Read the volume on the side and record it in your

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Unformatted text preview: e is used to measure volume. Unlike the graduated cylinder however the pipette is designed to measure one, and only one, volume, as indicated on the pipette. Never case a pipette that has a chipped or cracked tip, as these are no longer properly calibrated. Begin by cleaning the pipette according to standard methods. Be especially careful to avoid bumping the tip against the sink or other surfaces. If the pipette is clean, liquid should flow out of it smoothly without leaving spots. To use a pipette, begin by verifying that it is the correct type of pipette. Read the volume on the side (and record it in your notebook), and verify that it says “TD”, not “TC”. “TD” stands for “To Deliver”, which means that the volume that comes OUT of the pipette is exactly the amount that the pipette is calibrated for (NOT the amount the pipette will hold). Although “TC” (or “To Contain”; that is, it is calibrated so the amount of liquid actually IN the pipette is the recorded volume) pipettes are rare, they do exist. We will ignore the “TC” procedure, and focus only on the “TD” procedure, as these are the commonly used pipettes today. The bulbs we us e in the chemistry lab have a hard plastic base attached to a rubber bulb. These give the user more finesse and better reproducibility than a mechanically designed device or a “three-port” bulb. Notice that the bulbs we use are NOT designed to fit onto the top of the pipette; they are designed to be placed there and quickly and easily removed. Begin by holding the pipette vertically. Do not hold the pipette by the “fat” part of the glass; warmth from your finger will cause it to expand, and the pipette will lose it’s calibration. Instead, hold the pipette near the top (above the calibration mark) so it is easy to get your finger over the top. Squeeze the air out of the bulb (NOT on the buret) and place the bulb on the buret top. Place the buret into the liquid to be drawn up and slowly release the pressure on the bulb, allowing the vacuum created to suck up the liquid. If your bulb completely expands before the liquid is above the calibration mark at the top of the pipette, quickly remove the bulb and cap the top of your pipette with your INDEX finger (not thumb). Squeeze the air out of the bulb, put it back on the pipette, and continue to draw up the liquid. Dakota State University Page 38 of 232 Basic Laboratory Procedures General Chemistry I and II Lab Manual Once the liquid is above the calibration mark, take the pipette bulb off and cap the pipette with your index finger (again, not your thumb; you will have better control and get better results with your index finger). Put the bulb down, and slowly allow the liquid to flow out of the pipette until the bottom of the meniscus is right at the calibration mark. (If you are having trouble controlling the flow, try these few tricks; if you cannot hold the liquid in the pipette, moisten your finger slightly to get a better seal; instead of trying to lift your finger up to get the fluid to flow out, try rolling it slightly to one side instead; if the f...
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