pH lab outline and rubric

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pH lab outline and rubric

Dr. Kohnken AP chemistry

    Set 1 calibrate pH meters

    Care, use, and calibration of pH meters

     pH meters come in several shapes, but all consist of an electrode pair and a readout device. The

    electrode pair is usually in a thin glass probe termed a combination electrode. It is fragile. Contained in that

    probe is a saturated KCl solution and a reference electrode. Many electrodes have a vent hole near the top to

    vent H gas and allow refilling. Some electrodes have caps to fit over the end, which protects the electrode and 2

    holds in storage solution. Others must be stored in a separate container.

Storage: electrodes are stored with the end immersed in a mixture of saturated KCl and pH 7 solution, and the

    vent hole should be closed. Do not let the electrode, inside or out, get dried out.

    Use: rinse off the outside of the electrode with dHO. place electrode in the solution for which you seek to 2determine the pH. Turn on the meter and measure pH. When done, turn off the meter, remove the electrode,

    rinse it off, and return it to storage solution. If using a stir bar in your solution, make sure that it will not contact

    the electrode as they are fragile.

Calibration: Before use, you should calibrate the pH meter. Rinse off the electrode and place in standardized

    pH 7 solution. Turn on the meter. You may need to allow it to warm up some. Adjust the readout to 7.

     On the digital meters, there is a screw on top which is turned to adjust the pH 7 reading. On the needle

    deflection meters, there is a dial labeled „intercept‟ to adjust the readout to 7.

     Rinse off the electrode and put into pH 4 or pH 10 standardized solution (if I want to measure solutions

    less than pH 7, then I use pH 4 standard). Adjust the readout to 4 (or 10). On the digital meters, there is another

    screw on top to make this adjustment. On the needle deflection meters, there is another dial, labeled „slope‟ to

    adjust the readout or a screw in back which is somewhat awkward to get at.

    If you have to do a lot of turning of the screws, chances are you are nowhere near the proper calibration. Stop

    immediately and find me.

    When in doubt, read the instructions or ask the teacher!

Measure 3 of the pH solutions used for the indicator lab (nominally pH 2, 4, 6, 8, 10, and 12) using digital pH

    meters. Measure at least one of the solutions with an analog pH meter (they have better electrodes and are

    consequently both more accurate and precise). Measure pH of distilled water.

    Safety and disposal all chemicals may be flushed down the drain. Wear goggles.

Report: turn in your data. We will make a combined chart of the results. (10 pts)

Dr. Kohnken AP chemistry

    Set 2 indicator solutions

    There are 4 indicator solutions (0.04% bromphenol blue, 0.1% phenophthalein, 0.1% phenol red, and

    0.1% thymol blue) and 6 solutions of varying pH (approximately 2, 4, 6, 8, 10, and 12). Use 24 well plates to

    test all combinations.

     For your lab write-up, show what you observed and how you interpret it. When a color is constant,

    what information does that give you? When the color changes, what information does that give you? Indicate

    the pH range over which you think each indicator is useful for telling pH. How could you have done the

    experiment differently to get a more precise value for your pH range? The write-up for this set will be due


    Safety and disposal all chemicals may be flushed down the drain. Wear goggles.


    Observations - 12 pts

    Useful/interpretation - 6 pts

    More precise - 2 pts

We may have a lab practical in which you will have to determine the Ka of an indicator. If you‟d like to

    propose how you will determine the Ka of an indicator, and you‟d like some feedback, do so now before the

    actual lab activity! However, I'm likely to answer questions with questions - I'd like you to try to develop a

    procedure. My job is to guide you.

Dr. Kohnken AP chemistry

    Set 2.5 - pH challenge (just for fun!)

Think that you can mix 1 M HCl and 1 M NaOH and get pH = 7? Give it a shot. Use 50 ml of each. If you

    measure just right, you may get it. Check the pH with the pH meter.

    "You've got to ask yourself one question- 'Do I feel lucky?' Well, do ya, punk?" Harry Callahan (Clint

    Eastwood) in “Dirty Harry” (1971) Warner Bros.

    Safety and disposal all chemicals may be flushed down the drain. Wear goggles.

Dr. Kohnken AP chemistry

    Set 3 - Buffer

    Prepare a buffer and measure its pH discuss your choice with me first, and show me the calculations for its preparation! Consult the list of possible buffer compounds and their Ka values shown below. In this case, you

    get to choose the pH, the buffer, and the amounts of acid and base against which to test your buffer.

    Safety and disposal all chemicals may be flushed down the drain. Wear goggles.

Buffer preparation - Throughout the scientific community, buffered solutions are prepared for a variety of

    purposes. The particular choice of buffer depends on both the chemical attributes of the buffering substance,

    and the use of the buffer. For our purposes, I want you to be able to select and prepare a buffer at a given pH,

    and predict, based on your understanding of pH equilibria, the effect on pH of the addition of an acid and a base.

You will prepare a 200 ml solution of an appropriate substance buffered at _________ pH at 25 ?C. Total

    concentration of the two buffer components combined should be < 0.25 M.

Describe how you would make it. Show calculations and rationale to support your choice.

Separate your buffer into two 100 ml aliquots.

    What would the pH be after addition of __________ ml 0.1 M HCl to an aliquot? Show calculations.

What would the pH be after addition of __________ ml of 0.1 M NaOH to an aliquot? Show calculations.

What would be the pH of your 200 ml buffer if it were titrated to its equivalence point using 1M HCl or 1M 3-NaOH as appropriate? If you chose a polyprotic buffer, titrate to the final conjugate base (for example, PO). 4

    Show calculations.


Clearly describe what you did, why you did it, what calculations or other information formed your choices, and

    what you observed for every step. Be excruciatingly thorough. Make sure that all your calculations are

    clearly labeled with all units and identities of all materials I don‟t want to figure out what you mean!

Questions that I am likely to consider during grading:

    Did you choose an appropriate weak acid/base choice for buffer and justify that choice - 5pts

    Is your written preparation procedure complete, organized, and accurate - 15 pts

    Is your prepared buffer within 0.3 pH of calculated value - 15 pts

    Are your calculations for buffer preparation well-labeled, complete, clear, and correct - 15 pts

    Are your calculations for pH upon HCl addition well-labeled, complete, clear, and correct - 15 pts

    Are your calculations for pH upon NaOH addition well-labeled, complete, clear, and correct - 15 pts

    Are your calculations for equivalence point determination well-labeled, complete, clear, and correct - 10 pt

    Have you followed good laboratory procedures (safety, cleanliness, correct use of pH meter including cleanup

    and storage) I will monitor this - reduces your grade 5 pts for each infraction

    Have you turned the lab in on time - 15 pts

Dr. Kohnken AP chemistry

Buffers with Ka

Name Formula of acid Ka -3Phosphoric acid HPO 7.1 x 10 34-7- HPO 1.5 x 10 24-132- HPO 4.2 x 10 4-7Carbonic acid HCO 4.5 x 10 23-11- HCO 4.7 x 10 3-4Formic acid HCOOH 1.8 x 10 -5Acetic acid CHCOOH 1.8 x 10 3-2Oxalic acid (COOH) 5.4 x 10 2--5 HOOCCOO 5.4 x 10 -33+Iron (III) chloride Fe(HO) 5.5 x 10 26-2-Hydrogen sulfate HSO 1.3 x 10 4-10+Ammonium chloride NH 5.6 x 10 4-4Citric acid CHO 7.4 x 10 687-5- CHO 1.7 x 10 677-72- CHO 4.0 x 10 667-5Succinic acid CHO 6.2 x 10 464-6- CHO 2.3 x 10 454-3Tartaric acid CHO 1.0 x 10 466-5- CHO 1.5 x 10 456-2Maleic acid CHO 1.4 x 10 444-7- CHO 8.6 x 10 434-82+Ethylenediamine CHN 2.7 x 10 2102-11+ CHN 1.9 x 10 292-4Glycolic acid HOCHCOOH 1.5 x 10 2-10Boric acid HBO 7.3 x 10 33-13- HBO 1.8 x 10 23-142- HBO 1.6 x 10 3+-3Glycine NHCHCOOH 4.57 x 10 32+--10 NHCHCOO 2.51 x 10 32+-8Triethanolamine (CH(OH)CH)NH 1.58 x 10 223

Dr. Kohnken AP chemistry

    Set 4 - titration.

    Step one is to titrate potassium hydrogen phthalate (KHP, 204.2 g/mole) with 0.1M NaOH. KHP is a standard, that is, it can be obtained essentially pure and stable. With it, we can get an accurate (3 or 4 sig fig) determination of the concentration of the NaOH. This process is called standardization. Step two is to use the standardized NaOH to measure the moles of glycolic acid of an unknown concentration, so that we may

    determine its purity.

    Safety and disposal all chemicals may be flushed down the drain. Wear goggles.

Pre-calcs (10 pts - due before the lab)

    From the mass and molar mass of KHP, approximately how much 0.1 M NaOH do you need? Given that Ka is -5about 10, estimate the pH at the equivalence point from this information. How do you pick an appropriate

    indicator given this information? Given that glycolic acid is between 50 and 100% pure, can you estimate pH at its equivalence point and thereby pick both an indicator and estimate the volume of NaOH needed for titration?

Procedure and some things to include in write-up:

    1. obtain approximately 0.5 g, weighed accurately, of KHP. Transfer stoichiometrically (describe how you did this in your write-up) and dissolve completely in approximately 40 ml distilled water in a beaker. Add 5 drops of an appropriate indicator (see below).

    2. rinse your buret with 5 - 10 ml NaOH solution several times.

    3. titrate the KHP with NaOH to the end point. Repeat at least once. Go slower the second time so that you get a more precise measurement (within 0.1 ml) of the end point.

    4. calculate the more precise NaOH concentration: moles KHP = moles NaOH at end point. (5 pts)

    5. Next, obtain approximately 0.3 g, weighed accurately, of liquid glycolic acid (76.05 g/mole). Transfer stoichiometrically and dissolve completely in approximately 40 ml distilled water in a beaker. Add 3 drops of an appropriate indicator. Measure pH with your calibrated pH meter.

    6. Titrate with NaOH as before. Record pH at least once each ml, and more often near the equivalence point. Repeat at least once, and again, get within 0.1 ml of the endpoint in the second titration. 7. Calculate the moles of glycolic acid in your sample. From this information, and its molar mass, determine its purity. (10 pts)

    8. From a graph of your titration data, what would you predict to be the pKa of glycolic acid? (10 pts)

Other items

    a. Describe how you could determine molar mass from a titration. What would have been different in the

    experiment that you performed? (5 pts)

    b. pH meter handling. Did I observe you following the care, use, and calibration instructions? (6 pts)

Questions: (2 pts each)

    1. How would the following affect your conclusions regarding the purity of glycolic acid? Explain.

    a) Drops of NaOH cling to the side walls of the buret as it is drained during titration of KHP only. b) The buret is rinsed with water prior to filling with NaOH before first titration. c) The buret tip has an air bubble at the start of the titration of KHP.

    d) The buret is not read at eye level at any point in titrations.

    e) The NaOH is added too rapidly in the region of rapid pH change of glycolic acid titration. f) 50 mL of distilled water is used instead of 40 mL to dissolve glycolic acid. g) The original volume of base is not at 0.00 mL in the buret, but the student assumes it is at any point in


Dr. Kohnken AP chemistry

    Makeup for either buffer or titration lab

Perform the following titration and calculations. Describe what you did (10 pts) and show all your calculations

    (50 pts).

    Safety and disposal all chemicals may be flushed down the drain. Wear goggles.

Prepare 100 ml 0.1M NaPO solution and put in a beaker. Titrate with 0.3M HCl (I will supply this to you) up 34

    to 100 ml. Record pH vs volume at 1 ml intervals, perhaps more frequently near equivalence points. Show a

    graph of the data labeling the appropriate points.

Record the following information:

     Calculated pH Observed pH

    Starting point

    First buffer point

    First equivalence point

    Second buffer point

    Second equivalence point

For each of the 5 points above, what is/are the major species present in solution? What reaction(s) is/are

    affecting pH? Show the reaction that is the most prominent affecting pH after the second equivalence point?

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