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Subject: Stock solution for preparation of diphenylamine reagent for cellulose nitrate identification

Stock solution for preparation of diphenylamine reagent for cellulose nitrate identification

From: Scott Williams <scott_williams<-a>
Date: Tuesday, December 2, 2008
Convenient stock solution for preparation of diphenylamine reagent
for cellulose nitrate identification

A ready-made stock solution of 1% Diphenylamine (DPA) in
concentrated Sulfuric Acid is available in Canada from Caledon
Laboratory Chemicals and in the US from LabChem Inc in 125 mL
(CAN$30.64, US$25.10) and 500 mL (CAN$54.81, US$44.90) bottles.
Although the acid concentration is too high to use this stock
solution directly as received, dilution of four volumes of the stock
solution with one volume of water produces a test solution of 0.8%
DPA in 80% sulfuric acid which is suitable for detecting cellulose
nitrate in objects and photographic materials. This is a much more
convenient and less expensive method of preparing the reagent than
currently recommended procedures using solid DPA (US$25.10/g from
Chem Services) and concentrated sulfuric acid (US$39-29-500mL from
Fisher Scientific) reagent chemicals.

Recommended reagents vary in concentrations from 5% DPA in 100%
sulfuric acid to 0.5% DPA in 90% sulfuric acid in the conservation
literature, to 0.02% DPA in 80% sulfuric acid in the analytical
chemistry literature.  In 1994 I recommended 0.5% DPA in 90%
sulfuric in CCI Note 17/2 after testing various concentrations found
in the literature.  After publication of this note I started to
receive queries about ambiguous results for tests on photographic
materials.  Instead of the distinct and characteristic deep blue
colour of a positive test, dark brown and brown-black, or faint
green colours were being obtained. This test solution seems to be
unreliable for testing photographic materials.

While experimenting to find a suitable dilution of this commercial
stock solution of DPA, I tested cellulose nitrate and cellulose
acetate photograph films as well as cellulose nitrate objects, and
got the following results.

    1.  Test results for both objects and photographs are
        independent of DPA concentration between the limits of 0.2
        and 5% DPA.  The same deep blue colour was always obtained
        if cellulose nitrate was present.

    2.  Test reagents with sulfuric acid concentrations between 70
        and 90% gave the same blue colour reaction for objects.

    3.  Test reagents with sulfuric acid concentrations of 90% or
        greater gave ambiguous results for photographic materials,
        commonly showing brown or black colours which mask the blue
        colour of the positive test.  The brown and black colours
        seemed to be more common with more highly degraded material.

    4.  Test reagents with 70 to 80% sulfuric acid gave unambiguous
        blue coloured positive tests for cellulose nitrate
        photographic materials.

    5.  For concentrations of sulfuric acid less that 70% the test
        gives no colour or very slow colour development.

    6.  Some cellulose acetate film bases gave faint blue-green or
        greenish colour reactions.  This is a negative result
        indicating that the material is not made of cellulose
        nitrate.  Only deep blue indicates a positive test for
        cellulose nitrate.

    7.  Thin cellulose nitrate subbing layers in photographic
        materials give the deep blue colour, but because this colour
        is localized on the thin subbing layer only, it may be
        difficult to see.  For photographic materials this indicates
        that the film base is not cellulose nitrate, otherwise there
        would be a much stronger reaction.

Sulfuric acid is a dehydrating agent. It will absorb moisture from
air. It will extract water from organic compounds and cause
charring, with a concomitant development of brown and black charred
material (remember the sugar-sulfuric acid volcano we made as kids).
It seems that at sulfuric acid concentration above 80% the cellulose
nitrate and cellulose acetate photographic materials (especially
when degraded) are more susceptible than objects to dehydration and
charring to produce brown and black discoloured material that masks
the blue colour reaction. Perhaps the gelatin emulsion reacts with
the sulfuric acid. Dehydration, charring and brown or black colours
do not occur at sulfuric acid concentrations less than 80%.

The test depends on the release of nitrate ions from the plastic by
hydrolysis (not dehydration) in sulfuric acid solution.  At sulfuric
acid concentrations below 70% nitrate ion production by the
hydrolysis reaction appears to be too slow to give a positive blue
colour reaction within a few seconds as required for a positive

Recommended Test Reagent:

On the basis of these results and the availability of a prepared
stock solution of 1% DPA in concentrated sulfuric acid, the test
reagent for detection of cellulose nitrate that I now recommend is
0.8% DPA in 80% sulfuric acid.  This reagent is prepared from the
stock solution as follows.

Add 125 mL of stock solution to 31 mL water in a large container
with stirring. Since only one drop is used per test, this will
provide sufficient test reagent for hundreds of tests. Or, add 500
mL of stock solution to 125 mL water in a large container with
stirring. This will provide sufficient reagent for a gazillion

Always add acid to water.  Addition of sulfuric acid to water will
generate a lot of heat so stirring is required to prevent localized
boiling and spattering.  Store the prepared reagent in the original
stock solution bottle.  Polyethylene dropper bottles provide a
convenient method of dispensing a single drop of reagent to samples
in wells of spot plates or on microscope slides.  Do not store in
bottles with cap liners or bulbs made of rubber, paper, or metal.

Suppliers of stock solution of Diphenylamine, 1% in Sulfuric Acid,
containing: Diphenylamine (CAS 122-39-4) - 1%, Sulfuric acid (CAS
7664-93-9) - 96% w/w Water (CAS 7732-18-5) - balance:

    Caledon Laboratory Chemicals
    40 Armstrong Ave, Georgetown, Ontario, L7G 4R9, Canada
    877-225-3366 (1 877 CALEDON)
    Fax: 905-877-6666
    sales<-a t->caledonlabs< . >com
        Cat. No. LC13650-7  (125 mL)        $30.64
        Cat. No. LC13650-1 (500 mL)         $54.81

    LabChem Inc.
    200 William Pitt Way, Pittsburgh, PA 15238
    Fax: 412-826-5234
        Cat. No. LC13650-AG (125 mL)  USD25.10
        Cat. No. LC13650-51002 (500 mL)     USD44.90

R. Scott Williams
Senior Conservation Scientist (Chemist)
Conservation Research Division
Canadian Conservation Institute
1030 Innes Road
Ottawa, Ontario K1A 0M5
Fax: 613-998-4721

                  Conservation DistList Instance 22:35
                 Distributed: Monday, December 8, 2008
                       Message Id: cdl-22-35-004
Received on Tuesday, 2 December, 2008

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