Elemental Analysis of Ancient Roman Coins

Elemental analysis of ancient Roman coins, Part I

Chem 437, Walter Bowyer, Fall 2018

I have a set of “ancient Roman coins”.  Your assignment is to determine if they really are “ancient” and “Roman”.  One common way to know if an object is authentic is to determine its % composition.  Using the internet, you will determine what Roman coins were made of.  You will then design and perform experiments to analyze small samples taken from the coins using flame AA.  Finally, you will write a formal report on your conclusion about authenticity.  This experiment will take five weeks to complete.

Learning Objectives

By completing this lab, you will learn to:

1. Design an effective sampling strategy of solids by considering the necessary aspects of matrix and analyte (e.g. population vs. sample, phase of sample, quantity available, homogeneity, chemical stability, distributional vs constitutional heterogeneity).
2. Use qualitative tests to determine what quantitative analysis to pursue.
3. Design an experiment to digest and dilute samples.
4. Make standard solutions and prepare calibration curve.
5. Use the calibration curve to quantify analyte.
6. Apply statistics to determine uncertainty.
7. Arrive at appropriate conclusion considering limits of sampling technique and its impact on the uncertainty.

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Boss:  Please analyze this sample for me.

Analytical Chemist (cheerfully):  Sure boss!  But first, tell me what’s in it and how much.

Why is this funny?

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1. Initial research done on your own (individually, consulting no one but me). You must have references for all of your information.  You do not have to use SciFinder or Google Scholar, but you may if you prefer.  Before you begin this research, complete the Plagiarism Worksheet and be sure to apply what you learn.  For each of the following write at least a paragraph: it should be typed, clearly written, concise, and error free.
   1. What metals were used for making Roman coins?  (Hint: we will probably not be analyzing silver or gold coins.) 
   2. Choose at least two likely metals and find a qualitative microtest for each metal (using wet chemistry). Write it up as a set of directions to follow in lab.
   3. In what ways were Roman coins intentionally made heterogeneous? How might Roman coins have become heterogeneous in the intervening centuries? 
   4. What phase does your analyte need to be in to analyze by flame AA? How will you get it into the proper phase (give specific reagent)?  What concentrations are typical for flame AA analysis?  How will you get it to the proper concentration including needed glassware? 
   5. You could digest an entire coin. Why might you not want to do so?  What are some possible strategies for obtaining a sample of the coin?
   6. Hand in a hard copy of this on September 5 at the start of lab.

 

 

 

The following sections were developed for a similar lab in the Quantitative Analysis course.  They provide useful information to instructors interested in using the project as developed on the prior page.

Elemental Analysis of Roman Coins

By Walter Bowyer, Jonathan Thrall, and Aaron Weitgenant

To start this experiment, I would like for you to explore the following questions by using the internet:

• What metals were used for making Roman coins?  (Hint: we will probably not be analyzing silver or gold coins.)
• In what ways were Roman coins intentionally made heterogeneous?  How might Roman coins have become heterogeneous in the intervening centuries? 
• How will you get the coin into solution for analysis (give specific reagent)?
• You could digest an entire coin.  Why might you not want to do so?

Write up to four paragraphs and include references (website addresses are sufficient).  If needed, use quote marks appropriately.  You may not paraphrase.  You may not share websites or information with each other – I want you to do this research by yourself. 

Flame atomic absorbance (AA) is an excellent technique for determination of many metal ions in solution.  It typically offers high sensitivity and rapid analysis.  Unfortunately, like many spectroscopies, it offers a limited linear dynamic range. 

We will measure the absorbance of our selected elements by dissolving a small sample of the coin.  Just as we did for phosphate, we will prepare standard solutions that span our linear dynamic range (ldr).  We will aspirate them into the flame and measure the absorbance by passing a monochromatic beam of light through the flame.  We can then plot the absorbance vs. the concentration of analyte in the solution to make our calibration curve.  Finally, we will measure the absorbance of the solution of the dissolved coin.

Procedure:

All solutions except the stock (1000 ppm) Pb can go down the drain.

1. Take a photo of the coin including a scale. Drill a sample from the coin with the Dremel 3/32” bit to obtain approximately 20 mg of metal.  CAUTION:  Do the following only with strict supervision by me.  Hold the coin in a large pair of pliers, first padding the jaws of the pliers with Kimwipes.  Drill over the plastic basin lined with Al foil.  Transfer the metal to a closed vial.   
2. For both lead and copper the ldr is from 1 to 10 ppm. Your stock solution has been prepared for you and is 1000 ±10 ppm of metal. You may pipet 5.00 mL of the stock to prepare your first diluted solution. (Do not put pipet into bottle of stock solution.  Do not pour more than 6 mL out of the stock bottle.)   Calculate how you will prepare four standard solutions (in addition to your blank) spanning the ldr.  Use mostly 50 and 100 mL volumetric flasks.  Label one calculation appropriately and then write a table showing the size of pipet and volumetric flask for each solution.
3. We will dissolve the coin in a small amount of nitric acid and then dilute it. Assume it is 80% copper and 20% lead. What size of volumetric flask will you dilute the digested coin to?  You will probably need to do two serial dilutions to get the solution into the ldr with copper, but one may be sufficient for lead.
4. Measure the mass of coin shavings recording all significant figures and transfer it to a clean, dry 250 mL beaker. Add approximately 5 mL of concentrated metal grade nitric acid using a clean graduate cylinder. (Do not drip the acid. Do be sure that any glassware has had the nitric acid rinsed from it before taking it from the hood.) This should quickly dissolve all of the metal,.
5. Fill, the appropriate volumetric flask approximately half full with deionized water, then quantitatively transfer the coin solution to, being sure it is labeled. Fill to the line using deionized water.
6. Make your standard solutions using the 1000 ppm stock solution (do not put the pipet into the bottle of stock solution – pour no more than 6 mL into a small beaker and pipet from there). Dilute stock solution with the 1% nitric acid that Kathy has prepared for you.

Instrument procedure:

1. Turn on hood dimmer switch (on wall) to full. Remind yourself where the emergency shutoff is on instrument (upper left corner). Turn on air to >50 psi.  Check tank pressure on acetylene.  (CAUTION: Do NOT allow acetylene tank to drop below 80 psi – left hand gauge - or it will result in a $5000 repair bill when acetone floods the torch.  The flame burns about 20 psi/hour.) Turn on acetylene to deliver at <15 psi (right hand gauge).
2. Insert appropriate hollow cathode lamp. Turn on instrument power (switch beneath lamp). Wait 2-3 minutes for boot.
3. Start-up screen: don’t choose Hg, check diagnostics if needed, hit OK. Go to “lamp” tab. Tap white boxes to select element and line. Then tap “set up instrument” which sets default for slit and does alignment.  When complete (about 1 minute), the value of energy striking detector will be displayed.  Maximize energy by turning two knobs next to lamp to manually align lamp.  (Hint:  Go to the “Tools” tab to get recommended settings if needed.)
4. Go to flame tab and check interlocks. If not all are ready, go to “Tools” then “Diagnostics” to find problem.  Air flow is typically 10 and acetylene typically 2-2.5.
5. Using a white card, check that the beam passes over flame slot and adjust with two black knobs (begin with lower knob for in/out, then use middle knob to rotate burner head). For height, make sure the burner head is not blocking beam, autozero the signal, and raise burner head until it blocks the beam (as judged by signal) by turning (counter clockwise when viewed from above) the large horizontal wheel.  Then lower it until signal just returns to 0, then lower ½ additional turn.  Confirm that the nebulizer is clamped in to place.
6. To light the flame, tap the on/off button on the screen, then start DI water/1% nitric acid nebulizing. Once stabilized, begin nebulizing standard solution and optimize signal with in/out and rotator knobs on burner.  Do not optimize height (always use ½ turn below beam blockage).  Again nebulize DI water/1% nitric acid.
7. Go to the Parameters tab and tap “calibration” in upper right corner. Use “linear with zero intercept”.  Enter the concentrations of your standard solutions.  (Do not include the 0 as a “standard”.  The PE software calls this the “blank” and prompts you for it separately from the standards.)  Set to do four replicate measurements with 3 sec integration time and 5 second delay.  Record in your notebook the average absorbance for each standard including the blank.
8. After running the standards, look at the calibration curve on screen to confirm that it is linear and has reasonable scatter.
9. Analyze one of your middle standards and record the resulting concentration in your notebook. If accuracy is acceptable (<10% error), analyze your coin solutions (diluted as needed) recording the concentration of each.  Confirm that the resulting analyses fall into the LDR.  If they do not, prepare new dilutions and reanalyze.
10. Repeat your analysis of the same middle standard that you analyzed above. Analyze 1% nitric acid blank five times and record all five results.  Repeat your analysis of the same middle standard that you analyzed above (now for the third time) and confirm that the reproducibility is acceptable.
11. Remember to check acetylene pressure regularly throughout instrument use.
12. Shut off flame, change lamp to the other element, and repeat steps 6 - 11.
13. To shut down, run 1% nitric acid or DI for 1-2 minutes, then suck air for 1 minute to dry, then turn off flame (on screen on the flame tab). Turn off tanks (check psi for next time), bleed gases (on screen on flame tab).  Turn off power to instrument (hard switch behind instrument panel door below lamp.)  Turn off hood.

Calculations:

1. Plot the calibration curve and estimate precision from it.
2. Using the concentrations determined by the instrument for the solution containing the coin digest, determine the % Cu and Pb in your coin.
3. Calculate the limit of detection (consult and cite text for definition). Calculate the accuracy and precision using the results for your analysis of the middle standard solution.  Calculate how the accuracy and precision converts to apply to your analysis of %Cu and Pb in the coin.
4. Write you normal discussion.

Contributors and Attributions

• Walter Bowyer ([e-mail]), Jonathan Thrall and Aaron Weitgenant, Hobart Williams Smith Colleges
• Sourced from the Analytical Sciences Digital Library