🔬 UV Absorbance at 254 nm
Measurement of water absorbance at wavelength 254 nm as an indicator of organic matter content (aromatic and unsaturated compounds). Fast spectrophotometric method.
Overview
UV absorbance at 254 nm (UV254, SAC254 — Spectral Absorption Coefficient) is a fast, simple and non-invasive parameter used to assess dissolved organic matter (DOM — Dissolved Organic Matter) content in water. Organic compounds containing aromatic rings and unsaturated bonds (C=C, C=O, C=N) strongly absorb ultraviolet radiation at 254 nm.
UV254 is a valuable operational parameter in water treatment processes — it correlates with total organic carbon (TOC/DOC) content, potential for formation of disinfection by-products (trihalomethanes — THM, haloacetic acids — HAA during chlorination) and effectiveness of organic matter removal in coagulation, filtration and sorption on activated carbon processes. The SUVA result (Specific UV Absorbance = UV254/DOC × 100) informs about hydrophobicity and aromaticity of organic matter.
The method is described in standard PN-EN ISO 7887:2012 as method D (color measurement by spectrophotometric method in UV range) and consists of measuring absorbance of filtered water sample in quartz cuvette with optical path length 1 cm (or 5 cm for waters with low absorbance) at wavelength 254 nm relative to ultrapure water as reference.
UV254 is a fast and cheap parameter to perform — measurement takes several seconds, requires no chemical reagents and can be performed continuously (on-line) using immersion spectrophotometric probes. Therefore it is widely used for process monitoring in water treatment plants.
Method principle
UV radiation with wavelength 254 nm is passed through a quartz cuvette containing filtered water sample (0.45 µm filter). Organic compounds containing aromatic structures (e.g., humic substances, fulvic acids, phenols, ligno-cellulose compounds) and other chromophores (C=C, C=O bonds) absorb part of the radiation. The detector on the other side of the cuvette measures the intensity of transmitted radiation. Absorbance is calculated from Lambert-Beer law: A = -log₁₀(I/I₀), where I — transmitted intensity, I₀ — incident intensity. Result expressed as SAC254 in m⁻¹ or absorbance in cm⁻¹.
Applications
- Monitoring of water treatment processes — coagulation and filtration effectiveness
- Assessment of potential for formation of disinfection by-products (DBP — THM, HAA)
- Fast control of drinking water quality — operational parameter
- Monitoring of surface waters — organic matter load
- Control of sorption effectiveness on activated carbon (GAC/PAC)
- Calculation of SUVA (UV254/DOC) — characterization of organic matter
- On-line monitoring in treatment stations (immersion probes)
- Testing of water in pharmaceutical and electronics industry (purified water)
Key parameters
| Parameter | Value |
|---|---|
| Wavelength | 254 nm (UV) |
| Result | Absorbance [cm⁻¹] or SAC254 [m⁻¹] |
| Cuvette | Quartz, 1 cm (standard) or 5 cm (low absorbance) |
| Sample filtration | 0.45 µm (for dissolved organic matter) |
| Typical values (drinking water) | <0.1 cm⁻¹ (SAC254 <10 m⁻¹) |
| SUVA = UV254/DOC × 100 | >4: hydrophobic (aromatic), <2: hydrophilic |
Standard
- Standard number
- PN-EN ISO 7887:2012
- Title (PL)
- Jakość wody — Badanie i oznaczanie barwy
- Title (EN)
- Water quality — Examination and determination of colour
Step-by-step procedure
1. Spectrophotometer preparation
Turn on UV-VIS spectrophotometer min. 15 min before measurement (deuterium lamp stabilization). Set wavelength 254 nm.
2. Cleaning quartz cuvettes
Rinse quartz cuvettes with ultrapure water. If necessary soak in HCl 0.1 mol/L for 1 h, then rinse multiple times with ultrapure water. Dry.
3. Sample filtration
Filter water sample through 0.45 µm membrane filter. Discard first 50 mL filtrate (filter saturation). Collect filtrate in clean container.
4. Spectrophotometer zeroing
Fill reference cuvette with ultrapure water. Place in spectrophotometer and set zero absorbance (autozero / baseline).
5. Sample measurement
Fill measurement cuvette with filtered sample. Rinse three times with sample before measurement. Place in spectrophotometer. Read absorbance at 254 nm.
6. Duplicate measurement
Repeat measurement with second portion of filtrate. Difference between duplicates should not exceed 5%.
7. Result conversion
For 1 cm cuvette: SAC254 [m⁻¹] = absorbance [cm⁻¹] × 100. For 5 cm cuvette: SAC254 = absorbance / 5 × 100.
8. SUVA calculation (optional)
If sample DOC is known: SUVA [L/(mg·m)] = UV254 [cm⁻¹] / DOC [mg/L] × 100. SUVA >4 = high aromaticity, SUVA <2 = low aromaticity.
9. Quality control
Check zeroing every 10 samples. Measure reference absorbance filter. Repeatability control: duplicate every 10 samples, deviation <5%.
10. Cleaning and storage
Rinse quartz cuvettes with ultrapure water, dry. Store in dedicated case. Do not touch optical surfaces with fingers.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| UV-VIS Spectrophotometer | Hach DR6000, Shimadzu UV-1900i, Thermo Scientific GENESYS 150 | 15,000–45,000 PLN |
| Quartz cuvettes 1 cm | Hellma 100-QS, Starna 1-Q-1 — do not use glass or plastic cuvettes! | 200–600 PLN/pair |
| Quartz cuvettes 5 cm (optional) | Hellma 100-QS, path 50 mm — for waters with low absorbance | 400–1,000 PLN/pair |
| Vacuum filtration system | Sartorius, Merck Millipore — with 0.45 µm filters | 1,500–5,000 PLN |
| Membrane filters 0.45 µm | Cellulose MCE filters 47 mm, Whatman, Sartorius | 150–300 PLN/pack (100 pcs) |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Ultrapure water (Type I) | 7732-18-5 | For zeroing spectrophotometer and rinsing cuvettes. Absorbance <0.01 at 254 nm. Millipore Milli-Q, Elga Purelab system |
| UV absorbance standard solution | — | Optical filters or K₂Cr₂O₇ solution for verification of spectrophotometer absorbance scale |
| Diluted hydrochloric acid (HCl) | 7647-01-0 | 0.1 mol/L for cleaning quartz cuvettes from organic deposits |
Health and safety (OHS)
- UV radiation — do not look directly at deuterium lamp source. Close measurement chamber
- Hydrochloric acid (HCl) — corrosive, use gloves and goggles when cleaning cuvettes
- Quartz cuvettes — fragile and expensive, handle carefully
- Water samples — use protective gloves (potential contamination)