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Gravimetric Analysis of Silver in Pre-1965 Dimes

Categories: Chemistry Chemistry Stoichiometry

Analysis, Pages 6 (1284 words)

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Introduction:

The fascinating world of metallurgy and coinage intertwines in the analysis of U.S. Mint ten-cent dimes, specifically those minted before 1965. These historical coins, once composed of a unique alloy of silver and copper, hold not only monetary value but also serve as artifacts reflecting a bygone era. This study embarks on a comprehensive exploration, utilizing the precision of gravimetric analysis to unravel the percentage composition of silver in these pre-1965 dimes, providing insights into their material makeup and historical significance.

Chemical Reactions and Precipitation:

The crux of this experiment lies in the chemical transformations orchestrated by nitric acid.

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Acting as an oxidizing agent, nitric acid facilitates the dissolution of the dime, transforming the stoic silver (Ag) and copper (Cu) metals into their respective ionic forms:

Ag(s) → Ag⁺(aq) + e-
Cu(s) → Cu²⁺(aq) + 2e-
4 H⁺(aq) + NO₃^- + 3e- → NO(g) + 2H₂O(l)

This oxidation process is crucial, considering the inherently non-reactive nature of silver and copper, making traditional acids ineffective for dissolution.

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The resulting nitrogen monoxide (NO) and nitrogen dioxide (NO2) gases showcase the dynamic nature of these reactions, emphasizing the need for a controlled environment, such as under a fume hood.

To distinguish silver ions from copper ions, sodium chloride (NaCl) is introduced into the solution. The addition of sodium chloride precipitates silver chloride (AgCl) while theoretically leaving copper ions untouched. The reaction is depicted as follows:

Ag⁺ + Cl^- → AgCl(s)

This judicious selection of reactants ensures the isolation of silver ions, paving the way for further analysis.

Experimental Procedure:

Day 1

Commencing with the measurement of the mass of a pre-1965 dime using an analytical balance, the experiment embarks on a meticulous journey.
Under the shelter of a fume hood, the dime, now a piece of history, is submerged in a 100-mL beaker containing 6M nitric acid, initiating the process of dissolution.
Anticipating the need for precision, the amount of sodium chloride required for the subsequent precipitation of silver is calculated, measured to ± 0.001g on a sensitive balance, and then dissolved in 25 mL of distilled water.
The magic unfolds as the sodium chloride solution is slowly introduced to the dissolved silver, creating a visual spectacle and setting the stage for the overnight incubation in a drying oven.

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This extended incubation allows the silver chloride precipitate to grow larger, ensuring ease of filtration in the subsequent steps.

Day 2

With the anticipation of revealing the hidden secrets of the dime, a Büchner Funnel, Büchner Flask, and filter paper are assembled, heralding the commencement of the filtration process.
Caution prevails as the vacuum aspirator is activated, creating a seamless seal between the funnel and filter paper with the assistance of distilled water sprayed from a wash bottle.
The silver chloride-laden solution is poured into the funnel, with the vacuum aspirator ensuring an efficient filtration process. The wash bottle is judiciously employed to cleanse the funnel sides and the beaker, preserving the integrity of the precipitate on the filter.
Technical expertise comes into play as the vacuum aspirator is methodically halted, with the rubber tubing connecting the flask to the sink removed before turning off the water. This strategic maneuver prevents any inadvertent upward movement of the filtrate, ensuring the accuracy of the experiment.
The culmination of Day 2 involves the disassembly of the filter apparatus, with the funnel containing the precious precipitate ensconced in a drying oven, patiently awaiting the next phase of scrutiny.

Day 3

With a sense of anticipation, the filter paper, now a repository of the silver chloride precipitate, is delicately procured from the funnel using tweezers.
The analytical balance takes center stage, meticulously weighing the filter paper and its precious cargo, providing the crucial data for the subsequent analytical journey.

Observations:

Mass of Dime: 0.309g
Mass of Filter: 0.067g
Mass of Filter + AgCl: 0.435g
Mass of NaCl Used: 0.332g
During the dissolution on Day 1, a mesmerizing display of color change occurs, with the solution turning turquoise. Simultaneously, brown gas emanates, signifying the evolution of toxic gases, namely nitrogen monoxide (NO) and nitrogen dioxide (NO2). This chemical dance reinforces the necessity for the experiment to unfold under the protective wings of a fume hood, shielding the experimenters from potential hazards.

Calculations:

The journey through precision takes a quantitative turn, with calculations unveiling the hidden mysteries of the alloy composition:

Amount of NaCl necessary to precipitate silver x2:

0.309g Ag x 1 mol Ag x 2 mol NaCl x 58.443g NaCl = (0.334 ± 0.002)g NaCl

Mass of AgCl: (0.435g - 0.067g) = (0.368 ± 0.002)g AgCl

Mass of Ag: (0.368g AgCl x 107.87g Ag) = (0.277 ± 0.001)g Ag (0.5%)

Percent Silver in Dime: (0.277g Ag x 100) / 0.309g Dime = (89.6 ± 0.7)%

Conclusions, Discussions, and Evaluations:

As the experimental odyssey reaches its denouement, reflections on the journey provide insights into the nuances of the procedure, potential sources of error, and avenues for improvement. The experiment, despite its success in determining the percentage of silver in pre-1965 dimes, is not without its challenges.

The uncovered funnel during the drying phase poses a potential pitfall, with the risk of dust particles infiltrating the filter and mingling with the precious precipitate. This, in turn, may skew the mass measurements on Day 3, injecting an element of uncertainty into the results. A simple remedy to this predicament would be to cover the funnel with plastic wrap before subjecting it to the drying oven, shielding it from external contaminants.

Another point of contention emerges during the washing phase on Day 2. The potential phenomenon of "peptization," wherein minute particles form and can pass through the filter, raises its head due to the use of a wash bottle. To mitigate this risk, the addition of 2 mL of 6M HNO₃ to the 150 mL of distilled water in the wash bottle could serve as a preventative measure. This strategic addition ensures that even in the event of particle formation, they would not pass through the filter, fortifying the accuracy of the experiment.

Furthermore, the temporal dynamics of the experiment present an intriguing aspect for consideration. The allocation of a more compressed timeframe, perhaps within a two-hour window, encompassing both the laboratory procedures and the subsequent report compilation, could potentially enhance the reliability of the results. The continuity in workflow may contribute to a more cohesive and integrated experimental process, minimizing the potential for external factors to influence the outcome.

Why a Twofold Excess of Chloride?

A twofold excess ensures complete precipitation of silver ions, as NaCl and CuCl₂ are soluble and do not interfere with the experiment's integrity. This judicious excess serves as a safeguard, guaranteeing the comprehensive isolation of silver ions for subsequent analysis.

Funnel Coolness Impact on Percent Silver Calculation:

If the funnel is not cool during the mass determination, convection currents induced by the heat within the scale may lift the funnel, creating a false sense of reduced mass. This erroneous perception would consequently yield a lower calculated percentage of silver than the actual value, underscoring the significance of maintaining a cool environment during this crucial phase.

Calculating the Monetary Value of Silver:

The allure of silver extends beyond its chemical properties, delving into its economic worth. At the current market price of $0.56 per gram, the 0.277g of silver extracted from the dime equates to a monetary value of (0.277 x 0.56) = $0.1552 or approximately 15.52 cents. This economic perspective adds a layer of significance to the experiment, transcending the confines of the laboratory and reaching into the realm of real-world value.

As the experiment concludes, a holistic view emerges, encapsulating the scientific intricacies, the human precision, and the historical resonance of the pre-1965 dime. Beyond the realm of laboratory protocols and calculations, this exploration stands as a testament to the marriage of chemistry and numismatics, unraveling the mysteries embedded in the alloy composition of these iconic coins.