Determination of pH Value in Atmospheric Precipitation: Test Methods, Standards, and Applications

Accurate determination of the pH value of atmospheric precipitation—including rain, snow, fog, and other forms of wet deposition—is fundamental to acid rain assessment, air quality management, and ecological protection. This technical article explains the glass electrode (potentiometric) method for precipitation pH analysis in accordance with GB 13580.4, ISO 10523, and related standards, covering sample collection, instrument calibration, measurement procedures, quality control, and typical applications.

Technical Overview

The pH value expresses the hydrogen ion activity of a solution and is one of the most fundamental parameters in atmospheric precipitation analysis. Because natural precipitation is in equilibrium with atmospheric carbon dioxide, unpolluted rainwater typically exhibits a pH of approximately 5.6. Values significantly below this level indicate acidification, caused primarily by sulfur dioxide (SO2) and nitrogen oxides (NOx) that are oxidized in the atmosphere into sulfuric and nitric acids. Reliable pH data are therefore essential for identifying acid deposition, evaluating emission control measures, and protecting soils, forests, surface waters, and built structures.

In China, the determination of precipitation pH is standardized in GB 13580.4-1992 (Atmospheric precipitation—Determination of pH value—Glass electrode method), which forms part of the GB 13580 series covering sampling conventions, conductivity, and major ion analysis. At the international level, ISO 10523 (Water quality—Determination of pH) and ASTM D1293 (Standard Test Methods for pH of Water) describe equivalent potentiometric procedures, while ASTM D5464 specifically addresses pH measurement in low-conductivity waters such as precipitation.

Precipitation samples are weakly buffered and have a very low ionic strength, typically with conductivity between 10 and 100 µS/cm. This makes pH measurement technically demanding: electrode response is slow, readings drift, and liquid junction potentials become unstable. Consequently, the method places strong emphasis on correct electrode selection, careful calibration, strict sample handling, and disciplined quality control.

• Measurement principle: potentiometry using a glass indicator electrode and a reference electrode
• Typical sample range: pH 3.0 to 9.0, with the instrument covering the full 0 to 14 scale
• Readability and repeatability: read to 0.01 pH unit, duplicate agreement within 0.05 pH unit
• Governing standards: GB 13580.4-1992, GB 13580 series, ISO 10523, ASTM D1293, ASTM D5464

Testing Samples

Rainwater

Liquid precipitation is the most common sample type. It is collected on an event basis or as scheduled composites using wet-only automatic samplers that open only during rainfall, or bulk collectors that remain open continuously. Event-based sampling provides the clearest relationship between precipitation chemistry and emission episodes.

Snow, Sleet, and Hail

Frozen precipitation is collected with dedicated snow gauges or covered buckets. The sample is melted at room temperature in a closed container to prevent gas exchange and contamination, and the meltwater is then analyzed in the same manner as rainwater. The melted volume and precipitation depth are recorded together with the pH result.

Fog Water, Dew, and Rime

Occult deposition samples are collected with active fog collectors (string or impaction type) or passive condensing surfaces. These samples are usually far more concentrated than rainwater and can be markedly acidic, so they are analyzed with the same potentiometric procedure using small-volume cells when necessary.

Container and Preservation Requirements

Samples are collected into pre-cleaned high-density polyethylene or borosilicate glass bottles that have been rinsed with deionized water and, where appropriate, conditioned with sample. No chemical preservative is added for pH determination, samples are kept sealed and cool (approximately 4 °C), and analysis is performed as soon as possible, preferably within 24 hours of collection in line with GB 13580 sampling guidance.

Testing Items

The core test item is the pH value of the atmospheric precipitation sample. Accredited third-party laboratories typically report the following results and derived statistics:

• pH of individual precipitation events (event-based samples)
• pH of composite samples (daily, weekly, or monthly composites)
• Precipitation-weighted mean pH and the frequency of acid rain occurrence, calculated from the event data set
• Supporting parameters frequently tested in parallel: electrical conductivity (GB 13580.3), sulfate, nitrate, chloride, fluoride, ammonium, potassium, sodium, calcium, and magnesium, which support ion balance validation and source apportionment

For result evaluation, precipitation with pH below 5.6 is generally classified as acid rain. Monitoring programs commonly apply further grading, for example pH from 4.5 to below 5.6 as light-to-moderate acid rain and pH below 4.5 as strong acid rain, consistent with acid deposition monitoring specifications such as HJ/T 165.

Testing Methods

Principle — Glass Electrode (Potentiometric) Method

The glass electrode develops an electrical potential that is linearly related to the hydrogen ion activity of the solution, following the Nernst equation with a theoretical slope of 59.16 mV per pH unit at 25 °C. The potential difference between the glass electrode and a reference electrode is measured by a pH meter, which converts the signal into a pH reading after calibration with standard buffer solutions. This method is suitable for the full range of pH values encountered in atmospheric precipitation.

Apparatus and Reagents

• pH meter with a readability of 0.01 pH unit and automatic or manual temperature compensation
• Glass electrode suitable for low-ionic-strength, low-conductivity samples; a combination electrode with a double-junction reference system is recommended to minimize electrolyte leakage into the sample
• Standard buffer solutions traceable to national standards: pH 4.01 (potassium hydrogen phthalate), pH 6.87 (mixed phosphate), and pH 9.18 (borax)
• Clean polyethylene or borosilicate beakers, a thermometer, and a magnetic stirrer for gentle agitation

Calibration Procedure

Calibrate the meter before each batch of measurements using at least two buffer solutions that bracket the expected sample pH, normally pH 4.01 and pH 6.87 for precipitation work. Verify the calibration with a third buffer, and confirm that the measured electrode slope falls between 95 and 105 percent of the theoretical value. Rinse and blot the electrode between buffers, and recalibrate whenever the verification check deviates by more than 0.05 pH unit.

Measurement Procedure

• Bring samples and buffer solutions to the same temperature, preferably 20 to 25 °C, and record the sample temperature
• Rinse the electrode with deionized water, blot gently, and condition it with a small portion of the sample
• Immerse the electrode in a fresh sample aliquot sufficient to cover both the sensing bulb and the junction, stir gently, and wait until the drift is below approximately 0.05 pH unit per minute
• Record the pH to 0.01 unit and repeat the measurement on a second aliquot; the difference between duplicates should not exceed 0.05 pH unit
• Report the mean value rounded to 0.1 pH unit, together with sampling time, location, precipitation amount, and measurement temperature

Interferences, Precautions, and Quality Control

• Low ionic strength causes slow response and drifting readings; use electrodes designed for low-conductivity water and allow adequate stabilization time
• Minimize carbon dioxide exchange by keeping samples sealed and avoiding vigorous stirring, which alters the carbonate equilibrium and shifts the pH
• Prevent carryover between samples; potassium chloride leakage from the reference electrode can raise the apparent pH of small, dilute samples, so a double-junction design and consistent immersion technique are recommended
• Filtration is generally avoided before pH measurement, except for the removal of coarse debris, because filter media may leach or adsorb ions
• Maintain electrodes according to manufacturer instructions, check the slope daily, and store the glass bulb in the recommended storage solution
• Apply quality control measures including duplicate determinations, calibration verification with an independent buffer, certified buffer traceability, and regular participation in inter-laboratory comparisons

Applications

pH determination of atmospheric precipitation supports a wide range of environmental management, scientific, and industrial purposes:

• National and regional acid deposition monitoring networks, including trend reporting and compliance assessment
• Evaluation of SO2 and NOx emission control programs, such as the effectiveness of flue gas desulfurization and denitrification systems
• Environmental impact assessment and permitting for industrial facilities, power plants, and mining operations
• Ecological and agricultural research on soil acidification, forest health, crop damage, and surface water chemistry
• Corrosion and materials science studies concerning building facades, metal structures, coatings, and cultural heritage conservation
• Atmospheric chemistry research, including long-range and transboundary pollution transport and long-term climate-related trend analysis

FAQ

Why is pH 5.6 used as the benchmark for acid rain?

Natural rainwater in equilibrium with atmospheric carbon dioxide forms dilute carbonic acid, giving an equilibrium pH of about 5.6. Precipitation with pH below this value indicates additional acidity, most commonly from sulfuric and nitric acids derived from SO2 and NOx emissions, and is therefore classified as acid rain.

How soon after collection must the pH be measured?

Because precipitation samples are poorly buffered, their pH can change through gas exchange, biological activity, and container interactions. Samples should be kept sealed at approximately 4 °C and analyzed as soon as possible, preferably within 24 hours of collection.

Can a general-purpose pH electrode be used for rainwater samples?

A general-purpose electrode often gives slow, drifting readings in low-conductivity water. An electrode specifically designed for low-ionic-strength samples, preferably with a double-junction reference system, provides faster stabilization and better accuracy for precipitation measurements.

What sample volume is required for the determination?

A typical measurement requires approximately 20 to 50 mL so that both the glass bulb and the reference junction are fully immersed. Small-volume or flow-through cells can be used for limited samples such as fog water or single short rainfall events.

Which standards govern precipitation pH testing?

The primary standards are GB 13580.4-1992 for the glass electrode determination of precipitation pH, the GB 13580 series for sampling and related parameters, ISO 10523 for water pH measurement, and ASTM D1293 and ASTM D5464, the latter specifically covering pH measurement of low-conductivity waters.

How are the results reported and evaluated?

Results are reported to 0.1 pH unit together with temperature and sampling metadata. For monitoring programs, event pH values are used to calculate the precipitation-weighted mean pH and the acid rain frequency, with pH below 5.6 defining acid rain and pH below 4.5 typically indicating strong acidification.