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Chemical Engineering
Course Code
Joe Kim

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Measurement of Water Quality 2I03 Measurements pH Lab By: Amrit Aulak Student Number: 1150270 Experimental Work: Wednesday, January 23 , 2013 from 2:30 - 5:30 pm Date Submitted: Wednesday, January 30 , 2013 CHEM 2I03 Instructor: Dr. Jones 2 Purpose/ Introduction The purpose of this lab is to determine the variation introduced with sampling methods, specifically concerning a pH sensor. By adding an acidic substance in a large water container, factors such as mixing and height of sampling can be examined. The glass pH sensor operates as a potentiometric electrode as it measures the voltage created from the hydrogen ions. [1] Using the rearranged Nernst equation we can relate pH with electric potential: [1] Where the calibration electric potential (E’’) and electric potential (E) has the units volts and the temperature (T) is represented in Kelvin. Thus, the pH meter measures the electric potential and temperature, and outputs a varying value of pH. During the experiment, samples were taken every 120 seconds for three different heights and one pipette sample, and this was done for three trials. Results and Discussion The calibration curve is a straight line between the three buffers, as shown in Figure 1. This is a reasonable relationship as electric potential and pH are linearly related, as evidenced by the Nernst Equation [1]. Any small discrepancies were most likely caused by incorrect voltage readings, or experimentally if the pH meter was not rinsed thoroughly when being transferred to a new solution. 3 As evidenced by Figure 2, there was a large variation in pH with all the measurements. The range of pH spanned from 3.76 to 7.38, meaning a change in hydrogen ion concentration of approximately 3.5 orders of magnitude. The most significant error resulted from using tap water instead of distilled water during trial 2, altering the composition to the point where the lemon wedge had negligible impact on the pH. When mixed thoroughly, the pH noticeably increased three times, decreased twice and remained almost constant for the other four trials. Overall, there seems to be no pattern for the results of mixing the apparatus. The pH of the measurements taken at 180 mm consistently had a higher reading, caused by more volume between the sample points and the lemon. By using the non-graduated pipette, we displace more water and cause more motion in the water apparatus. [1] Comparing Figure 5, we can see that using a larger sample size actually created less error. This is proven with a standard deviation of 0.186 as compared to 0.207 of the sampling thief, and a variance of 8.49 as compared to the variance of 7.60 of the sampling thief. The pipette has a large volume, and when inserted, displacing more liquid, which makes it a more accurate representation of mixed condition. To solve for the difference in volume displaced we use the cylindrical volume equation: Where the volume (V) is in mL and the height/ diameter (d/h) is in cm. The average diameter was 2.35 cm for the glass vials, and the total height for the sampling thief and pipette is 34.1 cm and 83.7 cm respectively. 4 V s 148 mL and V = p63 mL. Thus, the difference in volume taken is 215 mL. When taking more frequent sampling, the solution is disturbed more often. This increases the chances of experimental error, caused by random fluctuations of the pH and error due to rushed sampling. Again, there is no clear relationship between the different frequency of the samples or the well-mixed solution. Tables Figure 1: Calibration curve comparing pH vs. potential voltage in millivolts (mV) of three buffers solutions stated as having pH values of 4, 7, and 10 respectively. Created from Table 1 data
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