[adenine135]

Briefly explain whether the following technique errors would result in a high or a low experimental determination of the molar concentration of CH3COOH in vinegar:

1) The student uses the pipet bulb to blow the last bit of vinegar sample from the tip into the titration flask, after the pipet has drained.

2) A student fails to clean the pipet first. After delivering the vinegar sample, the student notices a drop of vinegar clinging to the inner surface of the pipet.

3) After completing a titration, a student notices that the buret is not clean. Droplets of NaOH solution are clinging to the inner surface of the buret.


My attempts at answers (I would like help on how to phrase these better and any correction if necessary):

1) I know this has something to do with how a pipet is designed (these were 25.00 mL pipets by the way), but am not quite sure how this is done. I believe the excess solution left in the tip is factored into the 25.00 mL measurement. So you're actually delivering more than 25.00 mL of vinegar. Since molar concentration is inversely proportional to volume of solution, a volume that is too high would result in a molar concentration that is too low.

2) Having droplets cling to the inside of the pipet means that less than 25.00 mL of vinegar was delivered. Since molar concentration is inversely proportional to volume of solution, a volume that is too low would result in a molar concentration that is too high.

3) Same as number 2

[Mitch]

1)There are many kinds of pipettes available for use in the clinical laboratory, each intended to serve a specific function. In general, pipettes fall into two classes, volumetric or transfer pipettes and graduated or measuring pipettes. The volumetric pipette is calibrated for one specified volume measurement, either to deliver (TD) or to contain (TC). This distinction is clearly indicated on the pipette. A to deliver pipette calibrated for blowout has an opaque ring near the top. In this case the small amount of liquid remaining in the tip after free delivery has stopped is blown out and added to the initial volume. To deliver pipettes are calibrated for the volume delivered, with no attempt to wash out the film which adheres to the inside glass surface. To contain pipettes are calibrated for the total volume of liquid held in the pipette, and must be washed out completely for delivery of the correct volume.

[Borek]

http://www.titrations.info/pipette-burette

[adenine135]

So are my answers correct?

[Mitch]

So are my answers correct?

Only if the pipette used was a "to deliver" pipette.

[adenine135]

The pipette was used to deliver NaOH to a vinegar solution in order to neutralize it... is that what you meant by "to deliver"?

[Mitch]

Please read Borek's link, then comment.