Communication Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
pubs.acs.org/jchemeduc
Recovery of Silver Nitrate from Silver Chloride Waste James von Dollen, Sofia Oliva, Sarah Max, and Jennifer Esbenshade* Department of Chemistry and Physics, University of Tennessee at Martin, Martin, Tennessee 38238, United States S Supporting Information *
ABSTRACT: At many universities, silver nitrate is used in teaching laboratories, and a resulting waste product is often silver chloride. Silver chloride is not a compound that can be discarded into a sink as it has harmful effects to the environment; however, commercial waste disposal is costly, and replacement of the silver nitrate reagent is expensive. Silver chloride may be recycled back into silver nitrate, reducing costs for universities and burdens on the environment. For this study, multiple methods were tested to convert silver chloride into silver nitrate, a useful compound. The percent recovery, percent purity, cost, time required, facilities needed, and safety of the processes were compared, and an ideal method was identified to convert silver chloride waste into a useful species. This process could be used to reduce waste and cost for a university. KEYWORDS: General Public, Laboratory Instruction, Safety/Hazards, Green Chemistry, Laboratory Management
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INTRODUCTION Laboratory waste disposal is a common problem at universities.1,2 In particular, waste from teaching laboratories can be a costly and challenging issue. Many lab procedures are designed to use inexpensive reagents and allow for waste disposal down the drain; however, some procedures use costly reagents or produce environmentally hazardous waste that is undesirableand sometimes illegalto dump down the drain.1,2 This hazardous waste must be collected, stored, and eventually disposed of by paying for commercial waste disposal.2 For some waste, recycling may be more cost effective and environmentally friendly than disposal. AgCl is an example of such waste due to the relatively high cost of the Ag or AgNO3 (the produce of the recycling process),3 the expense of commercial waste disposal,2 and the environmental toxicity.4,5 Several methods to recycle AgCl into Ag or AgNO3 have been proposed in the literature, but many are lengthy or require specialized equipment. These methods include conversion by ion exchange,6 the use of small amounts of AgCl on the order of milligrams,7,8 electrodeposition with cyanide salts,9 use of extreme temperatures,10−12 forming potentially explosive ammoniacal solutions of silver,13−16 and forming additional environmentally toxic waste byproducts.10−17 However, two promising methods were identified that use only commonly available equipment and materials, have a simple procedure, and are relatively safe. These methods are most likely to achieve the ultimate goal of alleviating financial and environmental burdens. When using any of the methods to recycle silver chloride waste, caution must be employed. Depending on the method, there is risk of forming explosive compounds, so appropriate precautions must be taken.18,19 Silver fulminate Ag(ONC), silver azide AgN3, and silver nitride Ag3N are all shock sensitive explosives, and measures must be taken to prevent the formation of these species.18,20 One promising method15,16 involves dissolving the AgCl into ammonia and reducing the silver ions with copper metal. The silver particles can then be collected, dissolved in nitric acid, and purified as AgNO3. The second promising method18 © XXXX American Chemical Society and Division of Chemical Education, Inc.
involves dissolving the AgCl into NaOH and converting the silver ions into AgOH and eventually Ag2O. The Ag2O can then be heated to produce silver metal, which may be dissolved in nitric acid to produce AgNO3. This study presents the optimization and comparison of these two methods to covert AgCl to AgNO3 in order to reduce costs for the university and waste for the environment. These methods are compared for percent recovery, percent purity, time required, and cost. Safety issues and waste byproducts are additionally addressed in the paper.
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EXPERIMENTAL SECTION
Conversion of AgCl to AgNO3
Two methods were used. For method A, a modified version of Thomas’s method, AgCl was dissolved in ammonia and reduced with copper metal.16 For method B, a modified version of Willbank’s method, AgCl was converted using sodium hydroxide and then heated to reduce to silver.18 For both methods, the silver metal was then converted to silver nitrate by dissolution in nitric acid. Detailed procedures are provided in the Supporting Information. The AgCl used to test the methods were initially obtained both by combining NaCl and AgNO3 and collecting the precipitate. Further conversions were completed on AgCl obtained as lab waste from several quantitative analysis lab experiments (where the primary product is AgCl, and the purity of AgCl, while unknown, is expected to be >90%). All trials were completed by undergraduate students in their second or third year of study under faculty advisement. Analysis of Products
The percent recovery was calculated gravimetrically. AgNO3 purity was analyzed by dissolving the recovered product into a 2% v/v nitric acid solution to a concentration of approximately Received: September 15, 2017 Revised: January 30, 2018
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DOI: 10.1021/acs.jchemed.7b00713 J. Chem. Educ. XXXX, XXX, XXX−XXX
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10 ppm (calculated by assuming 100% purity) and analyzing the AgNO3 solution with an atomic absorption spectrometer (Varian Spectra AA-10) with a current of 4 mA, slit width of 0.5 nm, and wavelength of 328.1 nm. For the cost analysis, all prices for reagents were obtained from Sigma-Aldrich. Waste disposal costs were calculated from a quote for commercial disposal of 1 kg of solid AgCl.
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RESULTS AND DISCUSSION The two methods to convert AgCl to AgNO3 were compared for percent recovery, percent purity, time required, and cost. The advantages, disadvantages, and safety were also considered for both methods.
Figure 2. Comparison of the two methods for time to convert. Active time requires participation; passive time is time required for the conversion that does not require active work.
Percent Recovery and Purity
The percent recovery and percent purity of the conversions over multiple reactions is shown in Figure 1. The percent
Table 1. Comparison of the Two Methods for Dollar Amount Costs and Savings of a 25 g Sample of AgCl Results by Method, $ Processes or Materials savings costs
avoided AgCl disposal fees avoided AgNO3 purchase recycling reagents recrystallization reagents byproduct disposal cost net savings
A
B
+14.25 +109.57 −3.67 −9.90 −14.25 +96.00
+14.25 +119.41 −4.37 −9.90 −2.61 +116.78
disposal of the 25 g of AgCl are equivalent; however, the amount of AgNO3 made, and thus not necessary to purchase for later lab procedures, varies slightly based on the percent recovery. Because method A has a lower percent recovery, there is less AgNO3 produced and thus less savings. In addition, there is some byproduct waste that must be processed. Method A produces a concentrated copper ammine waste, which must be properly disposed of. Method B produces a basic solution, which must be neutralized before disposal. The cost associated with this is calculated from the cost of the nitric acid required to neutralize the waste. A typical gravimetric determination of chloride lab for 15 students with four replicates each would produce 50−90 g of AgCl (depending on the percent mass of chloride and sample sizes). Recycling from this lab alone would save a university up to $420 a semester. The quantitative analysis laboratory sequence at University of Tennessee at Martin incorporates three laboratories that produces AgCl: gravimetric determination of chloride, Fajan’s titration, and Volhard’s titration. These three laboratories with 10 students each semester produce approximately 165 g of AgCl in a year. Recycling all of this AgCl would save the university approximately $770 per year.
Figure 1. Comparison of the two methods for percent recovery and percent purity. Error bars represent 95% confidence interval.
recovery, at a 95% confidence level, for method B was statistically higher at 99.7 ± 3.2% (N = 5) than that for method A at 91.3 ± 4.4% (N = 5). The percent purity, at a 95% confidence level, for method B was not statistically different at 99.7 ± 0.6% (N = 3) than that for method A at 98.9 ± 0.5% (N = 3). Time for Conversion
The time required for both conversions is shown in Figure 2. Here we define active time as time in a process where participation or supervision is required. Passive time does not require supervision and includes time sitting in an oven or on a hot plate. The conversion using method A requires approximately 25 min of active time and only 20 min of passive time. The conversion using method B took less active time at 22.5 min; however, the conversion requires nearly 3 h for heating. While the total time for method B was overall longer, the additional time was passive time spent in an oven. For a process that is expected to be carried out a maximum of once or twice a semester, the extra passive time is negligible, and the two methods are roughly equivalent.
Additional Considerations
In addition to those already discussed, a few other considerations should be made, including conversion byproducts, facilities and equipment needed, and safety. Method A produces a concentrated copper ammine solution, which cannot be disposed of down the drain. The copper ammine byproduct must either be recycled, a time intensive process, or disposed of via costly commercial waste disposal. A summarized procedure for the recycling of this byproduct is in the Supporting Information. Method B produces a waste that is very basic. This solution may be neutralized with nitric acid and
Cost Comparison
As seen in Table 1, method A is less costly than method B, though the cost to run each conversion is very close. Note that the cost to run is based on an ∼25 g sample of AgCl waste, and the reagents are in stoichiometric excess. The savings on the B
DOI: 10.1021/acs.jchemed.7b00713 J. Chem. Educ. XXXX, XXX, XXX−XXX
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is easily disposed of. When considering the waste and ease of clean up, method B is superior. The facilities needed vary depending on the method as well. Method A requires standard laboratory equipment including general glassware and a hot plate. Method B uses similar glassware and a hot plate but also requires the use of a high temperature oven or muffle furnace (heated to 500 °C). The muffle furnace may not be readily available in a small or midsize university chemistry department; however, there may be such an oven available in an art department (for use with pottery) or in an engineering department. In the case where it is not available, a Bunsen burner may be used, though it has been found that the crucible in which the heating is done often cracks during the conversion, which is not ideal. Safety is also a concern when conducting these conversions. Previous literature has expressed concerns with method A due to the ammoniacal solutions of silver.3,16,18,21 When the AgCl is dissolved in concentrated NH3, there is a possibility of silver ammine complexes forming fulminating silver compounds.3,16,18,21 While this is unlikely if the solution is immediately reacted with copper (as the procedure instructs), it is much more likely if the solution is left unattended for an extended period of time.16 There are no published concerns with any intermediates produced in the method B conversion. For both methods, the AgNO3 must be collected. One publication suggests washing the AgNO3 crystals with ethanol.16 Upon further search in the literature, there is a possibility of forming explosive compounds when mixing AgNO3 and ethanol.19,22 Thus, these methods have been modified to wash the AgNO3 with chilled concentrated HNO3. This recycling process is intended to be used on relatively pure AgCl. When both methods were tested on lab waste AgCl (expected purity >90%), reduced percent recoveries were obtained. Similar purity was obtained. This process is recommended for quantitative analysis lab waste for laboratories such as gravimetric analysis resulting in AgCl, Fajan’s titration, or Volhard’s titration. The method is not recommended for qualitative analysis waste where there are potentially many additional metal ions that could interfere with the recycling reactions.
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CONCLUSION
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ASSOCIATED CONTENT
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Detailed procedure for each method and a detailed procedure for the recycling of copper ammine byproduct for Method A (PDF, DOCX)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jennifer Esbenshade: 0000-0001-9537-4061 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank the Department of Chemistry and Physics and the College of Engineering and Natural Sciences at University of Tennessee at Martin for support and resources. Additionally, the authors are grateful for the support the Bremer Family Undergraduate Research Endowment for summer undergraduate research, through which much of this work was done.
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REFERENCES
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Of the two methods, method B seems most promising. While it takes more time overall, the majority of the time is passive time where the sample is in an oven. Method B does require the use of a high temperature oven, however, most small to midsize universities will have something available either in chemistry, art, or engineering departments. Should an oven not be available, a Bunsen burner may be used in its place. Method A is also a useful method; however, the added safety concerns and the difficult byproduct waste makes it less attractive than method B. The best method will vary from university to university depending on the resources and time available. The information provided is to best inform the decision as to which conversion method to use to encourage a decrease in financial and environmental burdens.
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00713. C
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(17) Foust, D. F. Recovery of silver and cobalt from laboratory wastes. J. Chem. Educ. 1984, 61 (10), 924. (18) Willbanks, O. L. Reclaiming silver from silver chloride residues. J. Chem. Educ. 1953, 30 (7), 347. (19) Tully, J. P. Washing Reclaimed Silver Nitrate Crystals with Alcohol Leads to Explosion. Chem. Eng. News 1941, 19 (5), 250. (20) Shanley, E. S.; Ennis, J. L. The chemistry and free energy of formation of silver nitride. Ind. Eng. Chem. Res. 1991, 30, 2503. (21) Jenkins, I. D. Tollens’s test, fulminating silver, and silver fulminate. J. Chem. Educ. 1987, 64 (2), 164. (22) Henderson, K. O.; Garin, D. L. Procedure for recovering silver nitrate from silver-silver oxide residues. J. Chem. Educ. 1970, 47 (11), 741.
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DOI: 10.1021/acs.jchemed.7b00713 J. Chem. Educ. XXXX, XXX, XXX−XXX
