ANALYTICAL (pH) MEASUREMENT Laboratory Procedure Background InformationpH is the hydrogen...

ANALYTICAL (pH) MEASUREMENT Laboratory Procedure Background Information

pH is the hydrogen ion concentration of an aqueous (water-based) solution. Pure water ionizes to a very small extent to produce a few hydrogen and hydroxyl ions in equilibrium with the water molecules. Pure water always contains equal amounts of each of these ions and is considered neutral. An acid is a substance that yields hydrogen ions when dissolved in water. Bases are substances that yield hydroxyl ions when dissolved in water. The relative strength of an acid or base is found by comparing the concentration of hydrogen ions in solution with that of pure water. Any solution that contains equal concentrations of hydrogen and hydroxyl ions is neutral. Any solution that contains an excess of hydrogen ions is acidic, and any solution that contains an excess of hydroxyl ions is basic.

The pH probe is designed to selectively measure a hydrogen ion concentration difference between the inside of the pH sensitive glass probe and the solution it is immersed in. The small voltage (millivolt) produced by this probe is usually indicated on a meter marked 0-14 with 7 at midscale. Seven is considered to be neutral having equal amounts of hydrogen ions and hydroxyl ions.

Finally, each whole number on the pH scale represents a ten-fold change in concentration. That means that a pH of 5 is ten times more acidic than a pH of 6.

Materials Needed

• a pH meter and pH probe(s)

 NOTE: The pH probe(s) may be a combination probe (singular) or it may be two separate probes. In this lab, assume that the probe is a singular combination probe.

• five 250-ml beakers

• buffer solutions: 4 pH and 10 pH (Alternatively, rather than a solution of one material in different concentrations, use different liquids. Try fruit juices, milk, Coca-Cola®, and liquid detergent.)

• squirt bottle filled with distilled water for rinsing the probe

• distilled water or tap water to make up fertilizer solutions

• fertilizers such as Peters or Miracle-Gro®

Safety Requirements

Safety glasses are required in the lab.

Precautions

Glass electrodes are very fragile. Electrodes should be left in liquid.

Procedure

a. Check the calibration of the pH meter by immersing the probe in the two buffer solutions (first the 4 pH buffer solution and then the 10 pH buffer solution). Rinse the probe with distilled water between separate immersions. Always rinse the probe before placing it in another beaker.

b. If the calibration is NOT correct, follow the instructions provided by the instructor or read and perform the calibration procedure found in the instruction manual. Once the meter is correct, go on to the next step.

c. Add distilled water or tap water to a separate beaker. Place the probe into the water and record the reading.

 PH =______

d. Make up a 5% solution of fertilizer in the beaker. Record the new pH.

 PH =______

NOTE: To make a 5% solution, add 5 grams of fertilizer to the beaker and enough distilled water to bring the solution level to the 100-ml mark. Use this procedure to make up the other solutions. Another option is lo add the additional 5-gram increments lo the previous solution.

e. Make up a 10% solution of common fertilizer in the same or another beaker. Record the new pH.

 PH =______

f. Finally, make up a 20% solution of common fertilizer in the same or another beaker. Record the new pH.

 PH =______

g. Correlate the percent solution concentration to the pH recorded.

	Percent Solution	pH
1	5%
2	10%
3	20%

h. Draw a graph representing the percent versus the pH.

Additional Information

The actual solution concentration (chemical to water weight concentration) will probably not have a linear relationship with the resulting pH. Specific chemicals will have different effects on the water and its resulting pH. If you were careful while conducting the lab procedure, your data should support a reasonable conclusion.

Findings

Compile your observations and readings into a report that includes your recorded data, an explanation of the results of the experiment, and a conclusion. Include a paragraph explaining correlation of the observed pH for each concentration level.