The Finnpipette micropipettes used in this lab are a convenient way to measure and dispense small volumes of liquids. Disposable polyethylene tips are used with these pipettes. The teaching assistant will demonstrate how to attach the tips, how to pick up liquids and dispense them, and how to discard the used tips. In later lab exercises it will be important to be able to use these pipettes correctly in order to properly measure solutions and prevent contamination of samples.

         Check that the analytical balance you will use is level. Set the 40 - 200 µl pipette to 200 µl and use it to pick up a clean tip. Place a clean weighing boat on the balance and tare the reading to zero (depending on the balance you are using, you may have to open the glass doors in order to place things on the balance; it's a good idea to close the doors when taking readings to prevent interference from air currents). Add 200 µl of deionized water (DI water) and record the reading. Repeat this for a total of five measurements (either remove the weighing boat, empty it, dry it, and re-tare it, or continue adding water to the weighing boat and re-tare it in between measurements -- which method do you think is more accurate?). Set the 200 - 1000 µl pipette to 1000 µl and record 5 measurements. Record the temperature of the DI water. The teaching assistant will have a chart of the density of water as a function of temperature. Use this chart and record the density of the water you have used for these measurements.

         In order to start the calibration, first set the temperature. Turn on the pH meter and use the arrow keys to enter the temperature of the water you measured above. Then press the button on the bottom right hand of the meter marked "2nd" and then the button marked "cal". The display will show some numbers and then "P1", which means the meter is ready for the first calibration point. Rinse the probe with pH 7.00 buffer and place it in the test tube with the pH 7.00 solution. You should always use the pH 7 buffer for the first point of the calibration. Wait for the reading to stabilize (some of the meters beep when the reading is stable, flash the reading value and display the word "ready" next to the reading).

         These pH meters are set to recognize standard buffer solutions of 4.01,7 and 10. Once the reading stabilizes, you should only have to press the "yes" key twice. If a reading other than the one desired is shown, press the “yes” key once and then use the arrow/”yes” keys in combination to select the appropriate values. The display should flash the value of the buffer solution at the temperature you've set (see the side of the large pH buffer bottles to see how the buffer pH changes with temperature). The display should now show "P2", which means the meter is ready for the second calibration point. Rinse the probe with pH 4.01 buffer and place it in the test tube with the pH 4.01 solution and wait for the reading to stabilize and then beep. Again, you should only have to press the "yes" key twice. The two-point calibration is complete, and the display should show a slope value (disappears after a few seconds). The model 290A meter may ask for a third calibration point (P3). Press the "measure" key to skip the third point. Once calibration is completed, the meter will display a slope reading. This value should be at least > 0.90. If not, calibration should be repeated until a high enough slope is obtained (should be close to 0.997). If this takes more than two tries, consult the TA.

         When the battery is replaced, the meters automatically re-set themselves to shut off 10 minutes after the last button is pressed. This should not affect the calibration, but you should double-check the calibration by placing the probe back into a buffer to check after the meter has been turned back on.

         If something went wrong with the calibration, an error code will appear (e.g. E-23) accompanied by several beeps and you will have to perform the calibration again. If the problem persists, consult the TA. Checking the calibration periodically is a good idea. If one of the buffer solutions does not read within a couple one-hundredths of the calibration value, you should recalibrate.

Electrode care: In between measurements (or when the pH portion of this experiment is completed), leave the probe soaking in the pH 4.01 buffer. For storage, fill the electrode cap half-way with storage solution (KCl) and cap the electrode. To recondition a dried or drifting electrode, store in a test tube with storage solution for an hour. Do not leave the probe for an extended period of time in DI water. To check if electrode needs reconditioning or replacement, you can perform a millivolt test with pH 7 buffer (press the "mode" button once while reading, the value shuld be within -20 to +20 mV for best results; press "mode" again twice to return to the pH reading display), or you can perform a calibration and check for suitable values. It is also good to check the millivolt drift in pH 7 buffer to assess stability - should not vary by more than 3 mV relative reading.

         Now that the pH probe has been calibrated, you can measure the pH of the diluted acid solution the teaching assistant has provided. Rinse the pH probe with 0.5 ml of the acid solution and then place it in a test tube with approximately 1 ml of the acid solution. Take three measurements of the pH of this solution. Use the same tube with a fresh portion of acid each time, rinsing the probe with the dilute acid solution between each measurement.

         You now want to make three measurements of a 1/10 dilution of this acid solution. Prepare enough diluted acid (mix well!) to make three pH measurements, remembering you need to rinse the electrode with solution prior to each measurement. Make the pH measurements and record the results in your lab notebook (you should be recording all procedures and results there).

         Measure the pH of the DI water (three times) and record the result in your lab notebook.

         The teaching assistant has provided a bottle of an inorganic salt. The molecular weight of the salt is marked on the bottle. Each group should make a 0.1 M solution of this salt. When you're ready to make the solution, use one of the spatulas to transfer the salt from the bottle. Remember not to put any chemical back into the original salt container, and clean up anything you may spill on the balance. The teaching assistant can show you a way to steady your hand as you transfer chemicals from the spatula into the weighing boat.

         Recheck your calibration using fresh pH 7 and pH 4.01 buffers. Measure the pH of the solution you've made and record the results in your lab notebook.

         Using the statistical methods we've discussed in class and the material in Chapters 2-4 of the Fundamentals of Analytical Chemistry text by Skoog, West and Holler, answer the following questions:

1. What are the means and the 95% confidence limits for the pipette measurements?
   What is the 95% confidence limit of your pH measurements for cloud/fog samples with pH < 4? Use pooled statistics for data from your group and other AT 560 groups to obtain a pooled standard deviation.
2. Are the pH measurements of the two cloud/fog samples you measured different at the 95% confidence level? Be sure to perform a hypothesis test; don’t just check whether the confidence intervals overlap.
3. Compare your measurements of the cloudwater sample pH values with those obtained by other lab groups. Are your results statistically different?

• Comment on your experience with any difficulties measuring the pH of individual samples.
• What effect did the salt addition have on your ability to measure the deionized water pH value?
• Compare the pH measurements of the original and diluted acid solutions. Is the pH difference what you expected?