pH of Water

Introduction

The pH of a water sample will be found by using the TesTab^® color concentration method.

Concepts

• pH
• Acid–base concentration
• Water testing

Background

The pH test is a standard test used during water analysis. pH refers to the relative abundance of hydrogen ions in a water sample. In pure water, scientific measurements have shown that the [H⁺] is 1.0 × 10^–7 moles per liter. The negative log of the concentration of hydrogen (Equation 1) in pure water produces the value of 7.

As water becomes more acidic, the pH values decrease from 7 to 6 to 5 to 4 and so on. As the solution becomes more basic, the pH values increase from 7 to 8 to 9, etc. (Figure 1). A normal pH scale has a range of 0 to 14. Most aquatic organisms require a pH range between 6.5 and 8.2. At pH levels below 5 larval stages of insects and other small aquatic organisms may die off rapidly. Water with abundant algae and vegetation growth usually has a significantly high pH. This is due to the fact that rapidly growing algae and vegetation remove carbon dioxide from the water during photosynthesis. At pH levels above 9, fish may have a difficult time excreting ammonia from their bodies.

Materials

Safety Precautions

pH wide range TesTabs^® contain chemicals that may irritate skin or be harmful if swallowed. The TesTab reagents used in this kit were designed with safety in mind. The single-use, foil-packaged TesTabs are easy to dispense. Store TesTabs in a cool, dry place and only open when ready to use the tablet. A single tablet, either alone or reacted with a sample, is a very low health hazard; however, TesTabs should not be ingested. Wear chemical splash goggles and chemical-resistant gloves. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. In the field, reacted samples may be poured into a container for later treatment. Reacted samples may be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Procedure

1. Fill the water sample tube to the 10-mL line with a water sample.
2. Add one pH Wide Range TesTab^® to the tube.
3. Cap the tube and mix until the tablet has dissolved.
4. Compare the color of the sample to the pH color comparison chart.
5. Dispose of the reacted sample according to the instructor and rinse the water sample tube twice.

Teacher Tips

• Enough materials are provided for 15 individual pH tests.
• It’s a good idea to collect several water samples from each site that is tested. It is also wise to test the samples within one hour of collection if possible.
• pH Wide Range TesTabs contain a mixture of pH indicators that are sensitive to pH and undergo specific color changes with variation in pH.
• Positive pH results will be seen in the range of 4–11.
• TesTabs are a vendor product of the LaMotte Company. SDSs are available through the manufacturer website.