Clark Electrode

Introduction

• The importance of oxygen and its role in life was discovered as early as 1774 by Joseph Priestley of England.¹
• In 1874, Karl von Vierordt investigated the oxygenation of human blood by shining red light through a hand and observing differences in penetration.¹
• Early attempts at quantifying oxygen content in blood appeared in the 1920s; it was Leland C. Clark who created the first self-contained electrode for measuring blood oxygenation in 1954.²
• The Clark electrode is also used in many non-medical settings, including environmental science, industrial engineering, pharmaceutics, and brewing.

Physical Characteristics

• The Clark electrode consists of a noble-metal (typically platinum) cathode and a silver anode, suspended in a potassium chloride buffer solution (Figure 1).
• The first version of the Clark electrode suspended the cathode and anode directly in the blood sample; however, a reaction between the metal and blood would occur, causing protein deposits and inaccurate results.^3,4
• To prevent this, an oxygen-permeable membrane, typically Teflon, surrounds the electrodes and isolates them from the sample.⁵
• The gradient created between the Teflon membrane and the sample (blood) is proportional to the partial pressure of oxygen being measured.⁵

• When a sample is analyzed, an oxygen gradient will exist between the sample and the buffer solution, driving the reaction.
• The electrode requires an electrical current, and the following reduction (platinum cathode) and oxidation (silver anode) reactions occur:
  • Platinum Cathode: O₂ + 2H₂O + 4e^– = 4OH^–
  • Silver Anode: 4Ag⁺ + 4Cl^– + 4e^– = 4AgCl
  • Each oxygen molecule releases four electrons, which are then drawn to the cathode and generate a current.
  • The current in the buffer solution is proportional to the oxygen partial pressure in the sample, and a corresponding value is calculated and reported.

Modern Use

• Clark electrodes remain the gold standard for oxygen analysis and are utilized heavily in both medical and non-medical settings.
• Point-of-care arterial blood gas machines use a Clark electrode for oxygen analysis.⁶
• In anesthesia machines, Clark electrodes are used to measure oxygen levels during inspiration and expiration. They are frequently used with Serveringhaus electrodes, which measure carbon dioxide.
• Older anesthesia machines may only have one Clark electrode attached to the inspiratory limb (measuring inspired oxygen concentrations).⁷
• The Clark electrode is also occasionally used for transcutaneous oxygen analysis. This is typically achieved by applying an electrode or film to the skin. Downsides include local skin damage from pressure and thermal burns from the required operating temperature for the sensor to function as intended.⁸

Limitations

• Blood samples are ideally analyzed immediately, as oxygen content declines over time. To account for this, a correction factor is typically applied.⁴
• Clark electrodes require an external power source to provide an electrical current. Newer technologies, such as fuel cells, are used for oxygen analysis and provide their own power source.¹
• The oxygen-permeable membrane separating the electrode and sample degrades over time and must be replaced every 3 years. Furthermore, the Teflon membrane thins as it degrades, and oxygen permeability through the membrane can change over time.¹
• Another significant limitation is the requirement that the Clark electrode be maintained at a constant temperature (37°C). Many devices that use Clark electrodes include temperature regulators; however, these also require calibration and degrade over time.⁸
• Clark electrodes consume a very small amount of oxygen to drive the reaction. In most cases, this consumption is inconsequential. However, in cases of very low level of dissolved oxygen, or oxygen-depleted samples, this can provide a falsely low result.⁹