Calibration

evaluating and adjusting the precision and accuracy of measurement equipment

Buret

used in titration and is designed to measure the total volume of the solution delivered and permit drop-by-drop addition of the solution as the endpoint is approached

How is a measurement made?

by reading the level before and after draining liquid from the
buret and subtracting the first reading from the last reading

How do you read liquid level?

your eye at the same height as the top of the liquid, base from the bottom of the concave meniscus

How do you read liquid level if the liquid is colorless or transparent?

at the top of the meniscus

Parallax Error

error that occurs when your eye is not at the same height as the liquid

How to calibrate buret?

measuring the mass of water delivered by the buret and using the density of water to convert mass into volume

How to get rid of bubbles in tip of buret?

fill the buret with titrant and open the valve

Step 1 of Calibration

fill the buret with distilled water and force any air bubbles out the tip.

If drop clings on the wall?

clean with soap and water

If drop persists on the wall?

soak with cleansing reagent

Step 2 of Calibration

adjust the meniscus at 0.00 mL and touch the buret tip to a beaker to remove the suspended drop of water

Step 3 of Calibration

allow the buret to stand while you weigh a dry, clean 100-mL beaker fitted with a clean rubber or cork stopper. While weighing, hold the beaker with a tissue paper.

Why do we need to hold the beaker with a tissue paper?

to avoid fingerprint residue

What if the level of liquid in the buret has changed?

tighten the stopcock and repeat procedure

Step 4 of Calibration

Drain approximately 10 mL of water (at a rate of <20 mL/min) into the weighed flask. Allow about 30 seconds for the liquid on the wall to descend before you read the buret. Estimate all readings to the nearest 0.01 mL. Weigh the flask again to determine the mass of water delivered.

Why do we have to cap the flask tightly?

To avoid evaporation

Step 5 of Calibration

Without emptying the beaker or refilling the buret, again allow water to flow into the beaker until the reading is 20 mL and measure the mass of water delivered. Repeat this process at 10- mL intervals until the 50-mL mark is reached. Perform a second trial.

Step 6 of Calibration

Use table 2 to convert the mass of water to the volume delivered to compute for the true volume

Where should the correction agree on its approximate?

±0.04mL

Step 7 of Calibration

If the results do not agree, repeat the calibration process until they do. Average the buret correction results of the two trials and prepare a plot of the mean correction against the buret readings (at each
10-mL interval) using the EXCEL spreadsheet software

Step 8 of Calibration

After the final weighing, determine the temperature of the water. The initial and final temperatures should agree within about 10 C.

Why should the initial and final temperatures agree within about 10 degree celcius?

To ensure consistency in results and minimize systematic errors as well as having the thermodynamic equilibrium

True Volume Formula

Apparent mass of water * Density of Water

Buret Correction Formula

True Volume - Buret Reading