Acids and Bases Lab

Purpose:

   The purpose of this lab experiment was to see which substance was the most resistant to differences in pH levels by adding drops of acid and base solution. We were trying to find the best buffer from our substances.

Introduction:

   Acids and bases are formed when water dissociates into a hydroxide (OH-) and hydronium  (H+) ions.  The dissociation occurs because of the polar nature of water.  Water naturally will share electrons unequally between its hydrogen and oxygen atoms.  Sometimes, the electrons will be shared so unequally that the electrons will leave one of the hydrogen atoms and attach to another hydrogen and an oxygen.  That's where hydronium and hydroxide ions are formed.  Acids denote a substance that has an abundance of hydronium ions, and bases have an abundance of hydroxide ions.

     pH is a measure of the quantity of hydronium.  More specifically, it's defined by the negative log of the hydronium concentration.   The pH scale gives people an accurate way of categorizing a substance as an acid or a base.  The substances are placed on one of fourteen points, the lower the number, the more acidic the substance is. If a substance has an H+ concentration of 10 to the negative 7, then it would have a pH of 7.  Likewise, if it had a hydronium concentration of 10 to the negative 13, it would have a pH of 1.  Water is considered neutral with a pH of 7. 

     Buffers are compounds that prevent a substance from becoming too acidic or basic.  They function by either providing H+ ions when a substance becomes too basic, or accepting H+ ions when becoming too acidic.  Buffers are especially important in preserving life.  For example, blood needs to be at about a pH of 7.2.  If it varies too much, biological processes could cease to function.  If it would become too acidic, then a buffer would accept H+ ions from blood.

 Methods:

     First, we tested the pH of water with two separate Vernier probes in two separate beakers.  Then, we gradually added both an acid and a base to different beakers containing the water.  Periodically, we collected the pH of both the acid and base waters.  Next, we tested the pH of an antacid solution and a salt solution and repeated the same steps we took to test the pH of water. 

Data:

   

 Water

pH

 Number of Drops

 

 Salt

pH

Number of Drops

Antacid

pH 

 Number of Drops

Discussion:
     We tested the solutions of water, antacid and salt to see which would be the best at maintaing a consistent pH level when an acid (HCl) or base (NaOH) was added (best buffer).

Water (pH of 7)
(we had a reading of 6.9 pH for our water)

Acid added to water
-starting at a pH of 6.90 acids general trend was a decrease in pH for how much HCl was added. The largest change in pH occurred when we added 10 drops. At this point we observed a 1.55 drop in pH. The pH of the water dropped from 6.9 to 5.35. Through out the rest of the trial the pH would decrease at a rate being cut in half after every trial (ex 0.4 to 0.2 decrease to 0.1). At the end of the trials the pH ranged from 6.9 to 3.12. That's a change of 3.78 pH.


Base added to water
-starting with a pH of 6.90, we added a base to water to find that there was a general trend that was an increase of pH by no specific rate for how much NaOH was added. The largest increase of pH would be after adding 15 drops of base, here the pH increased by 2.26 (8.02 to 10.28). By the end of all the trials the pH ranged from 6.9 to 11.51  which is a difference of 4.61 in pH.

Salt (pH of about 6.5)

Our results for salt showed that without adding any other substances, salt would be between 6.25 and 6.5 on the pH scale.

Adding acid to Salt
 -After adding 5 drops of acid we observed a very slight increase of approximately 0.3 in pH of the acid beaker. For the next set of 5 drops there was relatively no change but it did go down to 6.18. On the contrary, once we hit the 15 drop mark, acid had a much larger change in pH. The salt beaker that was having acid added to it had a change in pH from 6.18 to 4.60. After the acid beaker reached 4.60 it would decrease in pH by increments of 0.2-0.3 for the remainder of the trials. The general trend here was a decrease in pH for how much HCl was added.


Adding a base to Salt
- For this trial there is a strange pH level in the first trial before we added any drops of the base. We had a reading of 9.25 pH, but in the acid portion of the same trial we had a reading for uncontaminated salt of 6.63. We should have this same number if we had done this trial correctly. We did have some human error (we spilled our first beaker and had to start over). I think the reason we had such a high pH at the beginning was maybe we didn't clean the beaker out well enough to get rid of the extra NaOH. Other than that the solution increased at recognizable rate of about 0.4 to 0.5 drop of pH. General trend was an increase for amount of NaOH added.


Antacid (pH of about 6.7)

Adding Acid to Antacid
- We had a starting reading of a pH 6.7. In this trial adding the acid wouldn't really effect the antacid solution. It would not fluctuate much at all. The largest change in pH was between trials of 5 & 10 drops, where the pH would go from 5.98 to 6.10. The highest the pH would be at 6.7 pH and lowest at 5.62 pH. The range of difference in pH was a modest 1.08. The general trend was that the solutions pH wouldn't really change that much at all no matter how much HCl was added.

Adding Base to Antacid
- This trial was another one where we experienced a strange reading in the unchanged solution. We got a reading of 9.11 for the 0 drops added trial. This unfortunately was another trial that we happened to knock over the beaker and spill the contents of the beaker everywhere. I think we go this reading because there was left over NaOH in the beaker. We should have gotten the 6.7 pH that we did in the acid portion. The biggest increase in pH came when the pH jumped from 9.11 to 9.89 a change of 0.78 pH. General trend was an increase of pH for the more NaOH added.

** Antacid is the most resistant to change in pH, which makes it the best buffer out of the three.

Did our findings add up to our hypothesis?

-Despite our human error in two of our trials our expectations were meet in this experiment! We had predicted that we would see a general trend when acid is added to a solution that the pH would decrease and if a base is added then the pH would increase. Although we were unsure which solution would be the best buffer.

Conclusion:

The best buffer we found to be was antacid although our data may have been skewed from spilling the basic substance twice, linearly the antacid was most resistant to the changes in pH level.

References: 
http://www.chemistry.wustl.edu/~edudev/LabTutorials/Buffer/Buffer.html