Laboratory Skills

Laboratory Skills

 

 Skill 1. Dilution

At times we need to dilute a solution to get the desired concentration. To do this,

We first make up a Stock Solution, which is at a higher concentration than any of the

desired concentrations. To make a stock solution you accurately, weigh out the required

mass of solid and prepare the solution using a volumetric flask as described for standard

solutions. The required dilutions are done by transferring an accurate volume of the stock solution by

pipette into a volumetric flask, and making up to the correct volume for the

flask by adding water. If less accuracy is sufficient, use measuring cylinders

to measure the desired volumes.

To calculate the volume Vstock of stock solution (of concentration Cstock) that should be

added to make a solution of volume Vsolution with a concentration Csolution, we use the

following formula:

In this formula the quantity Csolution/Cstock is termed the dilution ratio that is  the factor by

which the stock solution is diluted in order to prepare the required concentration of

solution. The above equation can also be used to dilute an already diluted solution

further. This is termed serial dilution. For example :  a 1 M solution can be diluted to a

0.1 M solution and then the 0.1 M solution can be diluted to a 0.01 M solution. Serial

dilution is a more accurate way of preparing solutions of low concentration than

weighing and dissolving a very small mass of solute.

 

Skill 2.  Use of the burette

The burette is a device for measuring the volume of solution used. It must drain cleanly and the tap must be in perfect order. Like the pipette, before use it must be thoroughly rinsed. Remove the burette from its stand, close the tap and pour in about 5 mL of solution. Hold the burette horizontally and carefully rotate it to rinse the entire inside surface thoroughly. Drain by opening the tap. Repeat this rinsing process twice more. Fill by pouring the reactant solution through a small funnel. DO NOT fill the burette above your eyes. Remove Clamp the burette in its stand with the good position  and with the tip 13 cm above a white tile. Carefully run liquid to waste, so as to remove any air bubbles below the tap and record the initial volume to two decimal places in your log book.

Skill 3. Use of the Indicators

An acid-base indicator is a weak acid or a weak base. Indicators used to determine when a reaction is complete. An Indicator does not change color from pure acid to pure alkaline at specific hydrogen ion concentration, but rather, color change occurs over a range of hydrogen ion concentrations. This range is termed the color change interval. It is expressed as a pH range. Two of the most commonly used indicators are methyl-orange (yellow in base, pink in acid) and phenolphthalein (red in base, colorless in acid).  
            Weak acids are titrated in the presence of indicators which change under slightly alkaline conditions. Weak bases should be titrated in the presence of indicators which change under slightly acidic conditions. For the example in the experiment of  titration acid-based we can use indicator phenolphthalein (PP) by adding 1-2 drop PP in the titran. The color of  solution can be cange become pink at a reaction complete as indicate equivalence point.

Skill 4. Use of the separating funnel

The steps to use separating funnel are this above:

1.      Set-up for the separating funnel

       Your separating funnel needs to be setup on your bench. Tap closed, solutions are poured in                 the top of the separating funnel.

2.      The separating funnel is carefully removed from the supporting ring and shake in both hands .with gently shaken, mixing the contents, with frequent venting.

3.      The two layers have separated and the lower layer is being removed. After shaking, the separating funnel is returned to the metal ring and the two layers are allowed to separate as shown

4.      Once the two layers have separated, the stopper is removed and the lower layer is run off into a conical flask by opening the tap. Careful control will ensure the entire upper layer remains in the separating funnel - the lower layer has been run off into the conical flask or been trapped in the passage of the tap. The upper layer is then poured out the top of the separating funnel into a clean and dry conical flask.

Skill 5. Attaching rubber tubing

Thick rubber tubing is used to connect filter flasks to the vacuum line. Thin rubber

tubing is used to connect water condensers to the water supply.  This simple procedure is one of

the major sources of injury in undergraduate labs so it is important that we learn

the correct procedure. If too much stress is put on the glass it will break and sharp bits

of glass will almost certainly get embedded in your hand. The steps attaching rubber tubing is :

1. Wet the end of the rubber tubing to be attached.

2. Wet the glass tube around the end where the tubing is going.

3. Hold the glass tube as close to that end as possible.

4. Gently slide the tubing on over the end. Do not twist. Keep your hand as close

to the end of the glass tube as possible throughout the process.