Method:
- Tare a clean top pan balance to zero.
- Place a weighing bottle on and record the value to 2 d. p
- Remove and add 2.65g of the solid. Weigh again. Record again onto a chart such as this:
Top Pan Balance:
- Remove the weighing bottle and place a lid on it. Reweigh it on an analytic balance. Make sure that the doors of the balance are shut
- Record the mass.
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Empty the weighing bottle into 100cm3, clean beaker and reweigh the empty bottle. Record the mass on a chart like this:
Analytical Balance:
- Transfer the entire solid content into the volumetric flask. This is achieved by dissolving the solid, in distilled water in the beaker, stirring with a glass rod.
N.B: The glass rod has been stirring the solid matter. It has atoms and ions on its surface. To transfer the full atom and the ions, leave the glass rod in beaker. This gives you an accurate transference.
- Get a clean funnel and place it into the volumetric flask. Pour the solution down the glass rod and into the funnel. This washes out the atoms and ions present on the glass rod. No drops should be lost, to guarantee accuracy.
- Wash out the beaker, glass rod and the funnel, with the distilled water to make sure every bit of the solid is completely transferred into the volumetric flask.
- Repeat this process, until you are certain that all content of the solid has been successful transferred into the flask.
- Make the solution up the mark on the flask by pipetting the last few drops for accuracy. Read from the bottom of the meniscus to the mark on the volumetric flask.
- Place a stopper at the top of the volumetric flask and invert the flask 17 times. The air in the bulb allows optimum mixing.
The Titration
The titration is against a strong acid, sulphuric acid and a weak base, sodium carbonate. For this type of titration the indicator needed is called the methyl orange indicator (i).
Appartus:
Clamp stand White tile
Methyl Orange Indicator
50cm3 burette 25cm3 pipette
250cm3 conical flask Pipette filler
Magnifier glass 50cm3 small Beaker
Standard solution Sulphuric Acid
Distilled Water
Method:
- Wash out your burette with distilled water. Repeat again with the sulphuric acid. Make sure to run the solution and distilled water though the jet too.
- Set up a clamp stand and place a white tile firmly by the leg
- Close the tap on the burette and pour the acid through carefully. Fill the burette up to zero. Place a small, clean beaker under the burette and run the acid though into the beaker. This fills the jet and check to see if there are any air bubbles present. (The acid in the beaker can be poured into the burette again)
- Connect burette to the clamp stand and read value off to 2 d. p using a magnifying glass. This is you initial reading.
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Rinse a pipette with distilled water and the solution to be pipette. Connect the pipette to its pipette fuller by holding the ends together and pushing firmly. Then pipette out 25cm3(up to the mark present on the pipette itself) of standard solution (the alkaline) and bring the conical flask to the pipette and fill by pressing on the lever.
- Wash out the sides of the conical flask by using distilled water, so all content are being titrated. Do this periodically for more accurate results.
- Put 5 drops of methyl orange in the conical flask. Do not exceed amount.
- Place the conical flask on to the white tile, under the burette
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At the beginning allow small volumes of solution to titrate and then swirl. Nearer to the end of the titration, open tap in sharp, 360o turns and allow your acid to run in to the solution below. Swirl content by the neck of the flask.
- Repeat till a permanent colour is seen. When satisfied at the colour of solution, in contrast to the white tile, read of value- this is your final reading.
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The first attempt is a rough, so over titrating is not going to affect the titre at the end as it is not considered in the average. The rough titration is to provide a general idea of whilst the end point is. Keep titrating until the titrations are with 0.1 cm3 of each other. Your chart should look like this:
ENSURING RELIABLITY:
PIPETTE-to measure any solution, a pipette should always be used, never a measuring cylinder, as it is not accurate. No air bubbles should be present. When a pipette is used, it should be washed out with distilled water and the solution to be used in the pipette. This is so that the potential moisture inside the pipette does not alter the concentration of the solution. When a solution has been pipette, never move the pipette, as drops can be lost altering the concentration of the solution. Always bring the beaker et al to the pipette. To ensure that the pipette is drained of solution, the tip of the pipette is to be touched on the side of the beaker et al. However there should always be one remaining droplet that is always left in the pipette.
BURETTE-always wash a burette put with the distilled water and the solution which it is going to be filled with, so it is covered well in that solution. This ensures the reliability so that the alkaline is the only solution being titrated. The burette must also be dry; otherwise the unknown liquid could also be titrated. No air bubbles should be present in the jet or the body of the burette. To get rid of any air bubbles, place a beaker under the burette and run the solution through. When the titration process has started using small volumes is appropriate, but when it comes towards the end to avoid over-titrating use 360o sharp turns. The possibility of over titrating is therefore eliminated. This is the drop by drop method. To read the initial starting and final point of the solution use a magnifying glass and read from the bottom of the meniscus.
CONICAL FLASK- needs to be cleaned with distilled water before used. Throughout the experiment the conical flask should be washed out by distilled water and swirled by the neck, into the main reservoir of solution, to allow all of the solution to be titrated.
VOLUMETRIC FLASK- the best equipment for best dilution.
WHITE TILE-ensures the reliability as you can see if any drop are present on it, informing toy that the experiment has lost some atoms and ions and therefore not accurate as it could be. Also when titration begins the colour can be easily contrasted against the white tile, indicating when to stop titrating. This eliminates the possibility of over titrating.
SUFFIENT INDICATOR-too much indicator could mean the titration process needs to titrate the indicator as well as the acid.
MENISCUS: a meniscus is the shape that the solution makes and is quite thick. Always be consistent and read from the bottom of the meniscus to the mark, as represented on the diagram below.
REPEATS- all experiments are independent to each other so neither experiment has an influence the tire. Repeating the experiment till the titres are consistent increases reliability as the answer is evidently within range.
A ROUGH TITRATION-the rough allows over titration to be made a, eliminating the possibility of over titration at the later stage.
RISK ASSESMENT
Sulphuric acid (H2SO4) - Between the moles 0.5-1.5, it is known irritant to the eyes and the skin. Wear goggles and gloves when handling. (ii)
Anhydrous Sodium Carbonate (Na2S04) - Irritant to eyes. Wear goggles when handling. (iii)
As several glass equipment is going to be used carefully handle them. Clear any spills.
REFERENCES
(i)- Information Sheet on The Use of Indicators In Acid Alkali Titration, OCR 2000, page 43
(ii)-
(iii)- Hazard Data Sheets, BDH, Product No: 5 7053 1S, page 942
