Report of:      Specialty Corrosion Testing of Dental Amalgams per ISO 24234 Annex A

Summary:     Five (5) sets of dental amalgam alloys – A1, A2, A3, A4 and A5 – were corrosion tested per Annex A of ISO 24234:2004, Determination of Immersion Corrosion for Dental Amalgam.  Reportable data included mass (micrograms, μg) of five metals – mercury, silver, tin, copper, and zinc leached into 0.1M lactic acid electrolyte – normalized to unit specimen surface area in 4 hours of total immersion time; and mercury exhalation into the vapor phase above as nanograms (ng) absorbed into an in-line sorbent trap in 4 hours.  The specification did not provide upper acceptable limits for the various analytes, so the data are presented without reference to conformance or non-conformance.  The rigorous ISO procedures, which included running method blanks to ensure reproducibility and prevent cross-contamination, were strictly followed – with the client-approved exception of substituting a specialized sorbent trap for a gold foil dosimeter as the medium for capturing vapor-borne mercury in the closed loop corrosion cell.

Experimental Method: 

Four specimens each of the following dental amalgam alloys – A1, A2, A3, A4 and A5 – were submitted.  The specimens had all been subjected to the specified air exposure prior to submission (§A.3 of specification). Each specimen was a solid cylindrical stub measuring approximately 4 mm diameter by 7 mm long (Figure 1).  The specimens were accurately measured (within 0.02 mm) to ensure a precise calculated surface area.  

The test electrolyte was 0.1 molar lactic acid aqueous solution (pH ≈ 2.3) and the test temperature was 37°C. The target pH was 2.30, and all batches were checked for conformance to 2.3 ± 0.2 pH units before using (§A.4).  All one-liter batches were made from the same reagent grade (ACS) 85% stock solution (supplier: GFS Chemicals), and as prepared they exhibited minimal deviation from the target pH. 

Due to the expense and lack of availability of gold foil dosimeter apparatus for collecting mercury leached from the specimens into the solution and then exhaled into the head space above the solution, an alternate method was required.  An acceptable alternative as allowed in §A.6 was, after client approval, to substitute an in-line mercury sorbent trap.  This medium was determined to possess the required nanogram-level sensitivity and accuracy, and consisted of two stages.  This device (Figure 2) is successfully utilized by a local energy resources company that performs time-weighted measurements of coal-fired power plant exhaust gases for air quality monitoring.   It was SKC Part Number ----, which is a higher-cost two-stage iodated carbon unit to ensure capture of ionic as well as elemental mercury. 

Each of the five sample sets required three “runs” – one “method blank” with no amalgam samples, plus two determinations.  Each determination required two duplicate specimens (randomly labeled A, B, C, and D from each set) immersed in the same corrosion cell, following a 24-hour “pre-corrosion” exposure in the same electrolyte at the same temperature.  The runs were called Blank, Determination #1 (A/B), and Determination #2 (C/D).  The testing time for each determination was 5 hours, including 4 hours of immersion time (for Determinations #1 an #2) and 1 hour with the samples raised above the liquid to allow time for air circulation to flush mercury vapor from the head space. 

Figure 3 shows the test schematic from ISO 24234, and Figure 4 shows several photographs of the Matco test setup, which corresponded exactly to the required setup except for the gold foil dosimeter.  The variable speed peristaltic pump (Omega Model FPUDVS2002) was adjusted to 42% of the lower capacity range to give an average measured flow rate through the mass flowmeter (Omega Model FMAA2302) of 5 mL/min.  The system was checked for uninterrupted air circulation (observed as bubbles in a small beaker of water) immediately downstream of the sorbent trap (at the point shown by the red circle in Figure 3).  This check was performed several times during the test to ensure that there were no air leaks in the system, and if there were leaks, to correct immediately. 

Procedures were strictly followed regarding acid washing of glassware and wholesale replacement of PVC tubing between runs in which different amalgam samples were tested.

The analytical methods were as follows: 

• Hg vapor absorbed in activated sorbent medium: Cold Vapor Atomic Fluorescence Spectrophotometry, or CVAFS
• Leachate Hg (in solution): Cold Vapor Atomic Fluorescence Spectrophotometry, or CVAFS
• Leachate Sn, Zn, Cu, Ag (in solution): Inductively Coupled Plasma / Mass Spectrometry, or ICP/MS

See Appendix for applicable limits of detection.  This information has been excluded from the example report.

Results:

The reportable results per §A.7.3.3 are listed in the table below.  The reported values are after subtraction of the method blank readings from the underlying concentration data. All fluid analytes are shown, although the specification states that only those elements that are present in the alloys in concentrations above 0.5 mass percent should be reported.  Alloy compositions were requested but were not provided with the amalgam specimens; therefore all of the five analyte metals are shown.  It is assumed that the client will utilize the data in this report as necessary for regulatory compliance or other purposes.  The attached appendix provides detailed, underlying test data for each of the five amalgam alloys.  This information in addition to the illustrations, appendices and diagrams have been excluded from the example report to maintain client confidentiality.