This document is a translation of "La mesure de l'acidité des produits volatils", originally published in the Journal of IIC-CG, vol. 17, pp17-25, 1992

Introduction

There are several cabinets, or carrying cases. These include wood, paint, plastics, adhesives, textiles, and many more. Some can release volatile products that are harmful to museum artifacts. Acetic acid and formic acid are among the most common products released. It is therefore important to detect acidic volatile products given off by a particular material or present in a given space.

To better illustrate the variety of materials that can release acidic volatile products, a list of products that release varying amounts of organic acids is given in Table 1. However, the release rates from various sources, as given in the documentation, do not always agree with the actual emission rates of vapours because they are often obtained under extreme conditions. For example, when a material is heated to induce the release of volatile products, this activity may itself introduce thermal degradation processes that do not normally occur.

The authors of the articles cited as references to this paper generally provide qualitative results. Quantitative results do exist for species of wood; however, the data on wood vary according to the sampling and the analysis method used.²⁰ Materials able to release aldehydes are included in Table 1 because formaldehyde in the presence of water converts to formic acid by the Cannizzaro reaction.²¹

Formic acid formed from formaldehyde is much more dangerous than formaldehyde itself. The transformation of acetaldehyde into acetic acid is also possible through oxidation in the presence of a metal surface.²² The organic acids of higher molecular weight (propionic acid, butyric acid, etc.) are also harmful, but to a lesser degree.

A number of artifacts are sensitive to organic acid vapours. Calcium-based artifacts, such as eggshells and seashells, can react with organic acids to form crystals of hydrated calcium formate or acetate. This type of damage is known as "Byne's disease".²³ Metals are also sensitive to organic acids. Much study has been done in the wood industry and in the conservation field on the corrosive effect of vapours released by various wood species and by modified wood materials on certain metals and their alloys.^{3,5,9,10,13,24,25,26}

Stulik and Grzywacz found corrosion of lead, bronze, brass, zinc, copper, and silver after 100 days in the presence of 1,200 ppb of formaldehyde (50% RH and 20°C ± 3°C).²⁷ Hatchfield and Carpenter⁹ showed the effect of formaldehyde on certain pigments and papers. Colour changes were observed, although the authors do not always state whether the discolourations were directly caused by formaldehyde or by formic acid formed by the Cannizzaro reaction.

Several detection methods have been developed to help in selecting the material that best meets conservation standards. One of the best-known tests is undoubtedly the Oddy test.^28,29 The Oddy test consists of putting the suspicious material, pieces of metal, and a humidity source into a hermetic container, heating the container for a few weeks, and noting whether the vapours released by the sample corrode the metals. This test does not explicitly indicate the nature of the volatile products. Hopwood³⁰ uses a pH indicator contai-ning an alkaline reserve rather than pieces of metal to detect the presence of acid gases (organic or inorganic); to some extent, he quantifies the concentration of acids in an enclosed space that is heated and humid. These two tests alter the ambient conditions to increase the emission rate of volatile products and thus to reduce the response time.

The rapid detection tubes made by Dräger³¹ can detect the presence of several types of gases, such as organic acids, hydrochloric acid, and formaldehyde. With this technique, the air quality of an environment can be determined in a few minutes by using a manual or automatic pump to force air into a tube containing a specific indicator. The air quality of a closed volume or the products released by a material in a container can be determined in a few hours using Dräger diffusion tubes. These techniques can quantify gas concentrations with a detection limit of the order of one ppm. The detection tubes have a lifetime of around two years.

Semi-quantitative measurements of the acidic volatile products released by a material or present in a confined space can also be made using a pH indicator impregnated with a glycerin-water mixture. This method is simple and inexpensive because it requires only pH indicators and glycerin, two products that are often already present in a conservation laboratory. Calibration curves were plotted for better interpretation of the results and a guide was established for determining deterioration potential.

Principle of measuring acidity using a pH indicator impregnated with glycerin

Acidity measurements using the logarithmic pH scale are normally done in an aqueous environment; the technique must be modified to measure vapours. Glycerin (or glycerol), a hygroscopic substance, forms a link between the pH indicator and the surrounding atmosphere. If a glycerin solution is applied to a pH indicator paper, the vapours present in the ambient atmosphere will be absorbed. The volatile compounds accumulate on the impregnated indicator until equilibrium is reached between the environment and the solution. The colour of the pH indicator changes with the amount of acid absorbed by the solution. This method is not intended to measure the acidity of a material, but the acidity of the emissions from it.

Procedure for measuring acidity using a pH indicator

The method of using a pH indicator with a glycerin-water mixture requires that certain preparations be made.

Preparing the glycerin-water mixture: Depending on the proportions of glycerin and water in the mixture, a specific relative humidity will be generated in a closed space. If only distilled water is applied to the indicator, the water will evaporate after a while and the pH indicator will no longer function. If pure glycerin is used on the pH indicator, a glycerin-water droplet will form on the surface of the indicator as the glycerin absorbs water vapour. This droplet could distort the results. Therefore, a glycerin-water mixture is necessary. According to Figure 1, a ratio of 20 mL of water to 80 g of glycerin is required to obtain a stable relative humidity of 50%.^32,33 This level of relative humidity was chosen because it is the value generally advocated for Canadian museums. It is recommended that the glycerin be weighed because this substance is relatively viscous. The glycerin-water solution must be kept in a well-sealed container, which will prevent the air from acidifying it or from altering the constituent proportions.

Figure 1. Relative humidity generated by a glycerin-water mixture32,33

Preparing the indicator: Using pH indicator paper that is graduated over a limited range allows for greater precision in the readings. An indicator that ranges from pH 4 to 7 is the most practical. Apply one or two drops of the glycerin-water mixture to the pH indicator. Then wait until the paper absorbs the liquid completely (about 15 seconds), and remove the excess by drawing it off with a pipette or by shaking the indicator gently. The pH indicator is now impregnated and ready to absorb the vapours present in the environment. If the colour of the pH indicator upon application corresponds to a pH lower than 6, the glycerin-water solution is contaminated and must be prepared again. It is recommended that two or three indicators be prepared for better reproducibility.

Evaluating a material: Place the material to be tested and the impregnated indicators in an inert container (with no cardboard or adhesive residue under the cover) that is dry and completely airtight. A container of 500 mL or smaller is recommended; the size of the container must be proportional to the dimensions of the sample. Ensure that the moistened indicators do not touch the walls of the container or the sample.

Evaluating a space: To measure the acidity of the volatile products present in a defined space such as a room, a cabinet, or a display case, the indicators must either be placed on a clean surface or be suspended.

Control pH indicator: A control pH indicator must be used as a basis with which to compare the acidity of an environment without a sample. To evaluate a material, prepare another indicator as described above, and place it in a container without a sample. To measure the acidity of the volatile products in a display case, place indicators in the case and place others outside (far from the case). To analyse the acidity of volatile compounds in a room, place indicators in the room being tested and place others in a control room with a similar temperature.

Measuring acidity: Twenty-four hours is sufficient to reach equilibrium between what the sample releases and what the glycerin-water mixture absorbs. After the required time, read the pH value using the colour scale on the indicator case. Do the same for the control indicators. The test is designed for a short term reading to avoid errors due to interferences or fading of the pH indicator color.

Results

It is almost impossible to obtain a neutral pH because the carbon dioxide in the air acidifies the ambient atmosphere and the pH indicator; a pH below 6 is often obtained. If the indicators accompanying the sample give the same values as those given by the controls, then the sample is not releasing any volatile acidic products. A pH reading lower than that of the control indicates that the sample is releasing acids or, in the case of a room, that the space contains volatile products that are more acidic than those in the control room. In rarer cases, where the pH obtained is higher than that of the control indicator, the material is releasing volatile alkaline products or the control indicator is in a more acidic environment.

Calibration curves

The pH values obtained from indicators impregnated with a mixture of glycerin and water have little meaning because, in the literature, sensitivity to acidic vapours is expressed in terms of concentration. The pH values obtained must therefore be converted by calibration. The calibration curves are given in Figures 2 and 3. Use a mixture of acid, water, and magnesium nitrate hexahydrate salt to obtain vapours of given concentrations. In this mixture, the acid solution generates acidic vapours at a given concentration and the salt maintains a stable relative humidity of 54%.³⁴ The pH indicators used for the calibration are from ColorpHast (BDH and Aldrich use the same indicator) (Figure 2) and Macherey-Nagel (Figure 3). The calibration curves provide different sensitivities and linear domains because the pH indicators, which are normally designed for aqueous solutions, behave differently in an environment where air and glycerin abound.

Figure 2. Correlation between readings of ColorpHast indicators impregnated with a glycerin-water mixture and concentration of acetic acid in the gas phase.	Figure 3.Correlation between readings of Macherey-Nagel pH indicators impregnated with a glycerin-water mixture and concentration of acetic acid in the gas phase.

The quantification of low amounts of acid is not very reliable because of the presence of physicochemical interference. Accordingly, a pH value higher than 4.7 or 5.5 for the Macherey-Nagel and ColorpHast indicators respectively indicates only the presence and not the amount of acidic vapours released by a sample or present in a given space. At high concentration, the quantification of acids is limited because of signal drift. This deviation from linearity is especially pronounced with the Macherey-Nagel indicators.

This test does not identify the nature of the acid emitted by the sample or present in the environment. The presence of inorganic acids, such as hydrochloric acid, can distort the quantification. Because these are strong acids, in identical amounts they give lower pH values than those obtained with acetic acid. Formic acid gives a pH reading quite similar to that obtained with acetic acid. However, a concentration of formic acid greater than 5 ppm causes bleaching of both types of indicators studied, and this can distort the pH reading.

Empirical rule of deterioration potential

Table 2 is intended to give an idea of the pH values that can be dangerous for museum artifacts. The zone divisions were made based on the performance of the ColorpHast pH indicator. At present, the zone divisions are not definitive standards, but are more of a guide for the choice and use of materials. In order to correctly define the pH zones, the deterioration rate for each type of artifact under specific ambient conditions would have to be known.

Examples of applications

In addition to measuring the acidity of volatile products released by a sample being tested or by materials in an environment, the test can be used to monitor changes in the emission of volatile products. The following are some examples of applications using the ColorpHast pH indicator.

Figure 4. Acidity released by silicone sealants as a function of time. Alkaline type: Silicone II, SE2122 from GE Silicone Acidic type: General purpose silicone from Dow Corning

Changes in acidic vapours released by silicone sealants

The use of a pH indicator with the glycerin solution allows changes to be observed in acidic emissions from various materials, such as the acidic type of silicone sealant often used in the construction of display cases. When it is applied, this sealant releases a large amount of acidic vapour as it solidifies. When it is used in a well-ventilated room, the product does not seem to release acidic vapours after one day, judging from the smell. Figure 4 shows the acidity generated by one gram of silicone in a volume of 50 mL. The samples were dried in ambient air at 23°C for periods of between one and 34 days, and then were kept in bottles for 24 hours before the pH reading was taken. The data indicate that a period of at least 25 days is required to obtain "relatively inert" silicone. After that, the silicone continues to release a small amount of acetic acid. This example shows that a minimum waiting period of one month is warranted before acid-sensitive artifacts are displayed in a case that has been sealed with this kind of silicone. However, there is an alternative that will solve this problem: alkaline-type silicone sealant (sometimes referred to as neutral or odourless). It releases methanol and ammonia. The waiting period can be shorter for the alkaline type of sealant because the amount of vapour released reaches a more acceptable level after 10 days.

Figure 5. Acidity released by wood samples coated with shellac (drying time: 14 days)

Evaluating the effectiveness of shellac as a vapour barrier

Coatings used to prevent the emission of organic acids from wood achieve varying degrees of success. The quality of the vapour barrier depends primarily on the type of coating, the number of applications, and the nature of the wood.

Shellac is often used to varnish wood surfaces or as a primer coat to cover knots in wood. Shellac does not release acidic vapours, but is a poor vapour barrier. As Figure5 shows, several coats of shellac are required to prevent the release of organic acids from oak that has been aged for five years and from 12-year-old mahogany. In view of these results, the choice of wood type remains an important factor. The least acidic wood should be used²⁰ or a better vapour barrier should be chosen.

Recommendations for the use of paints in museums

When paints containing oil and alkyd resins are applied, they release a large amount of volatile products, including peroxide, formaldehyde, and organic acids. This amount decreases as the paint film cures. The same phenomenon occurs for emulsion paints with an acrylic resin base or an acrylic-vinyl acetate copolymer. However, the amount of volatile products is smaller than that released by oil and alkyd resin paints. This is why the use of emulsion paints is preferable.

Regardless of the kind of coating, it is important to know the waiting period necessary to reach the minimum threshold of volatile product emissions to avoid deterioration. This waiting period depends mainly on the nature of the coating, the area of the painted surface, and the space in which the object is found. As determined by these parameters, the waiting period can be more than a month. This may sometimes be an unrealistic length of time for conservators and display designers, who often face a deadline of one or two weeks. The pH indicator impregnated with a glycerin-water mixture can help to solve the difficult problem of how long to wait before a paint is relatively inert and usable without danger to works that are sensitive to acids. A safe space can be obtained in a relatively short time in large, ventilated rooms, but patience is required to wait for a closed space without ventilation to reach a pH between 5 and 7.

Conclusion

The ideal solution for avoiding the presence of volatile products is to eliminate the problem at its source, an approach that is often difficult to maintain. To minimize damage caused by organic acids, it is valuable to be able to detect such acids. Of course, expensive advanced technology does exist for accurately detecting harmful gases, but is not accessable to most museum professionals. The pH indicator impregnated with a glycerin-water mixture determines actual acid concentrations of a given space and allows an evaluation of conditions without any extrapolation. Museum workers can thus determine, quickly and inexpensively, whether a material is compatible with artifacts that are sensitive to acidic vapour, and can decide whether a space is ready to receive museum artifacts.

Nonetheless, knowledge about the deterioration of art works by volatile organic acids still remains limited. It seems that the development of detection methods has advanced beyond our understanding of the real impact of these acids on artifacts.

Materials

Glycerin (99.5% solution): Fisher Scientific, 112 Colonnade Road, Nepean, Ontario, Canada K2E 7L6.

pH indicator (graduated from 4.0 to 7.0 and from 2.5 to 4.5):

• ColorpHast, EM Science, P.O. Box 70, 480 Democrat Road, Gibbstown, NJ 08027, USA.
• BDH Inc., 162 Barr Street, Ville St-Laurent, Quebec, Canada H4T 1Y4
• Aldrich Chemical Co., 1500 Stanley Street, Room 405, Montreal, Quebec, Canada H3A 1R3.

pH indicator (graduated from 3.6 to 6.1 and from 3.8 to 1.7):

• Macherey-Nagel, Aldert Chemicals Ltd, Suite 4, 4889A Dundas Street West, Islington, Ontario, Canada M9A 1B3.

Silicone sealants:

• General purpose silicone from Dow Corning, marketed by Lepage, Bramalea, Ontario L6T 2J4
• Silicone II, SE2122, from GE Silicone, 23000 Meadowvale Blvd, Mississauga, Ontario, Canada L5N 5P9.

Shellac (orange): Mohawk Finishing Products of Canada, 9290 Le Prado, St-Léonard, Quebec, Canada H1P 3B4.

References

1. Knotkova-Cermakova, D. and Vlckova, J., "Corrosive Effect of Plastics, Rubber and Wood on Metal in Confined Spaces," British Corrosion Journal, vol. 6, 1971, pp. 17 à 22.
   
   
2. Donovan, P.D. and Stringer, J., "The corrosion of metals by organic acid vapors," dans: Proceedings of the Fourth International Congress on Metallic Corrosion, éd. par N.E. Hammer (Houston, National Association of Corrosion Enginers, NACE, 1972), pp. 537 à 543.
   
   
3. Rance, V.E. and Cole, H.G., "Corrosion of Metals by Vapours from Organic Materials, A Survey," Admiralty and Ministry of Supply Inter-Service Metallurgical Research Council, Her Majesty's Stationery Office, (London, 1958), pp. 3 à 15.
   
   
4. Arni, P.C., Cochrane G.C. and Gray J.D., "The Emission of Corrosive Vapour by Wood. I. Survey of the Acid-Release Properties of Certain Freshly Felled Hardwoods and Softwoods," Journal of Applied Chemistry, vol. 15, juillet 1965, pp. 305 à 313.
   
   
5. Miles, C.E., "Wood Coating for Display and Storage Cases," Studies in conservation, vol. 31, 1986, pp. 114 à 124.
   
   
6. Blackshaw, S.M. and Daniels, V.D., "Selecting Safe Materials for Use in the Display and Storage of Antiquities," dans: Preprints, ICOM Committee for conservation, 5th Triennial Meeting, (Zagreb 1978), pp. 1 à 9.
   
   
7. Arni, P.C., Cochrane G.C. and Gray J.D., "The Emission of Corrosive Vapour by Wood. II. The Analysis of Vapours Emitted by Certain Freshly Felled Hardwoods and Softwoods by Gas Chromatography and Spectrometry," Journal of Applied Chemistry, vol. 15, octobre 1965, pp. 463 à 468.
   
   
8. Farmer, R.H., "Corrosion of metals in association with wood. Part 1. Corrosion by Acidic Vapours from Wood," Wood, août 1962, pp. 326 à 328.
   
   
9. Hatchfield, P. and Carpenter, J., Formaldehyde: How great is the danger to museum collections?, Center for Conservation and Technical Studies, Harvard University Arts Museums, (Cambridge Mass., 1987), p. 44.
   
   
10. Compton, K. G., Arnold, S.M. and Mendizza, A., "Effect of Packaging on Corrosion of Zinc Plated Equipment," Corrosion, vol. 7, 1951, pp. 365 à 372.
    
    
11. Anderson, IB, Lundqvist, G.R. and Molhave, L., "Indoor Air Pollution Due to Chipboard Used as a Construction Material," Atmospheric Environment, vol. 9, pp. 1121 à 1127.
    
    
12. Cawthorne, R., Flawell, W., Ross, N.C. and Pinchin, F.J., "Corrosion of Zinc by Vapours from Polyester Resins, II.-Prevention of Corrosion," British Corrosion, vol. 4, janvier 1969, pp. 35 à 38.
    
    
13. Ministry of Defence, "Guide for the Prevention of Corrosion of Metals Caused by Vapours from Organic Materials," Defence Standard, 03-13/Issue 1, 19 février 1977, p. 14.
    
    
14. Down, J., MacDonald, M., Tétreault, J. and Williams, R. Scott, "Adhesive Testing at the Canadian Conservation Institute - An Evaluation of Selected Poly(vinyl acetate) and Acrylic Adhesives and Approach Taken," Environment and Deterioration Report No. 1603, Institut canadien de conservation, (Ottawa, Canada, 1993).
    
    
15. Donovan, P.D. and Moynehan, T.M., "The Corrosion of Metals by Vapours from Air-Drying Paints," Corrosion Science, vol. 5, 1965, pp. 803 à 814.
    
    
16. Lewin, M. and Sello, S.B., Chemical Processing of Fibers and Fabrics, Functional Finishes Part A, Handbook of Fiber Science and Technology, Volume II, éd. Marcel Dekker Inc., (New York, 1983), pp. 367 à 426.
    
    
17. Wadden, R.A. and Scheff, P.E., Indoor Air Pollution; Characterization, Prediction and Control, John Wiley & Sons, (New York, 1983), pp. 23, 27 et 73.
    
    
18. Pilot Study on Indoor Air Quality, Environmental Protection Agency, Committee on the Challenges of Modern society, rapport n^o 183, (Erice, Italie, 1989), p. 17.
    
    
19. Godish, T., Air Quality, 2^e éd. Lewis Publishers, Inc., (Chelsea, Mi., 1990), pp. 347, 349 et 351.
    
    
20. Tétreault, J., "Matériaux de construction, matériaux de destruction," dans: La conservation préventive, Actes du 3^e Colloque international, éd. par D. Guillemard, (Paris : Association des restaurateurs d'art et d'archéologie de formation universitaire, ARAAFU, 1992), pp. 163 à 176.
    
    
21. Walker, J.F., Formaldehyde, 3^e éd., Robert E. Krieger Publishing Company, (Malabar, Fl., 1975), p. 107.
    
    
22. Wagner, F.S., "Acetic anhydride," Encyclopedia of Chemical Technology, 3^e éd., éditeurs: kirk et Othmer, John Wiley and Sons, vol. 1, (New York, 1978), p. 156.
    
    
23. Norman, H.T. and Baird, T., "The Deterioration of Mollusca Collections : Identification of Shell Efflorescence," Studies in Conservation, vol. 30, 1985, pp. 73 à 85.
    
    
24. Werner, G., "Corrosion of Metal Caused by Wood in Closed Spaces," dans: Recent Advances in the Conservation and Analysis of Artifacts, Jubilee Conservation conference, Summer School Press, éd. par J. Black, (London R.U.: University of London, 1987), pp. 185 à 187.
    
    
25. Berndt, H., "Measuring The Rate of Atmosheric Corrosion in Microclimates," Journal of Americain Institute for Conservation, vol. 29, 1990, pp. 207 à 220.
    
    
26. Clarke, S.G. and Longhurst, E.E., "The Corrosion of Metals by Acid Vapours from Wood," Journal of Applied Chemistry, vol. 11, novembre 1961, pp. 435 à 443.
    
    
27. Stulik, C. and Grzywacz, C., "Carbonyl Pollutants in the Museum Environment: An integrated Approach to the Problem," dans: La conservation préventive, Actes du 3^e Colloque international, éd. par D. Guillemard, (Paris : Association des restaurateurs d'art et d'archéologie de formation universitaire, ARAAFU, 1992), pp. 199 à 205.
    
    
28. Oddy, W.A., "An Unsuspected Danger in Display," Museums Journal, vol. 73, 1973, pp. 27 à 28.
    
    
29. Hnatiuk, K., Effects of Display Materials on Metal Artifacts, Gazette of the Canadian Museum's Association, été-automne 1981, pp. 42 à 50.
    
    
30. Hopwood, W.R., "Choosing Materials for Prolonged Proximity to Museum Objects," dans : Preprints of the 7^th American Institute for Conservation of Historic and Artistic Works, (Toronto, mai 1979), pp. 44 à 49.
    
    
31. Leichnitz, K., Detector Tube Hanbook : Air Investigations and Technical Gas Analysis with Dräger Tubes, 7^e éd. (Lübeck, 1989), 303 p.
    
    
32. Grover, D.W. and Nicol, J.M., "The Vapour Pressure of Glycerin Solution at 20C," Journal of Society of Chemical Industry, vol. 59, août 1949, pp. 175 à 177.
    
    
33. Union Carbide Corporation, "Carbowax polyethylene glycols," Union Carbide, Industrial Chemicals Division, Product Information Bulletin, (Danbury, 1986), p. 52.
    
    
34. Tétreault, J. and Sirois, J., and E. Stamatopoulou, "Study of Lead Corrosion in Acetic Acid Environment," Studies in Conservation 42 (1998), pp. 17-32
    
    

Zone of low deterioration potential
pH between 7.0 and 5.0:

(<1 ppm) When the material being tested must be used in a large room or in a ventilated location, volatile products are dispersed in the ambient air and the risks are therefore minimal. In a closed space (display case, carrying case), those artifacts that are most sensitive to acids (e.g., works on paper, lead and its alloys) may be affected if the pH value is more acidic than the control pH.

Zone of moderate deterioration potential
pH between 5.0 and 3.5:

(1 to 10 ppm) In a space that is large or that is provided with a ventilation system, the acidic gases present can be eliminated in the short or medium term. Even in the short term, however, these gases can cause deterioration of items that are sensitive to acids (e.g., shells, papers, metals, and certain textiles such as cotton). The ventilation must therefore be increased or one must wait until concentrations decrease before the room is used. The use of any material that releases this level of acidity is not recommended in a closed space. As a stopgap measure, consider reducing the relative humidity of the environment as much as possible, or use absorbent products.

Zone of high deterioration potential
pH below 3.5:

(>10 ppm) This acidic environment seriously affects many museum artifacts; in addition to those already mentioned, proteinaceous materials can be damaged. In this case, the artifacts must be removed from the area, the ventilation of the rooms must be increased, and there must be a waiting period until an acceptable acidity level is reached. In a closed space, the use of absorbent products is not sufficient. Prohibit the use of any material releasing this amount of acidity.