Identifying an Unknown Salt Using Cation-Exchange Resin Analysis
Using resin chemistry and quantitative analysis to uncover the identity of a mystery salt through an environmental forensics approach.
Course: ENV 4005L – Environmental Engineering Laboratory II
Lab Type: Group Laboratory
Role: Titration, molecular weight analysis, uncertainty evaluation
Focus: Chemical diagnostics using ion-exchange and acid–base titration
Overview
This experiment applied ion-exchange chemistry and analytical titration to estimate the molecular weight and identity of an unknown ionic compound. Using a cation-exchange resin (Amberlite IR-120, H⁺ form), the unknown salt’s cation was exchanged for hydrogen ions, producing an acidic eluate that was quantified through standardized NaOH titration.
By converting titration results into moles of exchanged ions, we calculated an experimental molecular weight of 93.23 ± 0.72 g/mol, most consistent with potassium nitrate (KNO₃). The experiment emphasized chemical diagnostics under uncertainty, rather than exact compound identification.
Engineering Context
Ion-exchange resins are routinely used in:
Nitrate removal from drinking water
Industrial wastewater treatment
Water softening and demineralization
Selective contaminant removal (e.g., heavy metals, ammonium)
This experiment provided hands-on experience with the chemistry that underpins those systems. By interpreting titration data and linking it to ion exchange behavior, we learned how environmental engineers identify ionic pollutants and evaluate treatment efficiency.
Methods Summary
All procedures, measurements, and materials followed the ENV4005L laboratory manual.
Resin Preparation
20 mL of Amberlite IR-120 (H⁺) resin packed into a rinsed burette.
Column washed with DI water until neutral pH.
Unknown Salt Preparation
0.1072 g of unknown salt dissolved in ~5 mL DI water.
Introduced into the resin column using a pipette.
Ion-Exchange Process
Cations bound to resin; released H⁺ ions acidified the eluate.
Eluate collected in an Erlenmeyer flask until neutral
Titration
Standardized NaOH (0.250 M) titrated into eluate using phenolphthalein indicator.
Endpoint reached at 4.60 mL NaOH.
Stoichiometry used to calculate moles of salt.
Molecular Weight Calculation
MW = mass (g) / moles (mol) → 93.23 g/mol
This value matched most closely with potassium nitrate (101.1 g/mol), giving a 7.79% deviation.
Results & Interpretation
Our calculated molecular weight closely aligned with potassium nitrate (KNO₃: 101.1 g/mol), showing a 7.79% deviation — well within a reasonable margin considering instrument tolerances and potential procedural variation.
Team | Mass (g) | NaOH Volume (mL) | Molecular Weight (g/mol) | Likely Salt | % Difference |
Team 2 (My team) | 0.1072 | 4.60 | 93.23 | KNO₃ | 7.79% |
The observed differences across teams demonstrate the sensitivity of resin titration to measurement precision — particularly burette tolerance, NaOH concentration accuracy, and resin-washing technique.
Uncertainty & Data Integrity
This experiment required careful control of variables to minimize systemic and random error. Major contributors included:
Burette tolerance (±0.03 mL)
Analytical balance precision (±0.0001 g)
Potential incomplete resin rinsing, air bubbles, or inconsistent flow rate
NaOH standardization error (±0.001 M)
Propagated total uncertainty yielded:
MW = 93.23 ± 0.72 g/mol
Despite these uncertainties, the inferred identity remained chemically plausible and environmentally relevant.
Reflection
This experiment reinforced how environmental engineers use ion-exchange systems as diagnostic tools, not just treatment units. The lab emphasized decision-making with imperfect data, careful uncertainty evaluation, and interpretation grounded in chemical behavior rather than exact matches.
These skills directly translate to contaminant identification, treatment verification, and forensic water quality analysis.
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