Justifications
The funnel was removed from the burette before reading it as drops of sulphuric (VI) acid could have fallen into the burette after the reading was taken which would have made the readings inaccurate and larger then it should have been. A white tile was placed underneath the conical flask so that any colour changes could be observed more clearly as the white would contrast with orange. The burette was read at eye level to avoid any parallax errors, as this is where the best view of the meniscus is, and from different angles the water can look different. A rough titration was carried out to see how much of the sulphuric (VI) acid was roughly needed to react with the hydrous sodium carbonate. The conical flask was swilled as the solution from the burette was added so that all of the solutions reacted with each other. The burette and pipette were washed out with the solutions that were to go in them before they were used to ensure that any other substances are rinsed out and they are the only substances in the glassware. If this was forgotten and the burette was not washed out the reading would be inaccurate and larger then they should have been as it would seem more sulphuric acid would be needed to neutralise the sodium carbonate. The conical flask was swilled with water to get rid of any impurities in the conical flask, the solution going into the flask already contained water as a solvent so no new substances were added. The solution was released from the burette drop by drop when the end point was near to find the exact drop at which the sulphuric (VI) acid stops reacting with the sodium carbonate solution. If it were let out very quickly there would be a larger percentage error, as the readings would have a large difference in values. The experiment was repeated until concordant results were obtained so that reliable and accurate readings were acquired. Approximately 5cm of the anhydrous sodium carbonate was used as this was a logical amount; not too much so that it would not completely dissolve, but not too little so that only a small amount of the sulphuric (VI) acid would be needed to neutralise it as a smaller reading gives a larger percentage error. 250cm3 standard solution of sodium carbonate was made so that the same solution could be used when doing repeats because this ensures that the concentration of the sodium carbonate solution used for each titration is the same. 25.0cm3 of sodium carbonate was used for the titration as this was a reasonable amount; enough to be able to stir it and see a colour change, not too much so that large amounts of sulphuric (VI) acid would be needed to neutralise it. Only a few drops of the methyl orange indicator was added to the sodium carbonate solution as if too little was added then the colour would be too faint to see; but if too much was added the colour would be too deep for the colour change to be seen clearly, which would lead to less accurate results.
Methyl Orange is a weak acid so just a small proportion of their molecules ionise in aqueous solution. We can show this as an equation:
HIn represents the methyl orange which is a weak acid, and In– the ion it forms when it ionises. HIn and In– have different colours: HIn is red and In– is yellow. Because it is a weak acid it will be mostly in the HIn form and therefore coloured red. But when added to the sodium carbonate solution, this will react with the H+ ions and then more HIn will ionise so as to replace them. So more In– will be formed and eventually the indicator will appear yellow. This will show up when the In– concentration is about 10 times greater than the HIn concentration.
The reaction is reversible, as we add the sulphuric (VI) acid from the burette the neutralsation will occur but because the sulphuric acid is a strong acid and sodium carbonate is a weak alkali the point of neutralisation for such a titration will occur when the pH of the solution is in the region of pH3.2 and pH4.4 which corresponds to the pH range over which point the methyl orange changes colour from yellow to orange, so it is appropiate to use this indicator
Risk assessment
As sodium carbonate is an irritant goggles and a lab coat must be worn to prevent any irritation to the eyes or skin. Try not to inhale vapours as sodium carbonate can irritate the respiratory system. If swallowed drink plenty of water and seek medical attention. If substance gets into eyes flood the eyes with gently running tap water for 10 minutes and seek medical attention. If sodium carbonate solution is spilt onto skin or clothes immediately remove contaminated clothing and wash skin with plenty of water. Soak contaminated clothing ands rinse repeatedly. If any of the substance is spilt in the lab scoop up the solid and if any solution is spilt add mineral absorbent and scoop into a bucket. Rinse the area of the spill and the cloth or mop thoroughly. If methyl orange is swallowed immediately wash out mouth, have a glass or two of water and seek medical attention. If the indicator gets into the eyes flood eyes with gently running tap water for 10 minutes and seek medical attention. If methyl orange is spilt on skin or onto clothes remove contaminated items and flood area with water and then wash thoroughly with soap and cold water. If spilt in the lab, wash the area thoroughly. Sulphuric (VI) acid is a corrosive and can cause burns to the skin so a lab coat must be worn at all times. If swallowed, wash out mouth and have a glass or two of water. Do not induce vomiting and seek medical attention as soon as possible. If splashed into the eye flood eye with gently running tap water for 10 minutes and seek medical attention. If the acid is spilt onto skin or clothes remove the contaminated clothing and quickly wipe as much liquid off the skin as possible with a dry cloth before drenching the area with a large excess of water. If a large area is affected or blistering occurs seek medical attention. If spilt in the laboratory wear eye protection and gloves. Cover with mineral absorbent and scoop it into a bucket. Add anhydrous sodium carbonate over the mixture and leave to react, and then add lots of cold water. Rinse the area of the spill several times with water.
Sources of information
AS Chemistry At Esher College Work Book 1 “The Elements of Life” and “Developing Fuels”, pg. 4-5
Hazcards, Cards 98, 95 and 32
http://www.chemistry-react.org/go/Faq/Faq_8219.html