Engineering Chemistry — 6-topic bundle

• Oxidation is the loss of electrons (OIL - Oxidation Is Loss). The species that loses electrons is called the reducing agent.
• Reduction is the gain of electrons (RIG - Reduction Is Gain). The species that gains electrons is called the oxidizing agent.

These reactions don't just happen anywhere; they occur at interfaces between an electrode (an electrical conductor) and an electrolyte (a substance containing free ions that conducts electricity).

1. Glass Membrane: This is the most crucial part. It's a thin, pH-sensitive glass bulb (usually soda-lime or lithium glass, about 0.05-0.1 mm thick) at the tip of the electrode. This glass is specially formulated to exchange ions with the solution.
2. Internal Reference Electrode: Inside the glass bulb, there's usually a silver/silver chloride (Ag/AgCl) electrode.
3. Internal Reference Solution: This solution fills the glass bulb and surrounds the internal reference electrode. It's typically a buffered chloride solution, like 0.1 M HCl, with a known, constant pH.
4. Electrode Body: This is the main structure, usually made of non-conductive glass or plastic, which houses the internal components.
5. Connector: A cable connects the internal reference electrode to a pH meter.

The most common method for measuring pH is using a pH meter, which consists of two main parts: a measuring electrode (typically a glass electrode) and a reference electrode. These are often combined into a single combination electrode.

The fundamental principle behind an ISE is the creation of a potential difference across a selective membrane. This membrane is designed to interact specifically with the target ion, allowing only that ion to pass through or generate a specific electrical response at its surface. This selective interaction leads to a potential difference that is proportional to the logarithm of the ion's activity (which is closely related to its concentration) in the solution, as described by the Nernst equation.

pH metry titration is a quantitative analytical method where you measure the pH of a solution using a pH meter as you add a titrant (a solution of known concentration). By plotting the pH against the volume of titrant added, you create a titration curve.

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Advanced ISEs are electrochemical sensors that convert the activity of a specific ion dissolved in a solution into an electrical potential. This potential is measured against a stable reference electrode, and the difference in potential (voltage) is proportional to the logarithm of the ion's activity, as described by the Nernst equation.

The Nernst equation for an ISE is:

$E = E^0 + \frac{RT}{nF} \ln a_i$