pH, Buffers and Osmolarity

Objective:

To measure pH and osmolarity and to explain in a short essay the importance of maintaining pH and osmolarity at critical values in the body, at the level of 85% proficiency for each student.

In order to achieve this objective, you will need to be able to:

Define pH, buffer, osmolarity, molarity.
Measure pH using different methods.
Observe buffer action and compare two buffers.
Measure osmolarity.
Prepare solutions of various electrolyte concentrations and thus of various osmolarities.

pH and Buffers

Materials:

Group Supplies

pH paper

pH meter

forceps

3 - 100 mL beakers

20 mL of each

Phosphate Buffer (Sorensen's buffer)

Milk

Vinegar

250 mL beaker for titration

250 mL beaker for rinse waste

ring stand with titration burette and clamp

0.0015 N HCl to fill the burette

50 mL of each

0.9% NaCl

Phosphate Buffer (Sorensen's buffer

Blood plasma

Safety glasses (as available)

Orange hazardous waste disposal bag

Methods:

Using the pH paper and the pH meter, measure and record the pH of the solutions listed in Table 1.
Using a 50 mL burette with 0.0015 N HCl, titrate acid into each of the following solutions: (1) 50 mL of 0.9% NaCl, (2) 50 mL of Sorensen's buffer, (3) 50 mL of Blood Plasma. For each solution listed in Table 2 take a pH reading before titration and after each addition of HCl in 10 mL increments.

Results:

Table 1 – Measurement of pH
Solution	pH paper	pH meter
20 mL of Phosphate Buffer
20 mL milk
20 mL vinegar

Table 2 – Buffering action of NaCl and Phosphate Buffer
0.9% NaCl (50 mL)		Phosphate Buffer (50 mL)		Blood Plasma (50 mL)
0.0015 N HCl Titrated	pH	0.0015 N HCl Titrated	pH	0.0015 N HCl Titrated	pH
0 mL		0 mL		0 mL
10 mL		10 mL		10 mL
20 mL		20 mL		20 mL
30 mL		30 mL		30 mL
40 mL		40 mL		40 mL
50 mL		50 mL		50 mL

Discussion:

1. If you know pH, can you determine the actual hydrogen ion concentration? Explain.

2. The pH scale allows us to use simpler numbers to express hydrogen ion concentration. Can you think of any disadvantages in using pH instead of molarity?

3. Describe some activities or situations that might decrease body fluid pH, increase pH.

4. What is a buffer? Do buffers actually get rid of hydrogen ions? Explain.

5. Using the titration data, plot the 3 curves on one graph, with the pH on the Y axis and the mL of 0.0015 N HCl on the X axis. Compare the graphs from each titration. What do these graphs reveal about the buffering capacity of each solution?

6. Name an actual body buffer system. Using chemical formulas illustrate how this buffer system works if excess hydrogen ions are present and if too few hydrogen ions are present.

7. Describe the significance of buffer systems in pH regulation. That is, what would life be like wihtout body fluid buffers?

Osmolarity

Materials

Group Supplies

100 ml graduated cylinder

3 - 250 ml beaker

3 - 50 ml beakers

Lab Supplies

0.05 M NaCl

0.15 M NaCl

NaCl

distilled water

balance

2 - 4 test tubes

20% potassium chromate solution (in dropper bottle)

2.9% silver nitrate (in dropper bottle)

[optional: vapor pressure osmometer, sample filters, 10 microliter pipeter, pipette tips, calibration standards]

Methods:

Determine the NaCl concentrations of the solutions listed in Table 3.
Prepare four concentrations of NaCl.as listed in Table 4. You must determine the number of grams of NaCl that must be added to distilled water to make 100 ml of solution with these concentrations. For each solution prepared calculate the actual osmolarity.

Determination of Sodium Chloride Concentration:

· Measure 10 drops of urine into a test tube using a standard medicine dropper. Add 1 drop of 20% potassium chromate solution to the urine.

· Add 2.9% silver Nitrate solution 1 drop at a time using the dropper in the bottle. Vigorously swirl the test tube after each drop of silver nitrate added.

· Count the drops of silver nitrate solution required to turn the solution from a bright yellow to an orange brown, color.

· Each drop of 2.9% silver nitrate required to produce the color change represents approximately 1 gram/liter of NaCI.

Results:

Table 3 – Measurement of NaCl concentration and calculation of osmolarity
Solution	measured NaCl concentration		calculated osmolarity
	gm/L	moles/L	mM
0.05 M NaCl
0.15 M NaCl

Table 4 – Preparation of solutions and calculation of their osmolarity
desired molarity (mM)	volume of solution to make	amount of NaCl needed (gm)	calculated osmolarity (mM)
100 mM	100 ml
150 mM	100 ml
200 mM	100 ml

Discussion:

1. What is the apparent relationship between molarity and osmolarity.

2. Predict the osmolarity of a 0.05 M solution of MgCl₂.

3. Predict the osmolarity of a 0.05 M solution of HCl. What is the pH of this same solution?