, fresh portion, of ethanol hydrocarbon blend sample for each measurement. Since many ethanol hydrocarbon blend samples are poorly buffered and have low ionic strength, it is important to use a new aliquot for each measurement for best accuracy. Place about 60 milliliters of sample in a 100-milliliter beaker containing an approximately sized magnetic stirring bar and either a thermometer or an Automatic Temperature Compensation-- ATC-- probe to ensure the sample is at 22 degrees Celsius, plus or minus 2 degrees Celsius. Remove the electrode from the pH 7.00 buffer and rinse the electrode with distilled water into a waste container. Blot the electrode to remove excess water. Insert the electrode into the stirred ethanol hydrocarbon blend sample at the proper depth to ensure an accurate reading. Some probes have an etch line that shall be submerged in a sample. Start the timer and measure the pHe at exactly 30 seconds, plus or minus 1 second. Stirring shall be rapid enough to cause a small to moderate vortex 6 millimeters to 8 millimeters deep. Note that the reading will still be changing slowly at 30 seconds due to dehydration of the glass pH electrode, which causes a shift in its output. If the reading is not rising at the 30-second point for low-pHe ethanol hydrocarbon blend, pHe less than 5, or gives poor repeatability in some solutions, those with low buffering capacity, the electrode probably needs to be cleaned or replaced. Remove the electrode from the sample and rinse it with distilled water into a waste container. Return the electrode to the pH 7.00 buffer to rehydrate the glass pH electrode. Soak the electrode in the buffer for 30 seconds or more if required to get the reading to fall between 6.90 and 7.10. Recalibrate to pH 7.00 if the pH is outside of the range of 6.90 to 7.10 after 5 minutes. Return to 9.2 of the written standard. Clean and rehydrate the electrode as described in 9.2 of the written standard after 10 alcohol samples have been measured. The electrode can give poor readability or high results in alcohols if not periodically cleaned and rehydrated in acid and base before use. Start with 10.4 of the written standard for the next sample.

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Duration: 3 minutes and 40 seconds
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Language: English
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Posted by: abuckmaster on Sep 14, 2018

D6423_10

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