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.
Project Overview
This experiment, completed as part of Environmental Laboratory II (ENV4005L), applied ion-exchange chemistry and acid–base titration to determine the molecular weight and identity of an unknown salt. Using a cation-exchange resin (Amberlite IR-120 H⁺), our group isolated the salt’s cation, generated an acidic eluate, and quantified it with standardized NaOH.
By converting the 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).
This laboratory project mirrors real techniques used in environmental engineering, particularly in contaminant tracing, groundwater forensics, and water-treatment diagnostics.
Relevance to Environmental Engineering
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.
Sources of Uncertainty
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
Even with these uncertainties, our value remained directionally consistent with the expected theoretical range.
Skills Demonstrated
Analytical Chemistry: titration, resin exchange mechanisms, stoichiometric analysis
Environmental Contaminant Identification
Experimental Design & Data Integrity
Uncertainty Quantification & Error Analysis
Team-based laboratory operation
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