Page 337 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Noble Metals (Chemistry) 491

the efficiency with which the blue photons are absorbed the proper ventilation. Caution must be used with osmium
and thus the efficiency with which the image is formed. because the metal is easily oxidized to OsO 4 .
The most popular compounds for chemical sensitization
are silver sulfide and gold salts (e.g., gold thiocyanate A. Gold
and K[AuCl 4 ]). Compounds such as K 2 PtCl 4 , PdCl 2 , and
Gold compounds are the most toxic of the copper, silver,
their phosphine/halide complexes have also been studied
and gold subgroup. When gold compounds are used medi-
as chemical sensitizers. Platinum salts were used in the
cally, it is found that their excretion is slow, approximately
early days of photography, in the same manner as that in
20% in two weeks and less thereafter. The side effects in-
which silver salts are used today, to yield excellent prints
clude urticaria (hives), itching, purpura, and skin rashes;
from the platinotype process.
damage to the blood forming organs and the nervous sys-
The silver halide, when exposed to light, is split into
temhasalsobeendescribed.Theeffectscanbelonglasting
silver metal and a halide radical that can migrate into
and require some treatment. Gold can be chelated with thi-
the gel and react or less likely, recombine with the sil-
olstoaccelerateitsexcretion.Antidotesforgoldpoisoning
ver metal. This trace amount of silver metal on the crys-
include 2,3-dimercapto-1-propanol(bal), sodium hyposul-
tal sensitizes it to further reduction. The silver present is
fite, calcium gluconate, sodium formaldehyde sulfoxylate,
termed the latent image and is too light to be detected vi-
vitamin C, and pencillamine.
sually. When the film is treated (developed) with a mild
reducing agent (e.g., hydroquinone, catacol, aminophenol,
B. Silver
p-phenylenediamine, or ascorbic acid), an enhancement
of approximately 10 10 to the latent image occurs. The Chronic exposure to silver or its alloys in poorly vented
reducing agent is of prime importance because it must areas can result in the irreversible absorption of silver into
be strong enough to reduce the centers with the silver the body tissue, where it precipitates as a gray to purple
metal but not strong enough to reduce the centers with- pigment. This condition, argyria, is disfiguring but appar-
out the metal (i.e., those centers that were unexposed). ently it is harmless. With proper hygiene and ventilation
This treatment will yield dark silver spots where the light this malady has become uncommon. Silver compounds,
impinged on the film and result in a negative image. At in general, can yield the skin effects discussed previously
this point, the unreduced silver halide must be removed as well as be an irritant to the skin and the mucous mem-
or it will develop on further exposure to light and thus branes. As a reminder, care should be taken with alloys
diminish the quality of the photograph. The silver salt is such as Ag–Cd because of the potential hazard due to
removed with a “fixer” solution, which is a complexing cadmium. The same caution should be taken with other
agent such as sodium thiosulfate that yields the complex alloys containing toxic components. Compounds of silver
[Ag(S 2 O 3 ) 2 ] 3− with a stability constant of approximately may be misleading in their lethal concentration data (e.g.,
13
1.7 × 10 . At this point, the photograph consists of a neg- LD 50 ) in that their insolubility may make them appear to
ative image, which is further processed to give the positive be less of a hazard than they may be.
image.
For color film, the silver halides are used in conjunc- C. Platinum
tion with a series of dyes and filters to allow for selective
Platinosis, the allergic reaction caused by small quantities
reduction by the various spectral regions. Color film is too
of anionic platinum salts, can be recognized by coughing,
complicated an area to be addressed in any depth here.
wheezing, running nose, tightness of the chest, shortness
of breath, and dermatitis. This is not seen with the metal it-
self, even in a finely divided state. Cisplatin, used in cancer
III. TOXICITY
treatment, can cause damage to the renal tubules, gastroin-
testinal epithelium, and spleen and can cause bone mar-
When determining toxicity, the metal should be consid-
row depression. In humans, it can cause nausea, vomiting,
ered in its bulk metallic state, the more reactive finely
peripheral neuropathy, and high-frequency hearing loss.
divided metallic state and its compounds, which should
Platinum compounds and other PGMs are generally toxic
be judged on a case by case basis. Unfortunately, for com-
to the kidneys. Also, as a general rule, as PGM valence
pounds that are not commonly used in industry, toxicity
increases the curarelike effects increase.
information may be difficult to obtain.
This section deals with generalities and is not intended
D. Palladium
as a substitute for a specific search regarding the com-
pounds or metal forms of interest. In general, the noble Palladium compounds, like platinum, can cause damage
metals themselves are nontoxic and no major problems to the organs (e.g., heart, liver, and kidney) and be dermal
have been reported when they are worked on in areas with irritants as well.