Acid Titration Curve (Page 3 of 4) in pdf

level down to 0.00 mL by draining out the appropriate amount via the stopcock (strictly speaking

you need not start at precisely 0.00 mL, but when making a titration curve, it is very convenient to

do so because then your volume READINGS will equal your “volume added”).

c. Stir plate/pH electrode/buret setup. You will find the pH electrode immersed in a buffer solution

“plug”. Remove it carefully from that solution (keeping it upright at all times) and rinse it with some

deionized water from your wash bottle. Put the plug in a safe place (you’ll need to replace it after

the experiment!). Carefully remove excess water from the electrode by lightly blotting (NOT

“wiping”) the tip with a tissue (Kimwipes). Calibrate the pH electrode using two buffers at specific

pH values (7.00 and 4.00). Place and center your beaker of acid solution on a stir plate, and get

your stir bar spinning at a reasonable rate by adjusting the “stir” knob on the stir plate (the knob is

very sensitive!) Once you get the stir bar spinning, never touch the knob again (unless of course

the bar stops spinning)--just let it stir constantly throughout the entire experiment.

Now lower the pH electrode (on the support arm holder) into your solution (in the 100 mL beaker)

such that the tip is completely submerged, but the bottom of the tip is above the level of the top of

the stir bar. The electrode need NOT be centered in the solution; it can be anywhere as long as the

tip is submerged. Move the ring stand with the buret so that the buret tip is over the 100 mL

beaker, AND in a place such that you can access the stopcock. Lower the buret so that the tip is

below the rim of the beaker, but well above the liquid level. You may need to adjust the position of

the pH electrode and buret stand to get things to “fit” okay (you may also need to rotate the buret

clamp on the ring stand to get things to work out; ask me). You are now ready to begin!

2. The titration.

Note: Remember that you should know approximately where to expect your equivalence point, and

thus your half e.p., (look closely at the calculation in III A)! Use this to help you gauge “where

you’re at” in the titration (see below). ALSO: your goal is not to stop when you reach the

equivalence point!!! This is not a standard “titration”—you want to generate a full “curve”.

First measure and record the pH of your solution BEFORE adding any NaOH solution (volume

added = 0.00 mL). Then add ~2 mL and record the new buret reading (precisely!) and the

corresponding pH. You will need to hit “READ” each time that you want to make a new reading.

Then add another ~2 mL, and so on, recording the pH values and buret readings each time. As you

get closer to the equivalence point, you will need to add smaller and smaller amounts of

NaOH or else you will likely “miss” the equivalence point. Note that your change in pH after

each 2-mL addition will initially be sizable (> 0.5 pH unit), then it should get smaller as you approach

the half-equivalence point (pH probably ≤ 0.1 pH unit around middle of buffer region), and then it

will begin to get larger again as you approach the equivalence point. As a guideline, after you’ve

passed the half-equivalence point, when a pH reading is more than about 0.25 pH units higher than

the previous reading DO NOT ADD ANOTHER 2-mL aliquot! You do not want to "miss" the

equivalence point, right? (Why not?) At this point, add titrant more judiciously, perhaps in 0.5 mL

increments at first, while closely monitoring the increase in pH with each addition. If the pH

increases by more than 0.2 units after an addition try making additions dropwise, and only record the

buret reading and pH when the pH is about 0.2 units above your last reading. Keep making

dropwise additions through the equivalence point--until it seems to take multiple drops to get a pH

increase of 0.2 units. At this point, you can start making progressively larger additions again since

the pH will begin to get less sensitive to additions of titrant. Ultimately make at least FOUR 2-mL

additions AFTER you see the pH reach about 11.5. (Please ask me to confirm before you “stop”!)

When you are done, remove the electrode, rinse it with deionized water, and put it back into the

buffer solution it was in originally. Carefully dump the titrated solution down the drain, making sure

to recover the stir bar and return it. Rinse any unused titrant down the drain as well, and rinse the

buret with deionized water before returning it.

D. Raw Data “Format”. Plot your pH/Volume raw data in Excel using an X-Y scatter plot (NOT a “line

plot”!). Make sure you plot pH (y-axis) vs. Volume (in mL) of NaOH solution added (x-axis).

Acid Titration Curve Page 3

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