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Titration instructions for SO2 analysis (also see notes)
1. - Remove the pear shape flask with the bubbler insert intact.
2. - Position the pear shaped flask so that the burette tip enters the side port of the flask and is positioned just above the liquid's surface. This allows the NaOH soln. to be delivered without depositing on the sides of the flasks and will provide clear visibility through the flask (see diagram).
2. - To start titrating, deliver the NaOH soln (titrant) by opening/turning the burette stopcock/tap slowly until a few drops at a time are delivered (see diagram)
3. - When the bluish/purple colour just changes to olive green, close the tap, release the liquid held in the bubbler tube by lifting it up and letting it drain. This can contain untitrated H2SO4 (sulphuric acid) and may bring the colour back to bluish/purple and continue titrating one drop at a time.
Placing a white piece of paper in the background allows the colour to be more clearly observed.
Note 1: The small reservoir of untitrated solution mentioned in step 4 can be used to bring back the colour to olive green, if you accidentally used more then one drop to reach the end point. This can be achieved by lifting the bubbler out of the pear shaped flask and jiggling it up and down.
Note 2: To gain experience, it pays to do a few practice runs. This saves you time, for if you overshoot the end point, all your preparations and 15 minutes of aspirating time will be wasted.
Note 3: For every 30 ppm SO2 expected ~ 2 ml (approx.) of ~ 0.01 M NaOH (approx.) is required.
Note 4: It will only take one drop of 0.01 M NaOH to change the colour to olive green, as it did in its preparation.
Note 5: If unsure whether the colour change has occurred, note the burette reading and add another drop, then -
- If no significant colour change is observed the noted reading is used (this is recorded in the interim column of your record sheet.
- Other wise continue adding a drop at a time until a significant colour change occurs.
Note 6: I recommended that you use the left hand to operate the burette tap (stopcock). This is achieved by cupping the stopcock to stabilize your hand and using your finger tips to turn the stopcock key to deliver a controlled amount of the burette contents. This method may seem awkward at first but with practice it provides better control and leaves the right hand free to hold and swirl the pear shaped flask for mixing (see diagram).
Another method is to turn the burette around so that the stopcock can still be turned with the left hand. Once the end point is reached, turn the burette back to observe the reading. This second method however does not steady the left hand around the tap.
4. - Proceed to calculations