Soren Sørensen and the Invention of pH: A Historical Look at His Career and Contributions

When we talk about modern aqueous chemistry, one name rises to the top with enduring influence: Soren (often spelled Søren) Sørensen. Best known for introducing the concept of pH in 1909, Sørensen’s career as a chemist helped shape how scientists think about acidity, basicity, and the behavior of solutions. This blog traces his career, the context of his work, and the lasting impact of his ideas on chemistry and beyond.

Early life and academic path

Søren Elias Sorensen (often listed in historical sources as Søren Sørensen) pursued his education and early scientific training in Denmark at a time when physical chemistry was rapidly evolving. Born into a period when the chemistry community was actively seeking quantitative descriptions of solution behavior, Sørensen found a fruitful intersection between experimental observation and theoretical interpretation.

- Education: Sørensen studied chemistry in Denmark, building a foundation in physical chemistry, analytical methods, and the emerging emphasis on quantitative descriptors of chemical systems.

- Influences: The late 19th and early 20th centuries saw rapid development in thermodynamics, electrochemistry, and acid–base theory. Sørensen was part of a generation that sought to formalize descriptors that could be universally applied across solutes and solvents.

The pivotal idea: pH as a quantitative measure of acidity

The concept

In 1909, Sørensen proposed the term pH to express the hydrogen ion activity of a solution, introducing a logarithmic scale to quantify acidity. He defined pH as the negative logarithm (base 10) of the activity of hydrogen ions, emphasizing that this scale would reflect the true acid-base character of solutions more reliably than simple concentration measurements in all cases.

- Core idea: pH = -log10(a_H+), where a_H+ is the activity of hydrogen ions in solution.

- Rationale: Activities account for non-ideal behavior in solutions (interactions between ions, ion pairing, changes in ionic strength), making pH a more robust descriptor than [H+] alone, especially in concentrated solutions or at higher ionic strengths.

The impact

- A new standard: pH quickly became the de facto standard for reporting acidity in chemistry, biochemistry, medicine, environmental science, and many industrial processes.

- Practical adoption: While the exact formalism of pH involves activity, in most practical laboratory work, scientists use pH meters and the revised Henderson–Hasselbalch framework with concentration-based approximations. Sørensen’s legacy lives on in the widespread use of the term and the concept of a scale that measures acidity logarithmically.

Sørensen’s broader scientific contributions

While the birth of the pH scale is the most enduring hallmark of Sørensen’s career, his work intersected with several themes that were central to physical and analytical chemistry in his era:

- Acid–base chemistry: Sørensen’s emphasis on quantitative descriptors of acidity helped catalyze the transition from qualitative to quantitative acid–base science. His ideas aligned with the broader move toward understanding solutions through thermodynamic and electrochemical perspectives.

- Analytical methods: In a period when chemical analysis was becoming more precise, Sørensen’s pH concept provided a practical tool for researchers and industry to monitor reactions, buffer systems, and solution quality.

- Education and dissemination: By introducing a simple yet powerful concept, Sørensen helped educate generations of chemists who would go on to apply pH concepts across disciplines—from biochemistry to environmental science.

The historical context and reception

- Reception among contemporaries: Sørensen’s pH concept was met with both excitement and scrutiny. Some colleagues pushed for a rigorous treatment involving activity coefficients, especially for concentrated solutions where deviations from ideal behavior become pronounced.

- Evolution of the concept: Over time, the pH scale has become a practical compromise between theoretical rigor (activities) and experimental practicality (pH meters calibrated to standard buffers). The modern formulation continues to rely on the fundamental idea Sørensen introduced: acidity is a measurable, transferable property of solutions.

Legacy and modern relevance

- Educational impact: The term pH is ubiquitous in textbooks, classrooms, and laboratories worldwide. Sørensen’s idea made acidity accessible to students and professionals alike.

- Scientific influence: pH remains central to countless fields: chemistry, biology, medicine, environmental science, geology, food science, and industrial process control. The concept underpins buffering systems, acid–base titrations, electrochemical sensors, and many regulatory standards.

- Ongoing refinements: Today, pH measurement still relies on the concept of hydrogen ion activity, and sophisticated methods (including traceable calibration, reference electrodes, and thermodynamic models) build on Sørensen’s foundational idea.

Concise timeline

- 1860s–1900s: Søren Sørensen develops ideas in physical chemistry and acid–base behavior in Denmark.

- 1909: Introduces the pH scale as a log measure of hydrogen ion activity, offering a quantitative descriptor of acidity.

- Early 20th century: pH gains rapid adoption across chemistry, biochemistry, medicine, and industry; discussions about activities vs. concentrations begin to influence teaching and practice.

- Mid–late 20th century: pH measurement becomes standard in laboratories worldwide; the use of buffers, pH electrodes, and standardized calibrations expands.

- 21st century: pH remains a foundational concept, with ongoing refinements in measurement standards and understanding of activity in complex solutions.

Final reflections

Sørensen’s contribution goes beyond a single metric; it represents a shift toward quantitative thinking about solutions. By articulating a practical and scalable way to express acidity, he provided a bridge between theoretical chemistry and everyday laboratory practice. His pH concept continues to resonate because it captures a fundamental property of chemical systems: how protons inhabit and influence the aqueous environment.