Laboratory 3: pH

Laboratory 3: pH

Introduction – Water and pH
In any aqueous solution (water-based solution), water molecules constantly break down into hydroxyl ions (OH-) and hydrogen ions (H+) at a specific rate (H2O→OH- + H+). At the same time, the opposite reaction (OH- + H+ →H2O) also occurs. The combination of these two reactions results in a specific amount of H+ being present in that solution.

The concentration of H+ in a solution will affect any biological process that occurs in that solution. Hydrogen ions alter the structure of proteins, and in nature, structural changes in a molecule will alter that molecule’s function. Increasing or decreasing the H+ concentration will change the shape of any protein in that solution.
pH is defined as the –log [H+], which translates to a scale that goes from 0 to 14 and this scale indicates the amount of H+ present in a solution. In a neutral solution (such as distilled water) the concentration of H+ results in a pH of 7, which chemically means that the hydrogen ions and the hydroxyl ions are balanced and thus cancel each other. For each whole number decrease in pH (ex. a change from a pH of 6 to a pH of 5), the concentration of H+ is increasing by a factor of ten.

A solution that has a pH below 7 is said to be acidic as it has a surplus of H+ while a solution that has a pH above 7 is said to be basic or alkaline, with a surplus of OH-. Low pH means very acidic, while high pH means very basic.

Most cells within the human body require that the pH be maintained at or around 7.4. Yet, substances are being produced (due to processes that are occurring within the body) that can lead to the production of acids and bases. Radical changes in pH would have drastic effects on the ability of the body to function properly. This would be a major problem. However, our bodies have mechanisms for controlling these changes. Within all body fluids are substances known as buffers. These chemicals mitigate changes in pH. Buffers act to absorb H+ when acids are added to a solution or they release H+ when bases are added to a solution. Consequently, a solution that contains a buffer will have a relatively stable pH regardless of the addition of acids or bases.
1. Though the buffers found in body fluids are very good, they can be overwhelmed. This can lead to conditions known as acidosis in which the body fluids have a pH that is too low or alkalosis in which the body fluids have a pH that is too high.
2. Most solutions found in the environment lack such buffers. The pH of these environments can change radically with the influx of acids or bases. Often the microorganisms living in a closed environment (stagnant pool, bottle of milk, etc.) will produce acids as they carry out their metabolism. This can lead to changes in the pH of the environment. These changes in pH ultimately kill the organisms that caused the change.
In the laboratory, chemicals known as indicators can be used to allow the pH of a solution to be determined at a glance. These indicators are chemicals that change color according to the concentration of H+ in a solution. Some of these chemical indicators can be embedded in paper strips to allow for a more precise determination of pH. The most precise way to measure pH is use of an electronic device known as a pH meter.

Exercise Part 1 Measurement of pH
Navigate to https://phet.colorado.edu/en/simulation/ph-scale-basics and open the simulation titled pH Scale: Basics. Use the simulation to determine the pH of the solutions listed below
Drag the end of the pH meter probe into the vessel.
Click and hold the red button on the dropper to fill the vessel to the 0.5 L mark with water
Record the pH value.
Using your mouse, drag open the valve at the bottom of the vessel to empty the contents.
Repeat the process with the other substances available from the pull-down menu.
Solution pH Acidic/Basic/Neutral Surplus of
H+ or OH-s
Water ________________ __________________ _________
Drain Cleaner ________________ __________________ _________
Hand Soap ________________ __________________ _________
Blood ________________ __________________ _________
Saliva ________________ __________________ _________
Milk ________________ __________________ _________
Chicken Soup ________________ __________________ _________
Coffee ________________ __________________ _________
Orange Juice ________________ __________________ _________
Soda Pop ________________ __________________ _________
Vomit ________________ __________________ _________
Battery Acid ________________ __________________ _________

Which of the above substances have relevance for living organisms?

Battery acid is _____ times more / less acidic / basic than vomit.

Exercise Part 2: Determining the Effect of Acids and Bases on Different Solutions
The experimental setup consists of two (2) beakers, one that contains Solution 2.1 and the other contains Solution 2.2. The researcher measures the pH of each solution and records these values. The researcher then adds 1 mL of an acid to each beaker, mixes and measures the pH of each. The pH values of each are recorded and this procedure continues until a total of 5 mL of acid has been added to each solution. Table 1 represents the data recorded for this experiment.

Identify the dependent and independent variables

Plot the information in the table below on the grid provided

Table 1
Volume of Acid added pH of Solution 2.1 pH of Solution 2.2
0 mL 10.31 9.62
1 mL 10.25 7.2
2 mL 10.16 5.2
3 mL 10.08 4.5
4 mL 9.99 4.3
5 mL 9.60 4

Follow up question

What can your deduce about solutions 2.1 and 2.2 from these results? Which of these is the buffered solution?