Friday, 2 October 2015

Use of a Burette

Buret with 0.1 mL incrementBuretA burette is used to deliver variable volumes of solution precisely and accurately (Figure 1). It is use for titrations, where the solution in the burette is called the titrant. When the stopcock valve is aligned with the burette, as shown in Figure 1, the valve is in the open position and the titrant flows from the burette into the collecting vessel. Rotate the stopcock valve 90 degrees to close the valve. A commonly used burette in our lab delivers a total volume of 50 mL and is calibrated in 0.1 mL increments. This means that the finest division on the burette is 0.1 mL (Figure 2).

When reading the burette, the last digit is the digit where you estimate to within the finest division. Therefore, a burette reading of the 50 mL burette is recorded to two decimal places.

Steps to prepare and use a burette:

1. A burette is usually stored with distilled water and capped.

2. Empty the burette of distilled water from the top and check that the stopcock is liquid tight. If the stopcock feels loose when it is turned, hold onto the stopcock handle and rotate the screw clockwise to tighten. The stopcock should rotate smoothly. If it doesn’t, you should disassemble the stopcock to clean the stopcock and where it touches the glass surface of the burette. Reassemble the stopcock with all its parts as laid out in Figure 3.

Stop cock assembly

3. Close the stopcock and using a small beaker, add approximate 10 mL of the titrant from the top. Hold the burette horizontally, rotate the burette to ensure the titrant touches the interior surface of the burette. Open the stopcock to allow the titrant to drain through the tip. Drain the restof the titrant from the top of the burette. This step known as acclimatization is done at least twice.

4. Fill the burette well above the zero mark. Rotate the stopcock to let some solution flow through the tip.

trapped air bubble5. Look for any trapped air bubble below the stopcock (Figure 4). To dislodge the air bubble, take your burette over to the sink, rapidly rotate the stopcock and allow small quantities of titrant to pass. Close the stopcock and check if the bubble is gone. If the bubble remains, open the stop stopcock and tap the burette near the stopcock, or give the burette with a quick up-and-down jerk. A combination of the above procedures should dislodge the trapped bubble (Figure 5).

Record the initial volume6. Finally, refill the burette to above the zero mark. Lower the level of the titrant below the zero mark. The eye should be level with the meniscus for a proper reading. The correct reading is where the bottom of the meniscus touches the scale. Record the initial volume (Vi) to 2 decimal places (Figure 6).

7. Place a receiving flask (usually an Erlenmeyer flask) under the tip of the burette. Adjust the burette height so that the tip of the burette is inside the neck of the Erlenmeyer flask.

8. Dispense the titrant through the burette until a desired volume is reached or until a chemical analysis is complete. Do not let the solution drain past the last mark on the burette.

9. Record the final volume (Vf). The volume of solution dispensed is found by subtracting Vi from Vf.


10. To clean up, drain the solution from the top of the burette into a proper waste container. Rinse the burette 2-3 times with distilled water. Each time, open the stopcock to drain some of the rinse water through the tip and then empty the rest through the top of the burette.

11. Store the burette with distilled water.

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