1.5: Lab Practices and Charts

Some Good Lab Practices

Measurement using any instrument

Make sure you are comparing the measurement value to the required unit. For example to measure the length of a book using a ruler in “mm”; with a ruler that has both “mm” and “cm” units on either side, make sure you are measuring using the right side or in the right unit (“mm”) that you need. If it is an electronic device check the “mode, and change it to the appropriate unit mode you need.
Using an electronic balance (scale)

An electronic balance is used to measure the mass of an object. Sometimes you are required to measure the mass of both the container and the object, and sometime you are required to measure the mass of the object only that is placed in a container. The first step as mentioned earlier is to check that the unit shown in the instrument is the one that you want. Otherwise press the mode button and bring it to the desired unit (it is usually the SI units). Then place the container and the object together to find the reading. Write down the measurement followed by the unit. If you do not want the mass of the container, keep the container on the scale, press the “zero” or “tare” button, so that the mass of the container is zeroed out. Now you can place the object in the container to find the mass of the object only.

Measuring volumes using graduated tubes or columns (volumetric measurements)

Volumes can be measured using either an instrument with a base like graduated cylinder or a tube like buret. In both the cases hold or place them so that the grading or marking on the tube and your eyes are on the same straight line (leveled). It is easier to keep the graduated cylinder on the work bench and kneel down to read the measurement. For a buret it can be clamped on a stand to read the measurement. For a colorless liquid the reading matching with the lower meniscus is taken, and for an opaque or colored liquid upper meniscus level is measured.

Using a pipet

Always use a mechanical or electric suction device to draw liquid into a pipet. Draw a little more than you need, and then slowly release the suction with great care to the nearest mark, pull it out from the stock container and then slowly release the liquid to the receiving container. 

Using a buret

Open the knob (keep the knob vertical) and place the tip towards the drain, and rinse with distilled water using a wash (squirt) bottle. Rinse the buret two or three times. Then hold the buret slanted over a waste bottle or collection beaker and slowly rinse the inside of the tube with the liquid that you want to fill it with. Rinse the buret with 3-5ml twice. Then close the knob (horizontal), and fill with the liquid you want. Fill it a little more than the top most reading. Hold the closed buret over the collection beaker again, and slowly open it (drip) to level with the top most reading (usually zero). This way you are removing the trapped air bubbles near the knob that could give rise to errors in your measurement.

Rinsing a glassware

Before you use any glassware make sure it is clean. Wash with a wire brush and soap until the glass is clean. After washing rinse it with 5-10ml of distilled water.

Washing used test tubes

Dispose the chemicals in the test tube into appropriate waste containers as instructed. Rinse with water and transfer the first two rinsing into the waste container itself. Then bring it to the sink and wash with wire brush, soap, and cold water. Then you can wash with hot water and soap if necessary. Rinse with distilled water. Make sure you wear gloves during the whole process. After removing gloves wash your hands thoroughly with soap and water.

Handling hot glassware

Never touch any hot glassware with your bare hands. Wear gloves, or use tongs or test tube holders as necessary. Never leave them on the table without a heat resistance mat or platform to cool off.

Diluting acids

Mixing acid and water will release heat. If you want to dilute an acid, take the required amount of water in bigger glassware in the fume hood and add the acid drop-by-drop into the water with vigorous shaking or mixing. It is also advisable to keep the water in an ice bath or cold water bath while you drip the acid into it. Wear corrosion resistant gloves, goggles, and lab coat.

Filtering a heterogeneous liquid-solid mixture

Find the appropriate filter paper with right dimension and mesh size. If it is a conical funnel, fold the filter paper into four or eight fold. Keep it in the funnel with the pointed side towards the bottom. Wet the paper with a very small amount of distilled water so that the paper sticks to the funnel. Slowly pour the top liquid in portions (never add more than the 3/4^th of the funnel height. Let the first portion drain, and then add the second. When the filtration is done rinse the remaining solid with fresh liquid, mix, and then repeat the filtration. Never touch the filter paper with sharp tips, like spatula or glass road.