Saturday, September 14, 2013

Acids and Bases Lab

Purpose: The main purpose of the lab was to measure the buffer on each liquid solution. The concepts we were testing was acid and base droplets measuring the buffer of the solution. The independent variable in the lab was the droplets of acid or base. The dependent variable was the pH of the one solution tested for reactions from both the acid and base droplets being added. Overall what our group was trying to do was to find the buffer range comparing the pH of acid and base affecting the one solution to another.
 Introduction: What will be considered a base is a solution whose pH is above 7, While for an acid when the pH is below 7. pH  can be solved with the equation of -log[H+] and is based off H+. This means that to find for hydroxide OH- one would subtract 14, which is the highest pH from the pH of the H+ solution. An example of this would be if a solution has a pH of 10 the OH- concentration would be 4. Last but not least is the buffer which minimizes the H+ and OH- concentrations depending on the situation which can be if the solution is excess or depleted in hydrogen ions. These things are important concepts to understand for the experiment.

Methods: After setting up the LoggerPro to collect data from the pH probes and gathering up the materials, we had two group members man the dropper bottles of 0.1 M acid and base, while the other two handled the LoggerPro and held the probes in place. For each step, the two with the droppers would drip five drops of their respective substances into the respective beakers at the same time, at which point the LoggerPro guy would add the new pH reading into the data table as a data point.
(Y-Axis: pH. X-Axis: Drops of base or acid added to beakers. Blue line: Beaker containing base. Red line: Beaker containing acid.)
(Y-Axis: pH. X-Axis: Drops of base or acid added. Blue line: Beaker containing base. Red line: Beaker containing acid.)
(Note: "Buffer acid" refers to buffered aspirin.)

Discussion: When we tested buffered aspirin, the beaker designated for bases was moderately basic (at 9.11 pH) and the beaker for acids was just slightly acidic (at 6.75 pH) already. This may have been a result of us forgetting to clean off the probes between buffers, so there was lingering acid and base still left on them. Now, as we added drops of the acid and base, both gradually became more acidic, even the beaker into which we were adding base. The acid ended up just a bit more acidic than before (6.05 pH), and the base was noticeably more acidic than it had been (7.90 ph). A similar phenomenon occurred with the orange juice test. While both beakers started out acidic (4.67 acid, 4.83 base), they both ended the experiment marginally more acidic than they had been, at 4.42 and 4.67 pH, respectively.
What can be definitively observed is that orange juice is a much stronger buffer, resisting pH change strongly so that the net pH difference was less than 0.5 pH for both beakers. The buffered aspirin is more difficult to place; while the acid beaker resisted the pH change rather well, only changing by 0.7 pH, the base beaker  had a larger change of 1.21 pH, which doesn't seem like ideal buffering. The pH also should not have been decreasing in that case, obviously, so there had to be some outlying error.
I think there would have been two errors that could have affected the outcome of the experiment, and both of them were the product of haste. First, we gave the mixtures very little time to settle before we collected data. Basically (har har), we added the drops, charted the pH change immediately, and then went straight to the next five drops. The readings would probably have been more accurate if we had given the machine more time to process the data. Additionally, we didn't rwally cleanse the probes between buffers. We did put the probes back in the "clean water" beaker after doing the tests for buffered aspirin, but we never changed the water to make sure it was clear of leftover acid or base that came off the probe, so we may have had the same residue from the first tests misleading the probe throughout the rest of the experiment.

Conclusion: According to our data, the most effective buffer that we tested was Orange Juice because it had the smallest change in pH when we added acid and when we added base. The change in pH in the acidic solution was .25, going from 4.67 to 4.42. The basic solution only became slightly more acidic at a change of .16, going from 4.83 to 4.67, although this may be attributed to a previous error n experimentation.

References: Pearson Publishing. " Campbell Biology: 9th Edition" 2011.

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