NBSIR 75-915

A Source of Error in Paper

Extract pH Determinations:

Contact Between Paper and

Reference Electrodes

Edwin J. Parks and R. L. Hebert*

Polymer Stability and Reactivity Section

Polymers Division

Institute for Materials Research

*TAPPI Research Associate

January 31, 1976

Final

Prepared for National Archives and Records Service

Washington, D. C. 20408

NBSIR 75-915

A SOURCE OF ERROR IN PAPER

EXTRACT pH DETERMINATIONS:

CONTACT BETWEEN PAPER AND

REFERENCE ELECTRODES

Edwin J. Parks and R. L. Hebert*

Polymer Stability and Reactivity Section

Polymers Division

Institute for Materials Research

*TAPPI Research Associate

January 31, 1976

Final

Prepared for National Archives and Records Service

Washington, D. C. 20408

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Contents

Page

1. Summary 1

2. Introduction 2

3. Experimental 3

4. Results and Discussion 4

5. Suspension Effect as a Source of Error 6

6. Short-circuiting as a Source of Error 7

7. Conclusions 8

Bibliography 9

1. SUMMARY

If the reference electrode of a pH meter is permxtted to

contact paper suspended in an aqueous medium, the pH observed

is lower than that observed with no contact. The effect has

been observed with papers of widely different pH, and varies

in magnitude with the proximity of contact and with different

papers. The problem can be avoided by taking measurements on

decanted equilibrium solutions instead of suspensions of paper.

The cause of this effect may be analogous to the suspension

effect in colloidal systems.
2

2. INTRODUCTION

The determination of paper acidity (pH) by standard methods

(1,2) involves a fairly simple measurement of the pH of aqueous

extracts, but results obtained by different operators vary widely

(3). The work described in this report demonstrates that

different pH values can be obtained on a single sample of paper

by varying the proximity of the reference electrode to the paper

in aqueous suspension and the effect of proximity can be a

major source of error in pH tests. The error can be avoided

simply by decanting the water after extraction and making pH

measurements on the decantate. The phenomenon seems to be

analogous to the well-known suspension effect of colloidal

systems (4-7)

.
3

3. EXPERIMENTAL

All samples were prepared for testing according to TAPPI

methods for cold or hot extraction pH (1,2).

The data in Table 1 were obtained on samples of paper made

from a kraft pulp that had been deashed with 0 . IN hydrochloric

acid and washed until free of HCl. The specimens were aged in

flowing air for six days at 90°C ± 0.1* and 0%, 5%, 20%, or 50%

relative humidity. The data in Tables 2 and 3 were obtained on

commercial book papers that had been used in interlaboratory

testing programs (3). Table 4 represents data on the same

samples as Table 1, in addition to an imaged specimen of the

same paper.

The pH electrodes were positioned as follows:

(a) distance between electrodes and paper was visibly

substantial, about 1 cm,

(b) close proximity, with light brushing contact,

was permitted,

(c) the reference electrode was in close contact with . the paper surface,

(d) measurements were made on decanted equilibrium

solutions

.

A research meter equipped with glass and calomel electrodes

was employed for making the pH measurements.
4

4. RESULTS AND DISCUSSION

Table 1 shows wide variations in pH values obtained by varying

the distance between the pH electrodes and the paper. Standard

cold extraction pH tests are made with paper cut into small pieces

and macerated with a glass rod. Opaque suspensions can result if

the paper is extensively disintegrated by this maceration, while

filler can make the resulting suspensions very opaque. It is

difficult to prevent close proximity between electrodes and fibers

when the suspensions are very opaque, so that conditions a, b,

or c might occur during a pH measurement. Substantially lower

pH values result when the reference electrode is brought into

close proximity with the paper. It was possible to control the

proximity of the electrodes to the paper in this study because

the aqueous extracts were relatively clear. Nevertheless, a great deal of care had to be taken to avoid errors as a result

of close proximity to the paper.

The data in Table 2 pertain to commercial book papers. These

samples show smaller variations in pH than those found in Table 1,

but again the pH readings are affected by the proximity of the

electrodes to the paper, except for the alkaline paper. The

magnitude of variation differs from paper to paper and indicates

that the effect is real and not due to instrument error.

The pH data obtained in interlaboratory testing programs

indicate that errors of the type due to close proximity or contact

may occur quite frequently. For example, sample 1 in Table 2 has

been subjected to analysis in a collaborative interlaboratory

testing program involving eleven different laboratories (3), and

the results are summarized in Table 3. The conclusion in reference

3 was that the most reliable pH values for sample 1 were 5.05

(cold extraction) and 4.50 (hot extraction). These pH estimates

resulted from a careful evaluation of data obtained in the

testing program over an extended period of time. The data contain

several points of interest.

(a) There are large differences between laboratory

means. The range of reported values for cold extraction pH is

4.51 to 5.66. That for hot extraction is 3.9 7 to 5.16.

(b) The most reliable pH values are very close to those

listed for the respective decantates in Tables 2 and 3 indicating

that decantates gave very good values.

(c) The two lowest values in Table 3 for both cold and

hot extraction pH suggest the analyst permitted the pH electrodes

to come into contact with the paper.
5

There are other unexplained sources of error reflected in

interlaboratory tests as well as those apparently due to the

closeness of the electrodes to the paper. Laboratories B and

H, for example, report a high pH value of 5.66 for sample 1

after cold extraction, where the most reliable value is 5.05,

suggesting incomplete extraction of the samples. The pH

procedure described in this report would not prevent errors of

this type, but high results would indicate procedural or

instrument errors.
5. SUSPENSION EFFECT AS A SOURCE OF ERROR

In colloidal systems, the so-called suspension effect is

the difference in e.m.f. obtained in the conventional determination

of pH in a suspension and in its equilibrium solution (4).

Theoretical (4) and more qualitative discussions (5,6,7) of the

pH of colloidal systems show that the position of the reversible

electrode (glass or hydrogen) is without influence on the measured

potential, but the position of the reference electrode is important.

Contact with the suspension results in greater potential readings

because of a greater charge density at particle surfaces than in

equilibrium solutions.

The similarity in the response of pH electrodes to paper

suspensions suggests that charges on the paper surface similarly

effect paper pH tests. These charges may be closely related to

the zeta potential of pulp fibers. Current standard methods do

not mention possible errors due to contact and analysts may not

be aware of the need to avoid it, but it is likely that a substantial improvement in pH determinations could be made by

testing decanted equilibrium solutions rather than suspensions of

paper.
6. SHORT-CIRCUITING AS A SOURCE OF ERROR

An alternative explanation of the contact phenomenon was

considered. A continuous matrix of fibers between the electrodes

might cause shorting which would result in unpredictable electrical

disturbances. This explanation was discounted when pH measurements

were made with electrodes in separate beakers connected by a salt

bridge. One beaker contained both paper and equilibrium solution;

the other, only equilibrium solution. The data in Table 4

resulted from varying the contact of the reference electrode with

the paper. These results indicate that shorting is not the cause

for the great variations in pH values caused by contact of the

electrodes with the paper.
8

7. CONCLUSIONS

1. The pH values obtained in the determination of the

acidity (pH) of paper extracts are highly dependent on the

proximity of the reference electrode to the test sample.

2. Contact between the reference electrode and charged

surfaces of paper may result in an increase in the measured

potential.

3. Error from this source can be minimized by making pH

measurements on the decanted equilibrium solutions.
9

BIBLIOGRAPHY

1. ASTM Designation D778-50, "Standard Method of Test for

Hydrogen Ion Concentration (pH) of Buffered Paper Extracts"

19 75 Annual Book of ASTM Standards, Part 20.

2. T-435 SU-68, "Hydrogen Ion Concentration (pH) of Paper

Extracts Hot Extraction Method" , TAPPI Standards and

Suggested Methods, Technical Association of the Pulp and

Paper Industry.

3. Report No. 30A, "Collaborative Reference Program for Paper"

,

E. G. Randall, Jr., J. Horlick, S. Prather, R. G. Powell,

and R. L. Hebert. U.S. Department of Commerce, National

Bureau of Standards, August 1974.

4. Overbeek, J. Th. G. , J. Colloid Sci. 8, 593 (1953).

5. Kruyt, H. R. , editor. Colloid Sci., Vol. 1, page 184,

Elsevier, New York (1952)

.

6. Eriksson, E. , Science 113 , 418 (1951).

7. H. Jenny, T. R. Nielsen, N. T. Coleman and D. E. Williams,

Science 112, 164 (1950)

.

USCOMM-NBS-OC
10

TABLE 1

EFFECT OF PROXIMITY ON OBSERVED pH OF AQUEOUS EXTRACT OF HANnSHFFTS

AFTER 6 DAYS OF ACCELERATED AGING AT 90*C AND VARIOUS RELATIVE HUMIDITIES

Relative Huraiditiea

of Aging Atmosphere

No

Contact Decantate

Close

Proximity Contact

0% A. 90 A. 90 A.A8 3.76

5% A. 86 A. 86 A. 50 A. 13

20%

' A. A3 A.AA 3.98 3.76

50% A. 17 A. 18 A. 16 3.56
11

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TABLE 4

COMPARISON OF pH VALUES USING

GLASS AND REFERENCE ELECTRODES IN (a) A SINGLE BEAKER

AND (b) SEPARATE BEAKERS CONNECTED BY A BRIDGE^

Observed pH

2 PTOxitnl 1. \j ^ ill ^ tv^ fa") X W W U \Z CL IV C-I-O tJlJJLLiCU. of Aging Atmosphere Condition Single Beaker by a Bridge

0% No Contact 4.70 4.85

Close Proximity 4.34 4.54

Cnn t" p t" 3.98 4.11

Decantate 4.65 4.80

5% No Contact 4.80 4.50

Close Proximity 4.34 4.28

Contact 4.12 4.01

Decantate 4.77 4.95

20% No Contact 4.53 4.46

Close Proximity 4. 38 4.24

Contact 4.00 4.02

Decantate 4.54 4.34

50% No Contact 4.35 4.52

Close Proximity 4.06 4.32

Contact 4.03 3.98

Decantate 4.28 4.42

Unaged Paper No Contact 5.30 5.42

Close Proximity 5.10 5.12

Contact 4.30 4.55

Decantate 5.28 5.46

The papers are identical to those in Table 1.

I Proximity conditions refer to the reference electrode.
14

ACKNOWLEDGMENTS

The authors wish to thank Dr. John Mandel for assistance

in statistical analysis and Mr. Edwin B. Randall for providing

the samples of book papers used to obtain the pH data listed

in Table 2.
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

D 1 D L 1 Uw K A r n 1 ^ U A 1 A SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 75-915

2. Gov't Accession

INO. 3. Recipient's Accession No.

4. TITLE AND SUBTITLE

A Source of Error in Paper Extract pH

Determinations: Contact between Paper and Reference

Electrodes

5. Publication Date January 31, 19 76

6. Performing Organization Code

7. AUTHOR(S)

E. J. Parks and R. L Hebert

8. Per for ming Organ . Report No. NBSIR 75-915

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Projcct/Task/Work Unit No.

11. Contract/Cirant No.

12. Sponsoring Organization Name and Complete Address (Street, City, Slate, ZIP)

National Archives and Records- Service

Washington, D. C. 20408

13. Type of Report & Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.)

If the reference electrode of a pH meter is permitted to contact

paper suspended in an aqueous medium, the pH observed is lower than that

observed with no contact. The effect has been observed with papers of

widely different pH, and varies in magnitude with the proximity of . contact and with different papers. The problem can be avoided by

taking measurements on decanted equilibrium solutions instead of

suspensions of paper.

The cause of this effect may be analogous to the suspension effect

in colloidal systems.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Acidity; contact, decantate; electrode; error; extract; paper; pH;

suspension

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