The pH scale measures the hydrogen ion concentration in a solution, determining its acidity or alkalinity. The scale ranges from 0 to 14, where 7 is neutral, lower values indicate acidity, and higher values denote alkalinity. Determining this value is a routine requirement across scientific fields, and the method chosen depends on the required precision.
Quick Assessment Using Color Change
The simplest way to estimate pH relies on color-changing chemical compounds known as indicators. These indicators are weak organic acids or bases that change their molecular structure in response to the hydrogen ion concentration. This structural change alters how the compound absorbs and reflects light, resulting in a visible color shift.
Indicator strips are absorbent paper infused with these dyes. When dipped into a liquid, the resulting color is compared against a standardized chart provided by the manufacturer. Liquid indicators are added directly to the sample, causing the solution itself to change color. While fast and inexpensive, these methods are semi-quantitative, offering an estimate usually accurate only to the nearest whole or half-unit of pH.
The Precise Electronic Method
For applications demanding high accuracy, the electronic pH meter is the standard instrument, relying on an electrochemical principle for measurement. The system consists of the meter and a combination electrode placed into the solution. This electrode is composed of two main parts: a hydrogen-ion-sensitive glass electrode and a reference electrode, which are often built together into a single probe.
The core measurement occurs at the glass membrane, a specialized bulb at the electrode’s tip. This thin membrane develops an electrical potential when hydrogen ions in the sample solution interact with its outer surface. Inside the glass electrode, a solution with a constant pH and a silver/silver chloride wire maintain a stable reference potential.
The reference electrode provides a constant, known electrical potential that does not change with the sample’s pH. When the probe is immersed, the difference in hydrogen ion concentration creates a measurable voltage difference across the glass membrane. The pH meter measures this voltage difference between the two electrodes.
Using the Nernst equation, the meter converts this electrical signal into the corresponding pH value, displaying it digitally with high precision. This electrochemical process establishes a direct relationship where a change of one pH unit corresponds to a potential change of approximately 59.16 millivolts at 25°C.
Ensuring Measurement Accuracy
Reliable results from an electronic pH meter depend on proper preparation and maintenance of the electrode system. The most important step for accuracy is calibration, which corrects for shifts in the electrode’s performance over time, known as drift. Calibration involves immersing the electrode in standardized buffer solutions—liquids with a known pH value, typically 4.01, 7.00, and 10.01.
Calibration Procedures
A two-point calibration, using pH 7.00 and a buffer that brackets the expected sample range, is often sufficient for general use. For the highest accuracy, a three-point calibration across the acidic, neutral, and alkaline range is performed. During this process, the meter measures the voltage for each buffer and adjusts its internal programming to translate the voltage readings into the correct pH value.
Electrode Maintenance
Regular maintenance is a prerequisite for accurate measurement and involves keeping the glass membrane hydrated. Electrodes should never be stored dry or in pure distilled water, which can damage the sensitive glass layer. Instead, they are kept in a specially formulated storage solution, often potassium chloride, to preserve the integrity of the glass bulb. Cleaning the electrode is also necessary to remove residue that could contaminate the measurement.