Performing Titrations

1. A known quantity of the unknown solution (HCl) is pipetted into a flask and several drops of an indicator are added. If phenolphthalein is being used as an indicator, the solution should remain colorless at this point. The flask is placed on white paper to make the endpoint easier to see.

2. Make sure the buret stopcock is closed and then rinse the inside with several milliliters of titrant (NaOH). The buret should be held nearly horizontally and rotated so that all of the inside surfaces are contacted by the titrant. Some titrant should also be run through the stopcock to clean it as well. Cleaning is normally performed over a sink.

3. Make sure the stopcock is closed. Place the buret in a buret clamp and fill it carefully with titrant. Use a beaker with a spout or funnel to reduce the possibility of spilling titrant.

4. Drain some titrant through the stopcock into a waste beaker. Make sure that no air bubbles remain in the stopcock.

5. Read the volume of the buret. This is your initial volume (14.62 ml in this case). Reading is made easier by holding a piece of dark paper behind the buret.

6. Place the flask containing the unknown under the buret. Slowly open the stopcock and add some titrant (usually a milliliter or so). You may notice a temporary color change in the solution near where the titrant was added. Stir the solution thoroughly. Any color change should disappear.

7. Continue adding titrant in small quantities. As the titration progresses, the color change described in step 6 will take longer to disappear. This signals that the endpoint is getting closer and that the titrant should be added in smaller and smaller quantities. Titrant should be added dropwise very close to the endpoint.

8. The endpoint of the titration is signaled when a permanent color change is observed (longer than 30 seconds). It is possible to overshoot the endpoint by adding too much titrant. A correct endpoint is shown on the left, an overshot endpoint on the right.

9. Record the volume in the buret. This is your final volume (26.48 ml in this case). Subtract the initial volume (step 5) from the final volume to determine the volume of titrant added (26.48 - 14.62 = 11.76 ml).

10. Use standard chemical calculation methods to determine the concentration of analyte.

Because the concentration of the stuff in the conical flask is irrelevant. You are trying to find the concentration of say the acid in the burette. The way you do this is you pour it into the conical flask and when you see a change in colour blah blah, you know when equal numbers of H+ and OH- have reacted. So in the conical flask you have a fixed number of MOLES of say a base in this case. So when the colour changes, you can work out the number of moles of acid that were poured in the volume measured by the burette and hence the concentration of the acid in the burette.

The concentration of the base doesn't matter- no matter how much water you add, the number of moles of base will remain UNCHANGED. Does that make sense?

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