Organisms and pH Lab

 

Introduction

The purpose of the lab is to explore how different biological substances from living organisms (liver, potato,egg white, and gelatin) compare to non-biological substances (tap water and a commercial buffer compound), in their ability to resist changes in pH. pH is the measure of the amount of acid or base in a solution.

 

KEY POINT OF LAB:

A cell’s chemical “tools” are called enzymes that are a necessary part of all biochemical reactions. Enzymes are an example of a type of organic compound called protein. A drastic change in pH and/or temperature can permanently damage the structure of proteins/enzymes. If the enzyme’s structure is damaged, so is its ability to function. Cells die if their enzymes don’t work. Ironically, most cellular activities produce a variety of acids and bases that in turn affect pH. The cell must resist these potential pH shifts. Cells protect their enzymes by using buffering molecules also produced by the cells.

 

The acid and the base you will be adding to the samples are an acid called hydrochloric acid (HCl) and a base called sodium hydroxide (NaOH). The concentration for each will be 0.1 M.

 

Your pre-Lab will need two blank data charts (one for HCl and the other for NaOH) with all columns and sample headings. Your pre-Lab will also have two blank line graphs (one for HCl and the other for NaOH). You will fill in the data charts during the lab and recopy for your final report. You will construct the line graphs using your completed data chart data

 

Materials

six 50 ml beakers

graduated cylinder

pH probe

egg whites

gelatin

liver

potato

tap water

commercial buffer

0.1M HCl

0.1M NaOH

 

Procedure

 

1.     Fill a 50 ml beaker with 25 ml of tap water. Place the probe in the tap water. Record the initial pH using the probe. Record the pH after every fifth drop. You will be adding 30 drops of 0.1M HCl. Use the probe to continuously stir the sample. Your HCl data table will have a total of seven recorded pH values for each sample plus the eighth column, which is the total change in pH. Indicate if the total change is +/-. These Data Charts will be included in the Data section of your lab report.

2.     Rinse the probe with distilled water. Repeat procedure #1 with each of the other five substances: commercial buffer, gelatin, potato, liver and egg white. Be sure to rinse and clean off the probe between each sample/run.

3.     After recording the data for each of the six sample/run using the acid, HCl, now repeat steps 1 and 2 using the base, NaOH. Fill in your second data chart (NaOH). You should have a total of 12 runs. Each substance should be tested once with the HCl and once with the NaOH.

4.     Clean up your station. Be sure to rinse the probe with distilled water and wash out all of the beakers.

5.     Using your pH data table create two line graphs (by hand and ruler or by using Excel) using your pH data for each of the runs (six with HCl) and (six with NaOH). Combine the six samples HCl data on the same line graph. The Y axis should be pH (1-14) and the X axis should start with "initial" and finish with 30 drops (Do not include the total change in the line graphs). Be sure to label each run/line clearly (Liver, Gelatin, etc.) Now do the same for the second line graph showing the sample’s results for NaOH. These graphs must be included in the Data section of your lab report.

6.     Follow all directions on the” Laboratory Investigation instruction sheet”. Whenever possible give quantitative answers.

 

 

 

Analysis Questions

 

Based on your data, answer the following questions in the Question section of your report. Your answers must be complete sentences that incorporate the question. This is very important. You will lose points if your answers are not complete sentences that incorporate the question.

 

 

1. How many pH points did tap water change after you added HCl? How many pH points did tap water change after you added NaOH?

 

2. How many pH points did each of the four biological materials change after you added the HCl? After adding the NaOH?

 

3. How did each of the four biological materials, especially liver and potato, compare to tap water in their response to the addition of an acid and a base?

 

4. How many pH points did the commercial buffer change after you added HCl? After adding the NaOH?

 

5. Is the pH response of the commercial buffer more like that of tap water or more like that of one of the biological materials?

 

6. Based on your data do you think the biological materials contain anything that behaves similarly to the commercial buffer? Based on your data do you think tap water contains anything that behaves similarly to the commercial buffer?

 

7.Which of the biological materials resisted pH shifts the best? Which resisted the pH the worst? Why might that be?

 

8. (Reread the Main Point of Lab, yes that means turn the sheet over and reread those 7 sentences) Do organisms need to resist shifts in pH? Why or why not? Does tap water need to resist shifts in pH? Why or why not?

 

Conclusion

 

(Reread the Main Point of Lab, yes that means turn the sheet over and reread those 7 sentences) Write a paragraph conclusion summing up the outcome of the lab. Statements like “interesting lab, I learned how to use a pH probe” are inadequate. The conclusion must state what you actually learned, describe what was learned, was the purpose of the lab accomplished, did you learn anything about how to use the computer? Were the substances better at buffering than tap water, how do you know? Which were the best, the worst? Did they buffer the acid the same as the base?

 

Don’t forget the last heading: Possible Sources of Error.

 

Attach to the back of your report the stamped pre-lab.