Materials and Resources

MWRA pH handout

Note: This text is made available courtesy of the Massachusetts Water Resources Authority. We have scanned in the original manual pages and converted the files to text. Unfortunately, the pictures were lost and there are some irregularities in the formatting. Our apologies for the inconvenience.

TEST 2: pH

Brief Summary

A relative measure of water's alkalinity/acidity. This test uses a liquid reagent and color comparator.

Note: To obtain the most accurate results, this test should be completed in the field.

Background

Chemically, pH indicates the number of hydrogen ions. At a pH value of 7.0, water contains an equal number of hydrogen ions (H+) and hydroxyl ions (OH-). If there are more hydrogen ions than hydroxyl ions, the substance is acidic and has a pH level lower than 7.0. IF there are more hydroxyl ions the substance is alkaline, and it has a pH value higher than 7.0.

The pH scale is logarithmic, so each one-digit change in the scale indicates a ten-fold change in acidity or alkalinity. In other words, a substance with a pH of 3 is 10 times more acidic than a substance with a pH of 4; 100 times more acidic than a substance with a pH of 5, and.10,000 times more acidic than a substance that is neutral. Substances at the far ends of the pH scale, therefore, are extremely acidic or alkaline.

Natural Factors That Influence pH

In nature, decomposition of organic materials releases carbon dioxide, and as that carbon dioxide mixes with water, it forms carbonic acid. Alkaline minerals, such as limestone, however, can buffer that acidity. At the time of their formation, most lakes and ponds had high pH, but over time carbonic acid has lowered their pH. Fresh water is much more susceptible to changes in pH than sea water because the minerals in sea water act as almost infinite buffering agents. Sea water, in fact, tends to maintain a steady pH of between 7.7 (in deeper waters) and 8.1 (at the surface).

Human Factors That Influence pH

The industrial revolution brought a variety of new combustion technologies which released varying amounts of sulfur dioxide (SO2), oxides of nitrogen (NOx), and carbon dioxide (CO2) into the atmosphere. These compounds go through a series of chemical, reactions in the atmosphere and eventually return to earth as add precipitation. Where limestone exists in the soil, the add is readily neutralized; where it does not exist, such as in large portions of the northeastern U.S., many lakes and streams have acidified. Interestingly, many of the acidic lakes are beautifully clear, but that clarity is caused by an alarming lack of life.

Testing for pH

Overview:	This test consists of a test tube of sample water to which you will add 10 drops of a wide-range indicator. You will then compare the color of the solution with colors in an octet comparator. The test contains two comparators: one ranges from 3.0 to 6.5; the other, from 7.0 - 10.5.
Length Of Test:	<5 minutes
Difficulty of Test:	Simple
Protective clothing	Eye protection, rubber gloves
Suggestion:	Reading the colors on the octet comparator is rather subjective. Given the ease and speed of this test, therefore, we recommend that each person in the group read the comparator, and that several different groups carry out the test. If the results vary, take a class average and discuss the inherent difficulties of testing variability.
Expected Results:	Sea water maintains a pH of around 8.0. In shallow coastal waters, that number may go as low as about 7.0. Fresh water in the region generally ranges from about 5.5 to 6.5. Snow and rainwater may be lower.

PROCEDURE

1. Fill one test tube with sample water to the level of the line marked on it (5 mL).

2. Carefully add 10 drops of the wide-range indicator to the test tube.

3. Cap the test tube with the small blue plastic cap and shake the solution gently until the color is uniform. (DO NOT cover the tube with your fingers. It is bad lab practice and it might change the pH of the solution.)

4. Select the appropriate octet comparator and insert the test tube into the appropriate slot.

   USING THE OCTET COMPARATOR

   • The test contains two comparators: one ranges from 3.0 to 6.5; the other from 7.0 - 10.5.

   • After adding indicator solution, the water sample will change colors. The color indicates the level of acidity/ alkalinity.

5. Hold the comparator so you are looking into a light source (preferably natural light) before comparing the color of the sample to the colors in the comparator.

6. Record the pH value on the Data Recording Form.

7. Have a lab partner (or a second person) take an independent reading so you can double-check each other.

8. The waste from this test is harmless to the environment and the water system, so you may pour it out in the field or wash it down the drain.

9. Clean and dry the equipment and carefully replace everything into the kit.

Interpreting Results

Did you predict the results you found? What conditions at the site or in the watershed area might have led to this pH level? What type of fish and plant life can and cannot survive at this level? Refer to Table 2 on page 38 for additional details.

Extension Activities

1. Test liquids throughout the school using the pH paper in the kit.
   Procedure for using pH paper
   1. Wet the pH paper in the sample.
   2. Compare the pH paper with the color chart in the box of pH paper.

   Test whatever sample liquids you want. Before testing a liquid, predict what you think the pH might be. Here are a few suggestions with some comments about their chemistry:

   Colas: All carbonated beverages contain carbonic acid, so they are acidic. Aspirin (dissolved in water): Aspirin is acetylsalicylic acid.

   Bicarbonate of soda (dissolved in water): People often take bicarbonate of soda to neutralize the unpleasant effects of too much stomach acid. What do you think its pH value is?

   Alka-Seltzer or Buffered Aspirin (dissolved in water): Both of the these substances contain acetylsalicylic acid and bicarbonate of soda, an acid and an alkali.

2. Think of ways to alter the pH of a sample. For example, you could make solutions of dilute lemon juice and bicarbonate of soda, and gradually add drops of those solutions to samples to see if they can change the pH.

3. Measure the pH of water in nature, such as rainwater, snow, and parking lot runoff. Let rainwater seep through a sample of soil in your area and compare the pH level both before and after.

4. Chart the pH level of precipitation during a single storm or over several storm events. Note whether the pH level changes during the storm and the direction from which the storm came. Does rain from a "'nor'easter"' (which comes from the northeast where there are few sources of industrial pollution) have a different pH level than rain from the south or west (where industrial regions emit sulfur dioxide and oxides of nitrogen)?

   Back to:

   • Materials and Resources