Silver iodide (SILL-ver EYE-oh-dide) is a light yellow crystalline or powdery material that darkens on exposure to light. The darkening occurs because silver ions (Ag+; silver atoms with a positive charge) are converted to neutral silver atoms (Ag0) that are dark gray in color. Silver iodide is used primarily in photography and in cloud-seeding experiments.
Binary salt (inorganic)
HOW IT IS MADE
Silver iodide occurs naturally in the mineral odargyrite (also known as iodyrite), from which it can be extracted. The compound is extracted by adding concentrated hydriodic acid (HI) to the mineral, which dissolves out the silver iodide. The silver iodide can then be recovered by evaporating the solution and purifying the product. The compound can also be made more easily by reacting silver nitrate (AgNO3) with sodium or potassium iodide (NaI or KI) under a ruby red light. The colored light is used to prevent oxidation of the silver ion to silver metal during the preparation: AgNO3 + NaI → AgI + NaNO3.
COMMON USES AND POTENTIAL HAZARDS
The primary use of silver iodide is in photography. The compound was first used for this purpose in the early nineteenth century by the French experimenter Louis Jacques Mandé Daguerre (1787–1851). Daguerre first covered a sheet of copper metal with a thin layer of silver. He then exposed the plate to iodine vapors, converting the silver metal to silver iodide. The photograph was taken by exposing the plate to light, which converted colorless silver ions in silver iodide back to grayish silver atoms. After exposure, the plate was treated with magnesium vapor, which adhered to the parts of the plate that had been exposed to light. The unreacted silver iodide was then washed off the plate, revealing an image on the plate. The fundamental principle invented by Daguerre is still used today in taking photographs with photograph film containing silver iodide.
The other major use for silver iodide is in cloud seeding. Cloud seeding is the process by which some foreign material—usually silver iodide or dry ice (solid carbon dioxide)—is dropped into a rain cloud. The crystals of silver iodide or carbon dioxide provide nuclei—tiny cores—on which water can condense to form water droplets. The process of cloud seeding was first developed in the 1940s by American chemist Vincent Schaefer (1927–1993). Schaefer used crushed dry ice dropped from an airplane into a cloud in his first experiments. His initial experiments proved to be successful, with rain or snowstorms resulting from this seeding.
For three decades, researchers attempted to modify and improve Schaefer's techniques so as to be able to use cloud seeding to produce rain on demand. One step in that direction was the effort by General Electric scientist Bernard Vonnegut (1914–1997) to use silver iodide crystals rather than dry ice for seeding. Although these experiments produced moderately successful results, many scientists eventually lost confidence in the process of cloud seeding as a reliable method for producing rain. By the early twenty-first century, the process was being used on only a small scale in isolated areas in efforts to produce rain. The problem is that assessing the success of cloud seeding is very difficult. Because the process requires that clouds are already in an area, it is difficult to determine whether rain or snow falls as a result of cloud seeding or as a result of natural processes. In 2003, the National Academy of Sciences reached the conclusion that no reliable scientific evidence exists to suggest that cloud seeding produces more rain or snow than would occur naturally. The American Meteorological Society has taken a somewhat more optimistic view, suggesting that cloud seeding may increase precipitation by up to 10 percent.
- The United States government spent about $19 million per year in the 1970s for cloud seeding experiments. By the late 1990s, that amount had been reduced to about $500,000 as doubts were raised about the effectiveness of the procedure.
Some concerns have been expressed about the environmental and health effects of using silver iodide for cloud seeding. However, only small amounts of silver iodide are released into the atmosphere. That which does fall to earth does not dissolve in water and so is unlikely to enter a community water supply. Tests have shown that the concen-tration of silver iodide in rainwater is far below the 50 micrograms per liter that has been deemed safe by the U.S. Public Health Service. The primary health concern for workers who handle silver iodide is a condition known as argyreia, in which the skin is stained a bluish black color by the compound.
FOR FURTHER INFORMATION
"Chemistry of Photography." Cheresources. http://www.cheresources.com/photochem.shtml (accessed on November 3, 2005).
Fukuta, Norihiko. "Cloud Seeding Clears the Air." Physics in Action (May 1998). Also available online at http://physicsweb.org/articles/world/11/5/3/1 (accessed on November 3, 2005).
"Silver Iodide." ESPI Metals. http://www.espimetals.com/msds's/silveriodide.pdf (accessed on November 3, 2005).
"Silver Iodide." H&S Chemical. http://www.hschem.com/msds/silverIodide.htm (accessed on November 3, 2005).
Zertuche, Casey. "Every Cloud Has a Silver Lining." The Daily Texan (April 12, 2004). Also available online at http://www.dailytexanonline.com/media/paper410/news/2004/04/12/Focus/Every.Cloud.Has.A.Silver.Lining-657354.shtml?norewrite&sourcedomain=www.dailytexanonline.com (accessed on November 3, 2005).