Microchemical Testing

Introduction

Listed below are five working protocols being used by the Microchemical & pH Testing Subgroup of the Materials Working Group (MWG) Testing & Standards Subcommittee as well as a description of a "Round Robin" testing of seven materials that were considered for use in exhibition/storage areas. Other Microchemical Tests are listed below those used by the MWG.

Contributors: Daniel Burge, Image Permanence Institute (IPI); Madeline Cooper, Winterthur/University of Delaware Program in Art Conservation (WUDPAC) Fellow; Marie Desrochers, Winterthur/University of Delaware Program in Art Conservation (WUDPAC)Fellow; John Dunphy, University Products; Fenella France, Library of Congress (LC); Catharine Hawks, National Museum of Natural History (NMNH); Susan Heald, National Museum of the American Indian (NMAI); Kelly Krish, Image Permanence Institute (IPI); Lisa Imamura, National Museum of the American Indian (NMAI) Mellon Fellow.

pH Pens

Pens for pH testing of paper and paperboards are available from several manufacturers. The inks in the pens are chemicals (indicator dyes) that are designed to change color in response to the pH of a paper or paperboard. The pens are not designed for use on any other types of materials. The actual color response is dependent upon the manufacturer’s choice of pH indicating chemicals.¹

¹ - The indicator in the Abbey pH Pen™ and Lineco pH Testing Pen is chlorophenol red. The pH Indicator Pen contains thymol blue, cresol red, and bromothymol blue.

Materials, Supplies, and Equipment

• Glass pane
  
• 91% isopropanol (available at pharmacies)
  
• Kimwipes
  
• Abbey pH Pen™ available from Gaylord should be stored bagged in a non-food grade polyethylene bag and refrigerated
• Lineco pH Testing Pen available from University Products should be stored bagged in a non-food grade polyethylene bag and refrigerated
• pH Indicator Pen (broad range pH) available from University Products should be stored bagged in a non-food grade polyethylene bag and refrigerated
• Reference materials: acidic, neutral, alkaline, newsprint, and archival papers
  

Procedure

1. Clean a pane of glass to use as a substrate for the tests with 91% isopropanol and a Kimwipe. Doing the test with other materials below the sample may produce erroneous results as the indicator dyes may react with the substrate rather than the sample.
2. Select a positive control. This should be a sample of a paper/board known to be acidic. Old newsprint is excellent for this purpose.
3. Select a negative control. This should be a sample of a paper/board that is known to be either completely free of acids, or is heavily buffered with an alkali. Newly purchased Permalife® paper products, which are buffered with calcium carbonate, are excellent for this purpose.
4. Select (or cut) three samples from a paper/board of unknown pH. This may be a new paper or board that you wish to test or may be old papers or boards that you suspect were initially acidic or may have absorbed acids over time. If testing a paperboard, remember to test both the surfaces, and to peel off the coverings and test the internal core of the board as well.
5. Place each sample, including controls, on the cleaned glass and mark 3 lines on each one with one of the pH pens. Observe the color of the lines over about a 1-minute period.
6. Repeat the process for other pH pens.
7. Compare the results of the lines on the samples with similar lines on your controls.

Interpretation

The results can be difficult or impossible to distinguish if the paper sample is any color other than white or off-white.

Some of the pens DO NOT DISTINGUISH between pH-neutral and alkaline-buffered papers. They can tell you only that the material has a pH below 7 or a pH of 7 or higher. One pen is designed to indicate a fairly broad pH range from acidity to alkalinity.

The pens tell you nothing about when a paper/board became acidic. For instance, if a paper has been in contact with an acidic material, it will absorb acidity from that material. This does not mean that the paper was manufactured with a low pH, only that it has a low pH at the time of the test.

On the other hand, if you are testing a new stock of paper or paper board, test several samples from within a ream or box (the ones at the top and bottom are likely to have been contaminated by packaging).

• If all these indicate a pH below 7 (use multiple pens to verify), the material probably is unsuitable for use with collections.
• If two of the pens indicate a pH of 7 or higher, you have no way of knowing if these papers/boards are acceptable for use with proteinaceous materials because these pens do not distinguish between neutral and alkaline materials. The broad-range pen may indicate the approximate level of alkalinity of a material.
• Based on test results, refer to the manufacturer’s specifications for the paper to make your decision about appropriate uses.

References

Abbey Material Safety Data Sheet
University Products Product Information Sheet

Surface pH test using pH indicator strips

Protocol based on Library of Congress method used by the Conservation Division. Revised at the November 2019 in-person meeting.

Materials, Supplies, and Equipment

• ColorpHast pH indicator strips range 4.0-7.0 should be stored bagged in a non-food grade polyethylene bag and refrigerated
• ColorpHast pH indicator strips range 6.5-10.0 should be stored bagged in a non-food grade polyethylene bag and refrigerated
• Hydrion pH test paper range 1-12 should be stored bagged in a non-food grade polyethylene bag and refrigerated
• Scissors
• Beakers
• Deionized water
• Distilled water
• Small weight
• reference materials: acidic, neutral, alkaline, newsprint, and archival papers

Procedure

1. Cut indicator strip into smaller strips to reduce tidelines.
2. Dip the reading end of the strip into a beaker of deionized water, then shake off excess water.
3. Place the reading portion of the strip face down on the paper and place a small weight to ensure good contact.
4. Wait five minutes, then compare to reference scale provided with pH indicator strip.
5. If testing a paperboard, remember to test both the surfaces, and to peel off the coverings and test the internal core of the board as well.

1. Repeat test with Hydrion pH test paper.
2. Repeat with distilled water for comparison.
   

Interpretation

This test tells you nothing about when a paper/board became acidic. For instance, if a paper has been in contact with an acidic material, it will absorb acidity from that material. This does not mean that the paper was manufactured with a low pH, only that it has a low pH at the time of the test.
On the other hand, if you are testing a new stock of paper or paper board, test several samples from within a ream or box (the ones at the top and bottom are likely to have been contaminated by packaging).

• If all these indicate a pH below 7, the material probably is unsuitable for use with collections.
• Alkaline materials are unsuitable for use with proteinaceous materials.
• Based on test results, refer to the manufacturer’s specifications for the paper to make your decision about appropriate uses.

pH & alkaline reserve test (Library of Congress-modified TAPPI test)

Protocol based on Library of Congress method developed by George Kelly of the Preservation Research and Testing Division. Revised at the November 2019 in-person meeting.

The Library of Congress developed this procedure as a modified version of the Technical Association of the Pulp and Paper Industry’s (TAPPI) test for pH of paper cold extractions (TAPPI 509/ISO 6588/ASTM D778) combined with an alkaline reserve test (TAPPI 553/ASTM D4988?).

Materials, Supplies, and Equipment

• pH Meter – for measuring pH
• Standard pH buffers – at pH = 4.00, 7.00, and 10.00 for pH electrode calibration
• Parafilm
• Reagents – 0.1N (0.1M) HCl (hydrochloric acid), 0.1N (0.1M) NaOH (sodium hydroxide)
• Water – conforming to ASTM Specification D 1193, Type I or II, Standard Specifications for Water, reagent grade or flushed DI water. Flush water until resistivity gets to 18.2 MΩ.
• Scale – to weigh paper sample
• Heating and Agitation – stand with magnetic agitation platform with heating option, magnetic stir bars
• Glassware – glass beakers, 400 to 500 ml, with corresponding size watch glass as covers; 100 ml beakers for HCL and NaOH; waste beaker; glass stirring rod; beaker to hold stirring rod; 1000 ml beaker for used slurries
• Wash bottle – plastic wash bottle, filled with water, as in section 1.4, for rinsing sample out of blender
• Titration Apparatus – stand with burette clamp assembly, 25 ml burette with orange stop-cock, 25 ml burette with blue stop-cock.
• Cutting Tools – scissors for cutting paper samples, board cutter for cutting card stock, utility knife with removable blade for thick boards
• Timer – to monitor measurement wait time
• Funnel – to transfer slurries
• Paper towel – to filter slurries
• Laboratory soap – for clean-up
• Blender
• reference materials: acidic, neutral, alkaline, newsprint, and archival papers

Procedure

Set-up

1. Prepare workspace to accommodate all equipment, reagents, glassware and tools so that procedure flows smoothly from start to finish without unnecessary delays. Timing of steps is critical in this study.
2. Set out all glassware, tools, and reagents to be used for all steps before starting the procedure.
3. Set up a stand with a clamp assembly to accommodate two 25 ml burettes.
4. Fill the burette with the blue stopcock with the 0.1N NaOH. Fill the burette with the orange stopcock with the 0.1N HCl. Flush the burettes by filling with the solution before first use if empty.
5. This set-up can be done before or after cutting the samples, described below.

Sample preparation

1. Cut paper/ board sample into 5-10 mm squares. Prepare enough sample for two replicates of 2.0g.
   1. Wear gloves when handling samples.
   2. Clean the blades of the scissors/board cutter/knife before and after cutting.
   3. Remove edges of the sample piece, to eliminate areas with contamination from handling/fingerprints.
   4. Cut specimens from different areas of the sample piece, to avoid uneven chemical distribution.
   5. Separate card, board, and corrugated board into layers for cutting.
   6. Collect cut sample in a clean container or piece of paper for transport.
2. Place weighing paper on scale, tare to zero. Add sample until weight reaches 2.00 (+/-) 0.01g. Record the weight.
3. Weigh two sets of samples to obtain replicate results for averaging.
   

Calibration – pH meter

1. Calibration may be performed before starting the procedure, during the initial set-up.
2. Calibration should be performed before adding water to the sample.
3. Prepare the buffer solutions (pH 4.0=red; 7.0=yellow, 10.0=blue): pour each into a 150 mL beaker at the 100 mL mark with a magnetic stir bar in the beaker; label all solutions (pH, your initials, date). Prepared buffer solutions can be reused for one month.
4. Turn on Accumet XL20 [150] pH/Conductivity Meter with the button on the right side panel of the instrument.
5. Remove the container with storage solution from the electrode by unscrewing the cap, and move cap/gasket to the top of the electrode. Rinse the electrode with deionized water over a waste beaker with a wash bottle.
6. Make sure there is sufficient electrolyte in the electrode (~1/4” below cap), solution 13-636-430
7. Open the fill hole on the pH electrode (rotate the purple collar at the top end of the electrode): the fill hole should remain opened during calibration and pH measurements.
8. Place pH meter in pH=4 buffer solution on a hot plate with stirring but without heat. Stir during measurement.
9. Wait for the pH reading to stabilize (~ 5 minutes). The pH reading should be close to the pH of the buffer solution (reading should be less than ± 0.1 from actual pH).
10. When the pH reading is stable for 30 seconds, select the “standardize” icon at the top right of the screen. Wait for the meter to display “STABLE” again. When the correct standard (beaker icon) is flashing, select “confirm” (icon top right of screen).
11. Remove electrode and rinse with DI water over the waste beaker; cover buffer solution with Parafilm.
12. Repeat calibration with buffer solution 10.0, and then 7.0.
13. Enter calibration date and record the slope in the log book. If slope < 94% replace the electrode.
    

pH measurement

1. Between measurements rinse electrode with DI water, do not wipe electrode bulb with any material. Place electrode in storage solution between measurements, the pH electrode bulb should be moist at all times.
2. Prepare beakers by using label tape to identify the sample and replicate number on each beaker.
3. Place sample into clean blender. Rinse blender and cap with DI water before use.
4. Add enough water to cover the blades of the blender.
5. Set timer and blend for 60 seconds. Start blender SLOWLY to avoid splashing sample on sides of beaker or into the cap.
6. Transfer the slurry to a clean 400 ml – 500 ml glass beaker, with a magnetic stir bar in the bottom.
7. Use the plastic wash bottle to rinse the blender and lid to transfer all the sample into the glass beaker.
8. Add water to reach a total volume of 200 ml, and place beaker on agitation stand.
9. Insert electrode into solution and gently start the stirring. Stir at a speed that will agitate all of the slurry.
10. End-point pH determination: Allow pH to stabilize for 5 minutes. After the 5 minute wait time, when the pH meter shows stable reading, wait an additional 30 seconds; record the final pH if there is no change in pH display. Record weight of samples and final pH in spreadsheet pH & Alkaline Reserve.xlsx, attached.
11. After recording the pH continue immediately to alkaline reserve titration, below.
    

NOTE: If the pH is less than or equal to 7, do not continue to alkaline reserve titration, as the sample is acidic or neutral and there should be no alkaline reserve. Alkaline reserve titration

1. Add 0.1N HCl from the burette with the orange stop-cock until the pH drops below 2.5. Record the volume of HCl used to achieve the target pH. Continue to stir the solution.
2. Remove electrode from the slurry, rinse with DI water, store as directed during heating phase of titration.
3. Place a watch glass on the top of the beaker and heat the sample to boiling, with mild agitation. After the solution reaches boiling point, let the sample boil continuously for 1 minute.
4. Cool the sample to room temperature for ~ 1 hr.
5. Place the beaker on the agitation stand, on a cool platform, with no heat.
6. Place the electrode into the solution and gently start the stirring. Stir at a speed that will agitate all of the slurry.
7. Check the initial pH. If the pH is ≥ 4.0, add HCl to bring the pH down to ≤ 3.0. Record the volume of the additional acid required to achieve the target pH.
8. Titrate the solution with the 0.1N NaOH in the burette with the blue stop-cock until the pH reaches 7.0.
9. End-point pH determination: When the pH meter shows stable reading, wait an additional 30 seconds.
10. Record the volume of NaOH required to reach the target pH.
11. Repeat the titration to achieve two replicates to average the results.

Shut down and clean up

1. Rinse the electrode with deionized water over waste beaker.
2. Place the electrode in the container with the storage solution: slide cap/gasket down the electrode, insert electrode midway into storage bottle, and gently screw on cap. If bulb is not submerged in storage solution, refresh solution (SE 40-1 Electrode Storage Solution).
3. Close fill hole on electrode by rotating purple collar.
4. Turn the pH meter off by pressing the lower left corner with the stylus. Select “Shut Down”.
5. Place a large funnel into a 1000 mL Pyrex beaker. Line the funnel with a paper towel as a filter paper.
6. Empty the beakers with the used slurries into the 1000 mL beaker with the paper-lined funnel to strain the paper pulp from the solution. Discard the paper towel with the strained pulp in a trash can. Discard the solution in the 1000 mL beaker in the sink. As the solution has been neutralized during the test process, it does not need to be treated as Hazardous Waste.
7. Add DI water to the waste beaker. Dispose of any remaining chemicals in the burets and beakers in the waste beaker to neutralize the solution. Flush the drain well with water. Buffer solutions in beakers can be covered with Parafilm, saved and reused for up to 1 month.
8. Return the bottles to their proper storage.
9. Flush burettes with DI water. Wash any glassware used with laboratory soap. Rinse with deionized water.
   

Calculation

1. Calculate result: Alkalinity – Weight % of alkaline species in sample, as CaCO3 equivalency.
2. Formula: Alkalinity (%) = 100 x 0.050 x [CHCl(N) X VHCl(ml) – CNaOH(N) X VNaOH(ml)] / Sample weight (g) Where, C = concentrations of HCl or NaOH, V = Volume.
3. Calculate result by completing blank lines on spreadsheet pH & Alkaline Reserve.xlsx, attached.
   

Report

• Report the average of the 2 replicates as calculated above. Report alkalinity as % CaCO3.
• Note: Unplug hot plates after use.

Test for sulfur using lead acetate paper and pyrolysis

Protocol published in Odegaard, Carroll, and Zimmt 2005 on pages 146 and 147. Edited for MWG use by Susan Heald. Revised at the November 2019 in-person meeting.

Pyrolysis of the sample causes the formation of sulfides, which react with lead acetate to form brown-black lead sulfide.

Materials, Supplies, and Equipment

• Pasteur pipettes
• Laboratory wrapping film
• Hydrion pH indicator paper
• Bunsen burner or alcohol lamp
• 3% hydrogen peroxide
• Pipette for hydrogen peroxide
• Lead acetate test paper (Macherey-Nagel) highly toxic carcinogen and mutagen; severe health rating
• Distilled water
• Glass beaker
• Eye protection
• Fume hood
• Scissors
• reference materials: wool fibers (positive); cotton fibers (negative)

Procedure

Wear gloves when handling papers. Use caution when using any type of open flame.
There should be no solvent containers in the vicinity or any materials that can catch fire.
Wear eye protection when performing this test. Perform test in well-ventilated area or fume hood if possible.

1. Seal the tip of a Pasteur pipette with a flame source.
2. Introduce the sample into the pipette and push it down into the tapered area.
3. Cut a strip of lead acetate paper about 2 cm in length.
4. Fold the strip along its long axis and quickly dip one end into distilled water.
5. Place the lead acetate strip (wet end towards the sample) just inside the open end of the pipette.
6. Place a 1 cm strip of pH indicator paper in the pipette.
7. Seal the open end of the pipette with laboratory sealing film (do not allow it to touch the test papers).
8. Gently heat the sample in the flame until fumes evolve.
9. Allow enough time for the fumes to reach the test paper.
10. Observe any color change in the paper.
11. If the paper turns a brown or black color, photograph the test paper according to photography guidelines.
12. If the paper turns a brown or black color, after photography, drop 3% hydrogen peroxide on the paper and note any color change.

Interpretation

The formation of a brown color on the lead acetate test paper confirms the presence of sulfur. Additional confirmation may be obtained by treating the paper with 3% hydrogen peroxide. This removes the color by converting the lead sulfide to lead sulfate, which is white. The addition of a pH paper strip in the reaction chamber (pipette) allows one to determine the pH of the fumes. If the test is negative for sulfur but the fumes are highly acidic, the material may contain poly(vinyl chloride), which can be confirmed with the Beilstein test.

Lead acetate and pH strips tested in tubes

References

Odegaard, N., S. Carroll, and W. Zimmt. 2005. Material characterization tests for objects of art and archaeology. London: Archetype Publications.
How to Test For Sulfur in Materials Using Lead Acetate Test Paper – Canadian Conservation Institute (CCI) Notes 17/5

Beilstein test for chlorine

Material containing bound or ionic halogens (Cl, Br I) reacts with a copper wire when heated in a flame to produce CuCl₂ and a green-colored flame.

Materials, Supplies, and Equipment

• Single-strand copper wire (12/14 gauge- available at hardware stores) a minimum of 12” long, with any insulation removed from about 6” inches to form a testing area
• Tongs or rubber grip pliers
• Heavy gloves (ideally leather or heavy cloth), even for insulated wires
• Propane torch (available at a hardware stores)
• Eye protection
• Cinderblock or other non-flammable material to support the torch
• Metal tray to collect melted material
• reference materials: PVC sheet (positive); Mylar polyester sheet (negative)

Procedure

Use caution when using any type of open flame. There should be no solvent containers in the vicinity or any materials that can catch fire. Wear eye protection when performing this test. Perform test in well-ventilated area or fume hood if possible. If a fume hood is not available, test outdoors. Testing on a slightly overcast day will improve ability to see colors in the flame. Support torch inside the openings in a cinderblock or other non-flammable material, inside a metal tray to collect any melted material.

1. Clean the wire of contaminants by holding it over the flame for several seconds. Hold the wire with some kind of holder as the entire length of wire will quickly become hot in the flame.
2. Check that the test is working by first testing the control samples
3. Sample the material by touching it with the hot wire.
4. Place wire with the sample at the air intake of the torch (located at the base of the nozzle). Observe the color that appears in the flame. Note: If no color is observed using the air intake when testing the positive reference, place the wire with the sample directly in the flame and observe the color.

Interpretation

A strong green color in the flame usually indicates the presence of halogens (chlorine, bromine, iodine, but not fluorine). The flame will burn green for a long period of time if chlorinated polymers such as PVC are present. Impurities, such as fingerprints, or surface treatments that contain chlorides may give a weak green flame that disappears quickly.

Flame test showing a positive result for PVC (green flame)

Flame test showing a negative result for polyester (yellow flame)

References

• Odegaard, N., S. Carroll, and W. Zimmt. 2005. Material characterization tests for objects of art and archaeology. London: Archetype Publications.
  
• Williams S. 1989. The Beilstein test: A simple test to screen organic and polymeric materials for the presence of chlorine. CCI Notes No. 17/1. Ottawa: Canadian Conservation Institute.
  
• The Beilstein Test: Screening Organic and Polymeric Materials for the Presence of Chlorine, with Examples of Products Tested – Canadian Conservation Institute (CCI) Notes 17/1

Round Robin testing of materials by Testing & Standards group

The goal of this work conducted by members of AIC's Materials Working Group starting in 2018 was to test a series of materials at several museums to see if a consistent result could be achieved everywhere. The reason for the goal is to make sure every workplace can follow the same guidelines and obtain the same result.

Materials tested in the 2019 and 2020 Round Robin

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