Sulphide

S²⁻

The ELIT sulphide electrode measures S²⁻ with very high sensitivity using a solid-state Ag₂S membrane. Requires strongly alkaline anti-oxidant buffer. Used in environmental monitoring and industrial effluent analysis.

Anion (−)ELIT 8225Valence: Divalent

Safety Warnings

Hydrogen sulphide (H₂S) is toxic and has a characteristic rotten-egg odour. Work in a well-ventilated area or fume hood.
Sulphide solutions oxidise rapidly in air — prepare fresh and keep alkaline.
Sodium sulphide is corrosive. Wear appropriate PPE.

Typical Applications

Environmental Monitoring Water Quality Monitoring Industrial Effluent Research

Electrode Specifications

Model

ELIT 8225

Membrane Type

Solid-state mono-crystalline Ag₂S membrane

Ion Type

Divalent anion

Molar Mass

32.065 g/mol — 1000 ppm = 0.0312 M

Physical Size

130 mm body (excl. contact) × 8 mm diameter. DC resistance < 2.5 MΩ at 25°C. Min. sample volume: 5 ml.

Operational Specifications

Concentration Range

0.003 to 3,200 ppm (1×10⁻⁷ to 0.1 Molar)

Electrode Slope at 25°C

27 ± 3 mV/decade (divalent)

pH Range

pH 13 to 14

Temperature Range

0 to 80°C

Response Time

< 30 seconds (90% response)

Potential Drift

< 5 mV/day in 100 ppm (8 hours)

Reagents

Reference Electrode

Double junction lithium acetate (ELIT 003n). Outer filling solution: 0.1M CH₃COOLi.

ISAB / Buffer

10M NaOH (Add 2% v/v) or SAOB (Sulphide Anti-Oxidant Buffer: 2M NaOH + ascorbic acid). Maintains pH > 13 and prevents sulphide oxidation.

Standard Preparation

Dissolve 7.491g sodium sulphide nonahydrate (Na₂S·9H₂O) in 1 litre deionised water (~1300 ppm S²⁻). Standardise by titration against lead perchlorate standard.

Interference

The Selectivity Coefficient (SC) represents the approximate apparent increase in the measured concentration caused by 1 unit of the interferent. Effect (% error) = (interferent concentration × SC / analyte concentration) × 100.

Interfering Ion	Selectivity Coefficient	Notes
Silver (Ag⁺)	variable	Will deposit Ag₂S on membrane — avoid silver in solutions
Mercury (Hg²⁺)	variable	Strong interference — forms HgS precipitate

Standard Analytical Method

Apparatus Required

Ion-Selective Electrode: ELIT 8225
Reference electrode: Double junction lithium acetate (ELIT 003n)
Dual electrode head (ELIT 201)
ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter
150 ml polypropylene beakers, 100 ml volumetric flask, 1, 2, 5, 10 ml pipettes

Calibration

Prepare all standards and samples in SAOB. Work quickly to minimise oxidation. Calibrate with 100, 10, 1, 0.1 ppm S²⁻ in SAOB.

Sample Preparation & Measurement

Fix samples immediately with SAOB on collection. Do not expose to air. Measure as soon as possible after collection.

Results

Results are displayed as ppm and mol/l. If buffer solution has been added equally to standards and samples, figures will not need adjusting as all are affected by the same dilution factor. Allow 2–3 minutes stabilisation after electrode immersion. Wash and dry electrodes between samples to avoid cross-contamination.

Analytical Notes

Must be measured at pH > 13 to ensure all sulphide is in S²⁻ form (not HS⁻ or H₂S).
Membrane may require polishing with fine abrasive if a black tarnish develops.
Highly volatile — prepare standards fresh and measure quickly.