Box 15-1 Systematic Error in Rainwater pH Measurement The Effect of Junction Potential

Combustion products from automobiles and factories include nitrogen oxides and sulfur dioxide, which react with water in the atmosphere to produce acids.17 

Acid rain in North America is most severe in the east, downwind of many coal-fired power plants. In the 3-year period 1995-1997, after S02 emissions were limited by a new law, concentrations of SOj and H in precipitation decreased by 10-25% in the eastern United States.18 

Acid rain threatens lakes and forests throughout the world. Monitoring the pH of rainwater is a critical component of programs to measure and reduce the production of acid rain. 

To identify systematic errors in the measurement of pH of rainwater, a careful study was conducted with 17 laboratories.19 Eight samples were provided to each laboratory, along with instructions on how to conduct the measurements. Each laboratory used two buffers to standardize pH meters. Sixteen laboratories successfully measured the pH of Unknown A (within 0.02 pH unit), which was 4.008 at 25°C. One lab whose measurement was 0.04 pH unit low had a faulty commercial standard buffer. 

A hypothesis was that variations in the liquid junction potential (Section 15-3) led to variations among the pH measurements. 

---

Box 15-1 Systematic Error in Rainwater pH Measurement The Effect of Junction Potential... 

---