Lab 4 (P) Flashcards
General info about this lab
Part A: pH meter titration of a weak acid with a strong base. (In pairs, 30 minutes)
the procedure for Experiment P, Part A is nearly identical to Experiment 0, Part A. The only difference is that a weak acid analyte will be used instead of a strong acid.
The pH meter may need to be standardized against two buffered solutions, as per directions on page 21.
Procedure for part A
pH meter titration of a weak acid with a strong base. (completed in pairs; this should take no more than 30 minutes)
After the pH meter has been standardized, you are ready to begin the titration.
Keep the pH electrode immersed in the storage buffer whenever you are not taking pH measurements.
Rinse and fill one burette with standardized NaOH and adjust the volume to 0.0? mL. Record the actual value.
Pipette 20.00 mL of stock acetic acid into a clean 150 mL beaker.
Add three drops of phenolphthalein.
Rinse the pH electrode with deionized water and immerse the probe (still wet with deionized water) into the solution to be titrated.
Leave the probe in the solution for the remainder of the titration.
Record the initial pH of the solution.
Slowly add NaOH with constant stirring (swirling or plastic stir rod) and collect data every 2 mL or 0.2 pH change increments, whichever comes first.
Close to the equivalence point, a few drops will cause a 0.2 pH change.
At the equivalence point, one drop will change the pH by several units.
Continue collecting data for 15 mL past the equivalence point at 2 mL or 0.2 pH change increments.
NOTE: Record on your Observation Sheet the pH region where the solution changes color.
Procedure for part B
Indicator titration of vinegar - the quantitative titation (completed individually)
The household product used for this experiment is Vinegar.
Use phenolphthalein as the indicator.
The quick initial titration will have been performed in Part B of Experiment 0, and you will use the results from this titration to determine the appropriate dilution required to achieve an endpoint between 15 and 40 mL of titrant in a careful quantitative titration.
You will perform the dilution and then carefully complete at least two titrations to accurately determine the concentration of the diluted vinegar solution and, ultimately, the original undiluted vinegar sample.
Given the available equipment, there are six possible dilutions you can make so that between 15 and 40 ml of titrant are used in the quantitative titrations.
Your choice is dictated by the volume of titrant (NaOH) used in the quick titration in Experiment O - Part B.
Quantitative titration:
If you used less than 15 ml of titrant in the quick titration
If you used less than 15 ml of titrant in the quick titration you must dilute the titrant so that between 15 and 40 mL of titrant are used in the quantitative titrations.
Pick the appropriate dilution:
• 2 to 3 mL titrant - pipette 40.00 mL of your titrant into a 100 mL volumetric flask.
• 4 to 9 mL titrant - pipette 20.00 mL of your titrant into a 100 mL volumetric flask.
• 10 to 15 mL titrant - pipette 10.00 mL of your titrant into a 100 mL volumetric flask.
Using proper technique, rinse and then pipette xx.00 mL of titrant (standardized NaOH) into a clean 100 mL volumetric flask.
This flask must be clean but does not need to be dry.
Dilute to the mark with distilled water and invert 20 times to mix: thorough mixing is critical.
In either of the above 3 cases, the diluted titrant (NaOH) will be placed in the buret for the quantitative titrations.
It is vital that the buret is cleaned and rinsed with this diluted titrant before the quantitative titrations are undertaken.
Keep in mind that you’ll have only 100 mL of this diluted titrant, so don’t use too much diluted titrant for rinsing.
Quantitative titration:
If you used more than 40 ml of titrant in the quick titration
If you used more than 40 mL of titrant in the quick titration you must dilute the analyte so that between 15 and 40 mL of titrant are used in the quantitative titrations.
Pick the appropriate
• 41 to 70 mL titrant - pipette 40.00 mL of your analyte into a 100 mL volumetric flask.
• 71 to 200 mL titrant - pipette 20.00 mL of your analyte into a 100 mL volumetric flask.
• 201* mL titrant - pipette 10.00 mL of your analyte into a 100 mL volumetric flask.
Using proper technique, rinse and then pipette xx.00 mL of analyte (Vinegar) into a clean 100 mL volumetric flask.
This flask must be clean but does not need to be dry.
Dilute to the mark with distilled water and invert 20 times to mix: thorough mixing is critical.
Once you have performed the appropriate dilution (one of the six outlined above), rinse and fill the burette with the titrant.
Set the initial level of the burette to between 0 and 2 mL, then read and record this initial reading on the burette.
Do NoT adjust your initial reading to 0.00 ml: a penalty will be imposed for this error.
Using proper technique, rinse and then pipette 10.00 mL of analyte into a clean 125 mL Erlenmeyer flask.
Add three drops of phenolphthalein.
Titrate the analyte by adding titrant from the burette.
The end point of the titration occurs when the indicator turns from colorless to a permanent colour with the addition of one drop of titrant.
Record the burette volume at the end point.
Repeat the titration on a second analyte sample.
If your volumes are not within ‡ 0.10 mL, repeat until you have two titrations within ‡ 0.10 mL.
Rinse the burette 3 times with hot tap water before putting it away.
