Introduction to pH Indicators
Experiment 1:
1. Fill in the table with the results from your lab notes.
Cabbage juice color
pH
water
Blue
7.00
acetone
Blue
7.00
5M citric acid
Pink
3.96
5% vinegar
Red
2.55
4M ammonia
Yellow
11.78
Bleach
Teal
10.52
2. How accurate is your cabbage extract, does it change to a different color for
every possible pH? The cabbage extract turned a different color for every pH
except for acetone, which remained blue.
Experiment 2 - Response of Solutions to the Addition of Acids and Bases
1. Fill in the pH values for the solutions before and after the addition of acid and base, then
answer the following questions.
Group 1
1 – 7.00 -> 2.01
2 – 4.68 -> 3.11
3 – 9.72 -> 9.72
4 – 7.21 -> 7.11
Group 2
1 – 7.00 -> 11.99
2 – 4.68 -> 6.26
3 – 9.72 -> 11.27
4 – 7.21 -> 7.39
2. Which solution was the most sensitive to the addition of acid or base? The solution most
sensitive was the first solution in group one, with 5 ml water and 2 drops of 0.5M
Hydrochloric Acid.
3. Which solution was the least sensitive to the addition of acid or base? The solution least
sensitive to the addition of acid or base was the third solution in the first group, consisting of
5
mL of 0.1M Sodium Dibasic Phosphate and 2 drops of 0.5M
Hydrochloric Acid.
4. Did any solution show a significant difference between its response to the addition of the acid
and the base?
The solution that did show a difference between its response to acids and bases was the third
solution: Sodium Dibasic Phosphate.
5. Which solution of the four is the best buffer against additions of acid or base?
The standard phosphate buffer (Sodium Dibasic Phosphate
and Sodium Monobasic Phosphate)
6. Within what range of pH did the best buffer perform well?
The range of pH in which the buffer performed the best was almost exactly neutral. The range
was 7.11 to 7.39.
Experiment 3 - The Buffering Capacity of a Phosphate Buffer
1. Record the results of the pH of the phosphate buffer solution vs. the number of drops of HCl
acid added to it.
Original phosphate buffer solution: 7.21
After HCI added: 7.16
Drop 2: 7.11
Drop 3: 7.05
Drop 4: 6.99
Drop 5: 6.91
Drop 6: 6.81
Drop 7: 6.69
Drop 8: 6.51
Drop 9: 6.21
Drop 10: 4.70
Drop 11: 3.15
Drop 12: 2.82 – pH has now fallen below 3.
Drop 13:
Drop 14:
Drop 15:
2. Record the results of the pH of the phosphate buffer solution vs. the number of drops of NaOH
base added to it.
Original phosphate buffer solution: 7.21
After NaOH added: 7.30
Drop 2: 7.39
Drop 3: 7.48
Drop 4: 7.58
Drop 5: 7.69
Drop 6: 7.81
Drop 7: 7.96
Drop 8: 8.16
Drop 9: 8.49
Drop 10: 9.72
Drop 11: 10.94
Drop 12: 11.26 – pH has now risen above 11
Drop 13:
Drop 14:
Drop 15:
3. Construct a graph of the pH vs. the number of drops of acid OR base added to the phosphate
buffer. Since, on the pH scale values below 7 are acidic while hose above 7 are basic, you can
construct a combined graph of both the acid and base data by doing the following:
 Use the initial pH of the carbonate solution for the Y value at the initial pH value of the buffer,
before any acid or base was added.
 Use the number of drops of base as positive X values
 Use the number of drops of acid as negative X values
The graph you constructed should give you a good idea of how much acid and base the
phosphate buffer solution can absorb before allowing the pH to change significantly.
Here's how to make a graph of your data:
Open the graphing utility by clicking on the graph button at the bottom of this
window page.
(a) Labeling Graph:
In the data area on the left, enter:
a title for your graph next to the phrase “Graph Title”
the X-axis units at the top of the X column
the Y-axis units at the top of the Y column
(b) Entering Data:
Click the + button for each set of data pairs that you want to graph. The - button will delete a
selected row of data pairs.
Click on the row to fill in the pair of x and y values.
To change any data, simply click on a value in the table and change its
value.
(c) Creating Graph
When all the data points have been entered, click the Draw Graph button
to view the graph.
If you update data after this point, click the Draw Graph button again to
see your changes.
(e) Saving Graph
To save your graph, click the Save as New button. The graph is saved
with your work with the title you gave it.
Use the My Saved Graphs dropdown menu at the top to view this or any
other graph you have saved for this lab.
Note: If you are editing a saved graph, click the Update button to overwrite
the original graph. If you click the Save as New button, the original graph
will stay intact and the updated graph will save as a separate graph.
(f) Submitting Graph
When you are satisfied with the graph, click Save Image to
Portfolio. Your instructor will only see graphs that you save to the
portfolio. You may save as many graphs as you like.
To view the contents of your portfolio, click the Return to
Assignments button at the bottom of this window, then select Portfolio.
4. How many drops of acid was the phosphate buffer able to absorb while keeping the pH
relatively stable?
5. How many drops of base was the phosphate buffer able to absorb while keeping the pH
relatively stable?
6. Hypothesis: The phosphate buffer has 2 components and a pH of around 7. One component
absorbs acids and the other absorbs bases. The buffer should be capable of absorbing up to half
its own 'weight' of excess acid or base.
Let's test this hypothesis.
To do so, we need to quantitatively compare the amounts of buffer and added acid and base in
the experiment. We use units of milli-mole (one thousandth of a mole), written as 'mmol'.
The phosphate buffer had 2.5 mL of 0.1M Na2HPO4, the acid absorber, where 0.1M means a
concentration of 0.1 moles per liter of solution. Convert this to mmol/mL as follows:
(0.1 Mole/L) x (1000 mmol / 1 Mole) x (1L / 1000 mL) x (2.5 mL) = 0 .25 mmol
The same amount of NaH2PO4 base absorber is present as well.
The amount of added acid or base is calculated from the number of drops, knowing that one drop
is approximately 0.05 mL, and the concentrations of the acid and base was 0.5M, as follows:
(0.5 Mole/L) x (1000 mmol / 1 Mole) x (1L / 1000 mL) x (0.05 mL/drop) = 0.025 mmol / drop
(a) Look again at the graph of pH vs. drops of acid/base and decide at what number of drops the
buffer failed to maintain a nearly constant pH.
(b) Record this pH value for the addition of both acid and base.
(c) Multiply the number of drops by the conversion factor above, and divide by the total volume of
buffer used (5 mL) to obtain the total buffering capacity, in mmol/mL. Your answer should be
written as follows:
Phosphate buffering capacity
= X mmol acid per mL of buffer
= Y mmol base per mL of buffer
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