6B: Acidity
INTRODUCTION
The acidity of a solution or substance is a way of describing the concentration of Hydrogen (H+) ions that it contains. This property affects important chemical processes that occur in living organisms, and must be kept in a narrow range to allow these processes to function properly.
The pH scale ranges from 0-14, with lower numbers being more acidic (i.e., having more H+ ions). The midpoint of this scale, 7, is considered neutral. The scale is logarithmic, which merely means that each change of 1 on it corresponds to a 10X change in the concentration of H+ ions. Thus, a change in pH of 2 indicates a 100X change in H+ concentration (10 x 10).
We will use a simple pH meter to measure the acidity of a variety of common substances.
SAFETY PRECAUTIONS
Be careful with glass containers, alert instructor to any breakage.
Be careful with the pH meter, a delicate piece of equipment.
Be careful with caustic household products (bleach etc.) and those which can stain.
MATERIALS
Common household substances.
pH meter.
Small beaker.
Distilled water.
PROCEDURE
Step 1: Speculate what you think the rank order of acidity will be among the household products, and arrange them in this order on the table. Generally, beverages are acidic while common household cleaning products are basic (with exceptions).
Step 2: Create an empty table with 2 columns x 11 rows in your lab notebook. Enter “Substance” and “pH” as column heads.
Step 3: Place 1/2 inch of tap water in a small beaker. Rinse the tip (bulb) of the pH meter wand in distilled water. Measure the pH of the tap water. Enter it in the first row of your table.
Step 4: Measure the pH of the substance you expected to be most acidic. Based on the behavior of the meter (how rapidly it settles on a value), come up with a rule to use to determine when you have arrived at your final reading, for future measurements. Write this in your lab notebook.
Step 5: Rinse the bulb and beaker with distilled water, and measure pH of the next substance in the order, and record in your notebook. Do you think it would be faster to do sequentially through your rank order of substances, or to alternate substances from opposite ends of the ranking? Why?
Step 6: After completing your measurements, identify the most acidic and basic substances. Too make a quick comparison of their concentrations of H+, round the lower pH value up, and the upper value down, and calculate their difference by subtraction. What is the difference in their rounded pH values? What does this translate into, in terms of how many times more H= one has than the other? Does the rounding process result in this being an under estimate of their difference, or an over estimate?
