Spot Testing: Faded Memories, Priceless Possessions
A Study in Paper Performance
I. Purpose: To compare the permanence of papers by testing pH, determining
presence of alkaline reserve as well as fold endurance, and testing
To indicate the strength by using a starch test as a comparison.
II. Background: On October 12, 1990 the permanent paper joint resolution was signed
into law. It recommends that federal, state, and local government agencies
require the use of acid-free permanent papers for publications of
enduring value and that all publishers note the use of acid-free permanent
paper in printed materials. To be considered acid-free the pH must be 7.5 or
Cultural institutions generally have laboratories equipped and staffed to
test the large amounts of paper for permanence. The smaller customer
often resorts to spot tests, which are indicators rather than accurate measurements. These spot tests include pH, alkaline reserve, lignin and
pH is the only property in the American National Standards Institute / National Information Standards Organization standard that can be easily tested by untrained people. Fortunately it is one of the biggest factors in permanence.Neutrality is pH 7 but paper of pH 6 and above is designated acid-free. A paper with a pH of 8 to 8.5 is referred to as permanent paper. Newsprint, by comparison, is pH4.5 when new, and pH 3.5 when degraded. PH is indicated by either indicator solutions or test papers. For the novice a pH pen is the best choice. A pH pen marketed by Abbey Publications contains the indicator, chlorophenol red. For dry paper testing it shows purple above a pH of 6.7 or 6.8 and yellow for pH below 5.5 0r 5.6, immediately after it dries. Between pH 6.0 and 6.8 it shows an intermediate color between yellow and purple. The reliable color change regardless of the solution concentration produces accurate results. A second pen available from Light Impressions appears to contain a modified bromocresol green and changes from a bright yellow to a bright blue. At pH 4 it begins to change from yellow to green is very definite green at 6, is bluish at 7 and above this a definite blue.
Alkaline paper is required to contain 2% by weight of calcium carbonate. Average paper contains at least 2% and occasionally contains as much as 30%. This filler keeps the pH from declining with age. Eventually alkaline papers will become acidic without an alkaline reserve. A titration procedure using methyl red, 0.10N solutions of HCl and NaOH can be used to determine the amount of alkaline reserve. The mere presence of carbonate can be detected by adding a few drops of acid and observing carefully for gas production (bubbling). The gas is carbon dioxide.
The presence of lignin causes paper to become brittle and to discolor on exposure to heat and light. In their natural state wood cellulose fibers are surrounded by lignin. The lignin is removed in the pulping process. As the cellulose fibers is purified the paper becomes "finer". Newsprint is made with pulp containing lignin. Permanence standards do not permit more than 1% lignin because as the paper darkens it is more expensive to microfilm or digitize. Also, important historical documents cannot be exhibited for long periods of time. Too, works of art on paper and paper collectibles lose significant market value if their appearance changes. In the laboratory phloroglucinol solution detects lignified fibers in amounts as low as 5%. Above that it changes from pink to red to brilliant magenta for newsprint and groundwood. Due to the precautions required in using the solution (corrosive and light sensitive) a better method is needed for the novice. University Products is now marketing a lignin-indicating pen, which makes a mark that turns bright orange when the paper contains 0.5% lignin according to the catalog for year 2000. The paper permanence standard specifies a maximum lignin content of 1%; therefore a positive test of 0.5% should not result in an outright rejection of the paper as permanent.
A folding endurance test can be performed to determine the degree of brittleness. A page corner is folded back and forth with the finger until it breaks. Two or four double folds are often used as the criterion for brittle.
A necessary quality in many paper products is strength. One way to increase dry strength is to add starch. To indicate the presence of starch a drop of iodine solution is applied. By visually comparing the degree of darkness, the starch content can be compared. The darker the stain the greater is the amount of starch.
III. Materials: numbered paper samples
0.10 M HCl
lignin indicating pens
IV. Procedure: A. pH test
With the pH pen make a thin line on each sample. Record the color and indicate if the sample is acidic, neutral, or basic.
B. Presence of alkaline buffer (carbonate)
Add a drop or two of 0.10 M HCl to each paper sample. Observe carefully for production of gas (bubbling). You may use a magnifier.
C. Lignin test
With the lignin-indicating pen make a mark on each paper sample. Record the color and indicate the amount of lignin present.
D. Folding endurance
Fold the corner back and forth with the fingers until it breaks. Back and forth is a double fold.
E. Starch content
Place a drop of the iodine test solution on each sample. Note any color changes.
Rank the tested samples in order of increasing starch content.
V. Data: Sample 1 2 3 4 5
A, B, or N
Addition of HCl
# of double folds
Intensity of iodine test
VI. Questions: 1. Which test most accurately predicts permanence? Explain.
2. Were there any samples in boxes marked "acid-free" or alkaline (permanent)?
3. Which paper provides the greatest brightness, whiteness and opacity?
4. Which papers will provide better ink holdout and printability?
5. Rank the samples in order of increasing permanence. Explain your reasoning for your decision.
VII. Hazards: Hydrochloric acid is corrosive and should be handled carefully. Wear goggles and aprons. In case of a spill, flush thoroughly with water. Iodine causes stains so avoid contact with the skin. The indicators in the pens may stain the hands so should be washed with water before they dry.
This activity was produced for Charleston County Public Library by Norma L. Ashburn, retired chemistry teacher (Hanahan High School, Berkeley County, South Carolina). It is intended for use in high school chemistry classes and is especially relevant to "Chemistry and Art", the theme of National Chemistry Week, 2001.