pH and Salinity » Practical: Testing pH of Different Waters

Introduction to pH and Water Testing

Water quality is crucial for marine life and two of the most important factors we can measure are pH and salinity. pH tells us how acidic or basic water is, whilst salinity measures the amount of salt dissolved in water. These measurements help scientists understand marine ecosystems and monitor environmental changes.

Key Definitions:

• pH: A scale from 0-14 that measures how acidic or basic a solution is. 7 is neutral, below 7 is acidic, above 7 is basic.
• Salinity: The amount of salt dissolved in water, usually measured in parts per thousand (ppt) or practical salinity units (PSU).
• Indicator: A substance that changes colour to show the pH of a solution.
• Buffer: A solution that resists changes in pH when acids or bases are added.

Equipment and Materials Needed

Before starting any practical work, it's essential to gather the right equipment and understand how to use it safely. Here's what you'll need for testing pH and observing salinity effects:

Essential Equipment

For accurate pH testing, you'll need several key pieces of equipment. Each serves a specific purpose in measuring water quality.

🧪 pH Meters

Digital devices that give precise pH readings. They need calibrating with buffer solutions before use.

🌈 Indicator Solutions

Universal indicator changes colour to show pH. Red means acidic, green is neutral, blue/purple means basic.

📌 pH Test Strips

Paper strips that change colour when dipped in solutions. Compare the colour to a chart to read pH.

Testing Different Water Sources

Different water sources have varying pH levels due to dissolved minerals, pollution and biological activity. Testing multiple sources helps us understand how human activities and natural processes affect water quality.

Common Water Sources to Test

Each type of water has characteristic pH ranges. Understanding these helps interpret results and identify unusual readings that might indicate pollution or other environmental issues.

🌊 Tap Water

Usually pH 6.5-8.5. Water companies add chemicals to make it safe to drink, which can affect pH. Chlorine treatment often makes tap water slightly basic.

🌞 Rainwater

Naturally slightly acidic (pH 5.6) due to dissolved carbon dioxide. In polluted areas, acid rain can have pH as low as 4.0.

🌋 Pond Water

pH varies with plant activity. During photosynthesis, plants remove CO₂, making water more basic. At night, respiration adds CO₂, making it more acidic.

🌊 Seawater

Usually pH 8.1-8.3 (slightly basic). The ocean acts as a huge buffer system, but climate change is slowly making it more acidic.

Step-by-Step Testing Procedure

Following a consistent method ensures accurate, comparable results. This procedure works for any water sample and gives reliable pH measurements.

Method for pH Testing

Step 1: Collect water samples in clean containers. Label each sample clearly with the source and collection time.

Step 2: If using a pH meter, calibrate it with buffer solutions (pH 4, 7 and 10). This ensures accurate readings.

Step 3: Rinse the pH probe with distilled water between each sample to prevent contamination.

Step 4: For indicator testing, add 2-3 drops of universal indicator to a small sample. Compare the colour to the pH chart.

Step 5: Record all results in a table, noting the temperature as this affects pH readings.

Step 6: Test each sample three times and calculate the average for more accurate results.

Understanding Salinity Effects

Salinity affects how organisms survive in water and influences pH measurements. Marine organisms have adapted to specific salinity levels and changes can be devastating to ecosystems.

Measuring Salinity

Whilst we focus on pH testing, understanding salinity helps explain why different water sources have different pH levels. Salt water conducts electricity better than fresh water and this property helps us measure salinity.

⚡ Conductivity Method

Use a conductivity meter to measure how well water conducts electricity. Higher conductivity usually means higher salinity.

💧 Evaporation Method

Evaporate a known volume of water and weigh the remaining salt. This gives an accurate salinity measurement but takes time.

Interpreting Your Results

Understanding what your pH measurements mean is crucial for drawing scientific conclusions. Results should be compared to expected values and environmental standards.

What Do Your Results Tell You?

pH results reveal important information about water quality and ecosystem health. Unusual readings often indicate human impact or natural environmental changes.

🔴 Acidic Results (pH < 7)

May indicate acid rain, mining pollution, or natural organic decay. Very acidic water can harm aquatic life.

🟢 Neutral Results (pH ≈ 7)

Pure water or well-buffered systems. Most freshwater fish prefer slightly neutral conditions.

🔵 Basic Results (pH > 7)

Common in seawater or areas with limestone. Very high pH can indicate algal blooms or chemical pollution.

Common Errors and How to Avoid Them

Accurate pH testing requires careful technique. Understanding common mistakes helps ensure reliable results and builds good scientific practices.

Troubleshooting Your Tests

Even experienced scientists make mistakes. Recognising and correcting errors is part of good scientific practice and leads to more reliable results.

⚠ Temperature Effects

pH changes with temperature. Always record the temperature of your samples and allow them to reach room temperature before testing for consistent results.

💧 Contamination Issues

Dirty equipment gives false readings. Always rinse probes and containers with distilled water between samples to prevent cross-contamination.

Environmental Implications

pH testing isn't just a laboratory exercise - it's a vital tool for monitoring environmental health. Changes in water pH can indicate pollution, climate change effects, or ecosystem stress.

Why pH Monitoring Matters

Regular pH monitoring helps scientists track environmental changes and predict future problems. This data influences policy decisions and conservation efforts worldwide.

Many aquatic organisms can only survive within narrow pH ranges. Fish, amphibians and invertebrates all have specific pH requirements. When pH changes rapidly, it can cause mass die-offs and ecosystem collapse.

Human activities like mining, agriculture and fossil fuel burning all affect water pH. By monitoring these changes, we can take action to protect aquatic environments before irreversible damage occurs.