Abstract
This experiment is designed to illustrate the
purpose of the different components of a
photographic developer. It will also illustrate
the chemistry of the toning process. Specifically,
you will determine the role of one component in
the developer and identify the importance of
reaction duration as a variable in the toning of
black and white prints. For the first part, be
specific. Indicate which component you are
omitting. Discuss the role of that component in
your conclusion section.
Introduction
Traditional black and white photography
utilized a chemical process that has been
observed since ancient times – that of the photosensitivity of silver (Ag) compounds. This
ability was harnessed by the19th century
photography pioneers by coating silver halides
(molecules of Ag and halide gases) in an
emulsion of gelatin onto a glass or plastic
backer. The process of making traditional film
has not changed much from the basic chemical
reactions discovered in the 1800’s. Initially,
elemental silver is reacted with nitric acid to
form silver nitrate. Complete and balance the
reaction below:
Ag (s) + HNO3 → AgNO3 + H2
Identify the element oxidized and that reduced
in the above equation.
The silver nitrate is then converted to the silver
halide (usually bromide) by fuming the film
with potassium bromide. Complete and balance
the next reaction:
AgNO3 + KBr →
The AgBr is an ionic compound that forms a
crystalline structure in grains. What does it
mean to be an ionic compound? The film now is
ready for exposure and the creating of the
intended image. Stored within a light sealed
container, the AgBr film is a uniform darkish
yellow. The light exposure occurs when an
opening allows light to enter the dark container
and strike the film. These “storage” containers
that subsequently allow light exposure are more
commonly referred to as cameras.
Procedure:
In the first section of this experiment you will
determine a set of optimum conditions for
developing a contact print using a light bulb for
exposure and a standard, already-prepared
developer. Two factors are important in
determining the optimum conditions. The first
is the amount of light hitting the photographic
paper. This depends on the intensity of the
light, the length of the exposure and the
distance of the light from the paper. The
greater the amount of light hitting the paper,
the darker the print since more silver halide
grains are exposed. This assumes all other
conditions are held constant. The directions for
the first part are listed in Section 1.
In the second section of the experiment you will
prepare a developer by weighing out and
mixing the necessary chemicals. You will then
use the conditions determined in the first part
to prepare a contact print with your own
developer. Most photographic developers have
several components in common. These are:
1. a reducing agent--to reduce the exposed
silver halide grains to metallic silver,
2. a preservative--to prevent reaction of the
reducing agent with oxygen from the air,
3. an activator--to activate the reducing agent,
4. a restrainer--to increase the contrast between
light and dark areas in the print.
In your developer the chemicals will be:
1. metol, the reducing agent.
2. sodium sulfite (Na2SO3), the preservative.
3. sodium carbonate (Na2CO3•H2O), the
activator.
4. sodium bromide (NaBr), the restrainer.
Section 1: Determining the Optimum
Conditions
Steps:
1. Obtain several sheets of photographic paper.
Caution: this paper is, of course, very sensitive
to light.
2. Place approximately 100 ml each of
developer, stop bath, fixer, and distilled water
into 400 ml beakers; label each beaker.
3. Set up the exposing light as shown in the
Figure below.
4. Using only safety lights, remove a piece of
photographic paper from its storage space and
place it directly under the light with the
emulsion side up. Place the negative on top of
the paper and put a glass slide on top of the
negative.
5. Turn on the light for 30 seconds to expose the
paper. Using only the safety light, observe the
paper. Does it appear to have changed?
6. Develop the print (still under safety light) by
placing it in the beaker containing the
developer for 60 seconds (caution: use tongs),
then the stop bath for 60 seconds, the fixer for 5
minutes and the distilled water for 5 minutes.
While the paper is in the developer and stop
bath, be certain that you agitate it. Place the
print on a paper towel to dry. Record the
temperature of the developer.
7. Try to improve the print. If it is too dark,
reduce the exposure or the developing time. The
exposure can be reduced by reducing the
exposure time, raising the bulb, or reducing the
setting on the Variac. If the print is too light,
the opposite remedies should be applied. Be
certain to keep an accurate record of your
conditions for each experiment. A Table such as
shown below works well. Your goal here is to
find the conditions for the best print possible.
You will use these conditions in the subsequent
section for your developer. Put 1/2 of each print
next to the conditions in youur Results section.
Section 2: Preparing your own developer
Steps:
Prepare your own developer by dissolving 2.0 g
Na2SO3, 0.25 g sodium bromide, NaBr, 0.6 g
metol, and 2.0 g of sodium carbonate
(Na2CO3·H2O) in distilled water and dilute to
100 ml. Be sure to dissolve these chemicals in
the order listed.
Expose and develop a contact print again
according to your optimum conditions of
Section 1. Record your results. Try to improve
the print.
How does this print compare with that of
Section 1?
Prepare 8 extra prints for the toning
experiments. Make these prints as soon as
possible after your determination of the optimal
conditions.
What happens if...? What happens if you
prepare a developer but leave out one of the
ingredients? Try it and record your results.
Section 3: Toning [Note: these experiments can
be performed in regular light].
Iron Toning:
Prepare an iron toning bath by mixing 10.0 ml
of ferric ammonium citrate (10% solution), 10.0
ml of K3Fe(CN)6 (10% solution) and 100 ml of
a 10% solution of acetic acid in a 400-ml
beaker. This solution can be safely disposed of
in the sink.
Place a print in the iron toning solution for 5
minutes. What happens? How does the length
of time in the toning solution affect the print?
Test this by placing a print in the toning bath
for only 2 minutes. Try another time for the
toning bath. Rinse the print in deionized water
briefly, and record your results.
Copper Toning:
Prepare a copper toning bath as follows.
Dissolve 0.54 g of K3Fe(CN)6 and 2.65 g of
potassium citrate in 100 ml of H2O. In a
separate beaker, dissolve 0.66 g of copper
sulfate and 2.65 g of potassium citrate in 100 ml
of water. Mix equal volumes of the two
solutions just prior to use. (Copper is a heavy
metal. Dispose of this solution in the aqueous
waste container).
Place a print in your bath for 5 minutes and
rinse What do you see? How does the length of
time in the bath affect the result?
Sepia Toning:
Place a print in a 400 ml beaker containing
about 100 ml of 20% sodium thiosulfate
solution for 5 minutes. Without rinsing,
immerse the print in a beaker containing the
prepared hydrochloric acid for 30 minutes then
rinse in distilled water. (Both of these solutions
can be put down the drain with the water
running). What do you observe? How does
length of time in the acid bath affect the result?
(Note: In your Results section, include the
prints by taping them in the appropriate
section).
For your conclusion describe the role of the
chemical which you left out of the developer.
Refer to the mechanism of the action of the
black and white developer as described by your
lab instructor and in the handout. Also draw a
conclusion about the effect of time on the
progress of the toning reaction.
Conclusion
For your conclusion describe the role of the
chemical which you left out of the developer.
Refer to the mechanism of the action of the
black and white developer as described by your
lab instructor and in the handout. Also draw a
conclusion about the effect of time on the
progress of the toning reaction.
References:
1. M. Philip, Advanced Chemistry (Physical
and Industrial) Published in South Asia by
Foundation Books New Delhi (2003) p. 168.
2. Chris Knud-Hansen, Conflict Research
Consortium (1994).
Abstract
Our vision is often described as a
molecular switch. The word switch
probably makes you think of a light switch.
In our eyes, the switch is much smaller
than a light switch, however, there is a
reaction in our eyes that works as a switch
to turn “on” and “off” our vision. To begin
this explanation, we will talk about the
source of this switch, a molecule called
retinal which is in all of our eyes. The
retinal molecule responds to light. As the
light hits our eyes, retinal “switches on”
our vision. When we close our eyes or are
in a very dark room with no light at all, we
are no longer able to see. However, the
objects in the room are still there even
though we cannot see them. We will
simulate this by using invisible ink.
Invisible ink cannot be seen by our eyes
until we add a special ingredient to make it
visible.
Invisible inks are used to send secret
messages. Secret messages are hidden
within an ordinary letter, and can be
revealed by the receiver. A secret message
prevents other people from reading the
message, or even knowing it is there. A
secret message should be used with a
believable normal message in order to
hide the secret message (a blank piece of
paper would be suspicious!). Invisible inks
can be used as a fun way to communicate
secretly with friends. An ideal invisible ink
should be easily made, hard to detect and
easily and safely revealed. Invisible inks
are normally non-greasy and not visible
under ultra violet light (a common
screening method for post). Commonly
acids and bases are used as invisible inks,
and these are revealed by indicators.
Indicators are chemical compounds which
are different colours when in contact with
either acids or bases. With acids,
indicators generally turn red, and with
bases they generally turn a blue/green
colour. In addition to indicators, heat can
also be used to reveal messages. A
substance applied to the paper that burns
at a lower temperature than the paper and
so the message is revealed. In this
practical you will investigate making up
solutions and then using them to create
your own secret messages!
Materials
• Baking Soda
• Paper
• Water
• Light Bulb (heat source)
• Paintbrush or Swab
• Measuring Cup
• Purple Grape Juice
• Paper
• Small Sponge
Activity
Making the invisible ink:
1. There are at least two methods to use
baking soda as an invisible ink. Mix equal
parts water and baking soda.
2. Use a cotton swab, toothpick, or
paintbrush to write a message onto white
paper, using the baking soda solution as
'ink'.
3. Allow the ink to dry.
4. One way to read the message is to hold
the paper up to a heat source, such as a
light bulb. The baking soda will cause the
writing in the paper to turn brown.
5. A second method to read the message
is to paint over the paper with purple grape
juice. The message will appear in a
different color.
Tips:
1. If you are using the heating method,
avoid igniting the paper - don't use a
halogen bulb.
2. Baking soda and grape juice react with
each other in an acid-base reaction,
producing a color change in the paper.
3. The baking soda mixture can also be
used more diluted, with one part baking
soda to two parts water.
4. Grape juice concentrate results in a
more visible color change than regular
grape juice.
Procedure:
1. Mix a small amount of water with
cornstarch (use about 2 tablespoons of
cornstarch and 4 teaspoons of water) in
the pan and stir until smooth.
2. Heat the mixture for several minutes.
Stir. (heat each mixture separately)
3. Dip a toothpick into the cornstarch and
water mixture and write with the mixture
on paper. Let the paper dry.
4. To observe the message, dip the small
sponge into the iodine solution and
carefully wipe the paper. Do not get the
paper too wet. The message should appear
purple.
More Information
What is an acid-base indicator?
An acid-base indicator is a weak acid or a
weak base. The undissociated form of the
indicator is a different color than the
iogenic form of the indicator. An Indicator
does not change color from pure acid to
pure alkaline at specific hydrogen ion
concentration, but rather, color change
occurs over a range of hydrogen ion
concentrations. This range is termed the
color change interval. It is expressed as a
pH range.
How is an indicator used?
Weak acids are titrated in the presence of
indicators which change under slightly
alkaline conditions. Weak bases should be
titrated in the presence of indicators which
change under slightly acidic conditions.
What are some common acid-base
indicators?
Several acid-base indicators are listed
below, some more than once if they can be
used over multiple pH ranges. Quantity of
indicator in aqueous (aq.) or alcohol (alc.)
solution is specified. Tried-and-true
indicators include: thymol blue, tropeolin
OO, methyl yellow, methyl orange,
bromphenol blue, bromcresol green,
methyl red, bromthymol blue, phenol red,
neutral red, phenolphthalein,
thymolphthalein, alizarin yellow, tropeolin
O, nitramine, and trinitrobenzoic acid. Data
in this table are for sodium salts of thymol
blue, bromphenol blue, tetrabromphenol
blue, bromcresol green, methyl red,
bromthymol blue, phenol red, and cresol
red.
References:
1. Sciencekids.com
http://www.sciencekids.co.nz/experiments/
invisibleink.html
2. Foster, K., Ghering, C., Light, M. and
McCollum, M. (1999). Secret methods and
techniques – invisible ink. Retrieved 20
November, 2009 from,
http://www.si.umich.edu/spies/methodsink.html
3. Narasimhan, B. R. V., Prabhakar, S.,
Manohar, P. and Gnanam, F. D. (2001).
Synthesis of gamma ferric oxide by direct
thermal decomposition of ferrous
carbonate. Retrieved 20 November, 2009
from,
http://www.sciencedirect.com/science