The recording intervals of 15 seconds is quite a good length of time as it enabled enough oxygen to be produced for a clear graph to be plotted. The trials will last for 180s rather than 120s because some of the concentrations that will be used, for example 0.5% may be quite slow and so a better range of data can be collected if results are recorded for 180 seconds. To ensure there is a complete reaction, the conical flask will be swirled once, this will mix the reactant particles together thoroughly. The concentrations will range from 0 to 3% as this has been proven to produce a good set of data (class information). The 0% will be done as the control to show that when no substrate is present, no oxygen is produced. To get sufficient data to plot on the initial rate of reaction graph, the concentrations will increase in 0.5% so there will be six concentrations to test in total. The pH used, as informed by the preliminary experiment will be pH7 as this is the optimum pH and so will produce the best set of results.
Variables
Concentration table:
This concentration table shows how much of each substance will be needed in order to make up the different concentrations. They will be made up separately for each repeat because then if one of the concentrations is made incorrectly, it can easily be spotted without affecting the results too much. It will be a clear anomaly, which can then be ignored in the results, as there will still be the other two repeats.
Apparatus
Method
- Collect all apparatus required
- Put a piece of masking tape on the trough approx ¾ of the way up and fill trough with water up to the bottom of the masking tape.
- Set up the clamp stand so that the burette will be just above the bottom of the trough. (See diagram)
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Fill the burette with water using a 50cm3 beaker ensuring that the tap is closed as this is done. Then attach the burette to the clamp.
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Using the 2cm3 syringe, get 2cm3 of catalase. After, fill up the 10cm3 syringe with 10cm3 of pH7 and then pour 12cm3 (see concentration table) of H202 into the 25cm3-measuring cylinder.
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Pour the pH7 and the H202 into the conical flask.
- Carefully open the burette tap to allow the bubbles out and record the initial volume. Then put the end of the delivery tube into the bottom of the burette.
- Put the catalase into the conical flask and immediately place the bung on the top. Swirl the solution once and start the stop clock.
- Record the readings of the burette every 15seconds for three minutes.
- Repeat the entire method for each of the repeats and for each of the concentrations.
Diagram
Risk Assessment
Analysis
Unfortunately, my results had no correlation whatsoever between the substrate concentration and the volume of oxygen produced. This meant that data was given to me to analyse and evaluate; the data in the results table is the data given to me and the original data is in appendix 1.
From the initial rate of reaction graph, I can see a strong positive correlation between the two variables demonstrating that substrate concentration does affect the volume of oxygen produced. As the substrate concentration increases, the volume of oxygen produced increases proportionately. This is just what was predicted on the graph showing the rate of reaction with increasing substrate concentration.
Above in the table is the initial rate of reaction at 30s, which is plotted on the graph. It shows that the initial rate is 1/10th the concentration however, there are some exceptions. For the 1% concentration, the initial rate is 0.05cm3s-1, which is half of what the expectation is. This can be explained by the fact that at 30s on the 1% concentration graph, the point is lower than the line of best fit. The other anomaly on the initial rate of reaction graph is at 2%. This is also explained by the original 2% concentration graph as at 30s, the point is on the lower side of the line of best fit showing that slightly less oxygen was produced than there should have been.
Evaluation
There were various sources of error in the method. The temperatures were different on different days. As all the trials and repeats could be not done in one hour lessons, the experiment had to be carried out on four separate days. The temperatures on these days, although they only differed by a few degrees centigrade from one another, may have made a significant difference on some of the results. A higher temperature will increase the rate of reaction, increasing the volume of oxygen produced.
The clamp stand occasionally restricted the view of the burette, not allowing me to read the volume and forcing a judgement to be made. In addition to this, the catalase enzyme is bought as powder and is mixed with a liquid in order to use in experiments. The amount of liquid added could affect the speed at which the enzyme works as it could be too dilute or too concentrated which would affect the results significantly given that only 2cm3 of catalase was used for each trial. It was quite difficult to put the bung on the conical flask as soon as the catalase was put in and at the same time as tuning the stop clock on and swirling the conical flask, leading to a loss of oxygen. In addition to this, when the concentrations were made, they were made separately for each repeat. This was not very accurate as there is much more room for error- the concentrations would have been identical if the repeats were made together.
When the conical flask was swirled, more kinetic energy was given which meant the rate of reaction would increase, as there was a bigger chance that the molecules would collide with one another. This, in turn probably increased the amount of oxygen that was produced. Nevertheless, it was a systematic error and so that much more oxygen would have been added to all of the final volumes, which wouldn’t affect the comparison of results. Also the water was displaced, bubbles were produced, which often overlapped one another, making it very difficult to read off the burette. This was made worse when the concentrations were faster and even more water was displaced in order to make room for the increasing oxygen production.
The main source of error was the delay of putting the bung on the conical flask. The graphs show that out of a total of four anomalies, three of them are at 15s. This supports the idea that the insertion of the bung has caused these errors because by putting the bung on the flask slightly after the reaction has started, when the reaction is fastest, quite a lot of oxygen is lost. The anomalies at 15s on the 2, 3 and 5% concentration graphs are all underneath the line of best fit, illustrating that the volume of oxygen recorded was less than expected. The logical explanation for this is that the bung was after the reaction had begun and so some of the oxygen had already escaped. The fourth anomaly is on the 1% concentration graph at 30s. This value is also below the line of best fit, demonstrating less oxygen was produced than usual.
To ensure precision in the data, the most suitable apparatus was used for each substance. This made sure that there was less room for errors as the larger the piece of equipment, the larger the calibrations. For example a 2cm3 syringe was used for 2cm3 catalase. Masking tape was put on the trough ¾ of the way up, as when the water is the same amount, there is no change in pressure. If the pressure were increased by the presence of more water, there would be more displacement of water. A stop clock was used to make sure the volume was recorded every 15s.
The error bars demonstrate reliability as they show the higher and lower limits of each point whilst taking all the repeats into account; the larger the error bar, the less reliable. For 1, 2 and 3% concentration, the error bars are generally on the smaller side whereas for the 4 and 5% concentrations, the error bars are mostly quite large, suggesting the 4 and 5% concentrations results were less reliable. The most likely reason for the error bars being so different is the displacement of water. When the concentration is higher, more oxygen is produced and at a faster rate. This makes it hard to read as the bubbles produced speed past one another. When the concentration is lower, the oxygen is produced at a much slower rate and so the bubbles do not tend to move as quickly, making it easier to read. This leads to larger error bars in the higher concentrations as is more difficult to tell the exact volume of oxygen.
The standard deviation shows the spread of the data from the mean. Most of the values do not vary very much. It is only really in the 5% concentration graph from 45s onwards that there are large standard deviations. This means that the range is very big and so these results are not very reliable as the repeats were not very consistent like the rest of the data.
The validity depends on the range and reliability of the measurements. The conclusion drawn from the results and graphs is that the substrate concentration is directly proportional to the volume of oxygen produced, supporting my hypothesis. The range of my planned values were good, however from the given data it appears that I could have planned to use up to 5% concentration as it worked rather well. The experiment was quite fair and the control variables were all successfully controlled and the method was rather precise. It was also quite accurate despite all the possible inaccuracies. Therefore I think my results are valid enough to support my hypothesis, although many more repeats would need to be carried out in order to prove the hypothesis.
Bibliography
Jones, Fosbery and Taylor, 2000. Biology 1, pp 42-49 Cambridge University Press.
Toole & Toole, 2004. Essential AS Biology for OCR, pp 46-51 Nelson Thornes.
Class Information