Experiment 4: Comparison of Two Chloride Determinations
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Experiment 4: Comparison of Two Chloride Determinations Objective: The objective of this experiment is to determine the percentage of chloride in an unknown sample. The chloride content will be determined by a volumetric chloride titration and a gravimetric determination. Both methods will be compared and evaluated statistically using a student T test. Background: The volumetric chloride determination uses Fajans method to determine the chloride content. Fajans method is a precipitation titration involving an adsorption indicator and a colloidal precipitate form with the analyte (anion) and titrant (cation). The indicator forms a complex with the precipitate. Before the equivalence point, the anion analyte and indicator form a layer around the precipitate. After the equivalence point, the cation titrant attracts the indicator forming a layer on the precipitate surface. The gravimetric analysis portion of the experiment will use precipitation gravimetric analysis to determine the chloride content of the unknown. The acidified chloride solution will be precipitated with silver nitrate. The chloride content will be directly measured. Procedure: Three Gooche crucibles were prepared using an acidic and basic wash, HNO3 (5 mL, 0.1 M) and NH3OH (5 mL, .01 M), followed by a rinse with distilled water. The crucibles were heated in an oven (120oC) for 1 hour. An unknown soluble chloride (0.2000 g) was dissolved in distilled water (150 mL) and HNO3 (1.00 mL, 6 M). Silver nitrate (29.0 mL) was added to the solution and gently stirred and heated until the supernatant was clear. A solution was prepared for each Gooche crucible. The solution was vacuum filtered through the crucible and washed twice with HNO3 (5 mL, 0.1 M) and distilled water. The product and crucibles were dried in an oven (120oC) for 2 hours. Data: Reaction Equations π΄πππ3 (ππ) + πΆπ − (ππ) → π΄ππΆπ(π ) + ππ3 − (ππ) π΄ππΆπ(π ) → π΄π+ (ππ) + πΆπ − (ππ) Table 1: Reaction equations Silver Nitrate Standardization Mass NaCl (g) Trial 1 Trial 2 Trial 3 Volume Initial (mL) 0.1467 0.1462 0.1464 Volume Final (mL) Volume Used (mL) 24.42 23.95 23.72 23.95 23.58 23.7 Volume Final (mL) Volume Used (mL) 15.5 30.77 46.14 15.40 15.27 15.37 0.47 0.37 0.02 Table 2: Standardiztion of AgNO3 solution Silver Nitrate Unknown Titration Mass Sample (g) Trial 1 Trial 2 Trial 3 0.1015 0.1012 0.1014 Volume Initial (mL) 0.1 15.5 30.77 Table 3: Unknown chloride titration data Gooch Crucibles Trial 1 Trial 2 Trial 3 Mass Sample (g) Mass Empty Crucible (g) Mass Mass Crucible/Product Product (g) (g) 0.2024 0.2012 0.2016 17.413 17.083 16.6364 17.8746 17.5397 17.0937 0.4616 0.4567 0.4573 Table 4: Gravimetric determination of chloride data Calculations: Mass of NaCl needed for standardization Equation 0.1 πππ π΄πππ3 1 πππ πππΆπ 58.44 π πππΆπ 0.025 πΏ π΄πππ3 × × × = 0.1461 π πππΆπ πΏ 1 πππ π΄πππ3 1 πππ πππΆπ Calculation 1: Mass of NaCl needed to standardize AgNO3 Moles of Silver Nitrate Equation πππ πππΆπ 1 πππ π΄πππ3 π πππΆπ × × ππ πππΆπ 1 πππ πππΆπ = πππ π΄πππ3 Calculation 2: Moles of AgNO3 used in titration Trial 1 Example πππ πππΆπ 1 πππ π΄πππ3 π πππΆπ × × ππ πππΆπ 1 πππ πππΆπ = πππ π΄πππ3 Molarity of Silver Nitrate Equation πππ π΄πππ3 = π π΄πππ3 π π΄πππ3 Trial 1 Example πππ π΄πππ3 = π π΄πππ3 π π΄πππ3 Calculation 3: Molarity of AgNO3 Mass of Chloride (Titration) Equation πππ π΄πππ3 1 πππ πΆπ − π π΄πππ3 × × πΏ 1 πππ π΄πππ3 ππ πΆπ − × = 0.1461 π πππΆπ πππ πΆπ − Trial 1 Example πππ π΄πππ3 1 πππ πΆπ − πΏ π΄πππ3 × × πΏ 1 πππ π΄πππ3 35.45 π πΆπ − × = 0.1461 π πππΆπ 1 πππ πΆπ − Calculation 4: Mass of chloride from the volumetric determination experiment Volume of AgNO3 Needed Equation π ππππππ€π π ππππππ€π = π π΄πππ3 πππ‘πππ‘πππ π π΄πππ3 πΊπππ£ππππ‘πππ Example π ππππππ€π 0.2 π ππππππ€π = 14.00 ππΏ π΄πππ3 πππ‘πππ‘πππ π π΄πππ3 πΊπππ£ππππ‘πππ π = 28.00 ππΏ π΄πππ3 Calculation 5: Volume of AgNO3 needed for gravimetric analysis Mass of Product (Gravimetric) Equation πππ π πΆππ’πππππ+πππππ’ππ‘ − πππ π πππ’πππππ = πππ π π΄ππΆπ Trial 1 Example 1 πππ π΄ππΆπ 1 πππ πΆπ − 35.45 πΆπ − π π΄ππΆπ × × × 143.32 π΄ππΆπ 1 πππ π΄ππΆπ 1 πππ πΆπ − = πππ π πΆπ − Calculation 6: Mass of AgCl product Mass of Chloride (Gravimetric) Equation πππ π΄ππΆπ πππ πΆπ − ππ πΆπ − π π΄ππΆπ × × × ππ π΄ππΆπ πππ π΄ππΆπ πππ πΆπ − = πππ π πΆπ − Trial 1 Example 1 πππ π΄ππΆπ 1 πππ πΆπ − 35.45 πΆπ − π π΄ππΆπ × × × 143.32 π΄ππΆπ 1 πππ π΄ππΆπ 1 πππ πΆπ − = πππ π πΆπ − Calculation 7: Mass of chloride from gravimetric analysis experiment Mass % of Chloride in Unknown Equation πππ π πΆπ − × 100 = πππ π % πΆπ − π π πππππ Calculation 8: Mass percent of chloride in the unknown Trial 1 Volumetric Determination Example πππ π πΆπ − × 100 = πππ π % πΆπ − π π πππππ Mean (xΜ ) Example Na2S2O3 Molarity Equation xο½ x 1 ο« x 2 ο« x 3 ... ο« x n n 0.0710694 M ο« 0.0711064 M ο« 0.0712872 M 3 x ο½ 0.071029 M xο½ Calculation 9: Mean Calculation Standard Deviation (S) Equation n Sο½ ο₯ (x i ο1 1 Example EDTA Molarity ο x) 2 n ο1 S=√ (0.0710694 M-0.071029 M)2 + (0.0711064 M-0.071029 M)2 + (0.0712872 M-0.071029 M)2 2 S = 2.4823×10−4 Calculation 10: Standard deviation calculation F Calculated Equation π12 = πΉπππππ’πππ‘ππ π22 Example 0.16678622 0.165529082 = 1.015247 Calculation 11: Fcalculated comparison of standard deviations S Pooled Equation Example π 2 (π − 1) + π22 (π2 − 1) √ 1 1 = πππππππ π1 + π2 − 2 π 2 (π − 1) + π22 (π2 − 1) √ 1 1 = πππππππ π1 + π2 − 2 Calculation 12: Case 2 student T test Spooled calculation t Calculated Equation Example Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ |π π1 × π2 1 − π2 | ×√ = π‘πππππ’πππ‘ππ πππππππ π1 + π2 Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ |π π1 × π2 1 − π2 | ×√ = π‘πππππ’πππ‘ππ πππππππ π1 + π2 Calculation 13: Case 2 student T test tcalculated calculation Results: Molarity of Standardized Silver Nitrate AgNO3 Molarity 0.1055 M ± 5.3621E-4 Table 5: Silver nitrate molarity Mass Percent of Chloride from Analysis Mass % Chloride - Volumetric Analysis 56.64% ±0.1668 Mass % Chloride – Gravimetric Determination 56.22% ±0.1655 Table 6: Calculated mass percent of chloride from the volumetric and gravimetric analyses F Calculated S Pooled Student T Case 2 Calclulated 1.015247 0.166159 3.102091 Table1 19.0 N/A 4.303 Table 7: Statistical analysis Discussion: This experiment compared two separate methods for determining the percentage of chloride in an unknown sample. Calculated values from a volumetric determination involving a direct precipitation titration and direct precipitation gravimetric analysis were compared. The calculated mass percent of chloride from the volumetric and gravimetric analyses were 56.64% ±0.1668 and 56.22% ±0.1655 respectively (Table 6). To determine if there was a statistical significance between the two values, a case 2 student T test was performed (Table 7). A F calculation was done to ensure there was not a statistical significance between the standard deviations (Calculation 11). F calculated (1.015247) is less than F table (19.0) indicating no statistical significance between the standard deviation values. For the student T test, case 2 was used because two separate methods were being compared (Calculation 13). T calculated (3.102091) was less than T table (4.303) at a 95% confidence interval indicating the results are the same. There is no statistical significance between the two methods tested. Despite no statistical significance, the volumetric determination was more accurate because the volume of silver nitrate needed to completely react with the unknown chloride could be directly measured. In the gravimetric analysis, the volume of silver nitrate used to react with the unknown chloride was approximated. It was difficult to tell if all of the unknown chloride was precipitated. Conclusion: Overall, the experiment was successful. The mass percent of chloride was accurately calculated for each method, and the statistical analysis showed no statistical significance between the volumetric and gravimetric methods. To ensure the maximum accuracy for the gravimetric analysis, adding silver nitrate in excess may be beneficial to ensure all of the chloride is precipitated. 1 Harris, D.C. Quantitative Chemical Analysis, 7th ed.; W.H. Freeman and Company: New York, 2007.
