Figure 3-32. Freshly precipitated (left to right) pale yellow AgI, creamy white AgBr, and white AgCl (source
Silver halides play the central role in sliver-based photography. Like many other silver salts, silver halides are photosensitive
, decomposing to silver metal under sunlight. Although bulk Ag metal is silver in color, in a finely dispersed form silver and other metals are black. The areas on the film that are exposed to more light darken stronger due to the formation of Ag metal from the silver halide in the sensitive layer. Watch this excellent video
demonstrating how AgCl is precipitated and subsequently used in imaging. 3.3.10. Halogen Compounds Around Us.
We encounter various derivatives of halogens on a daily basis. Below are selected examples of halogen compounds that we use most frequently. Fluorine.
Fluorine in the form of fluoride is critical for healthy teeth, especially in children. Many brands of toothpastes contain fluorine, usually in the form of sodium fluoride (NaF) or sodium monofluorophosphate
F). Tin difluoride (SnF2
) is used in some toothpastes for sensitive teeth. In many areas, fluoride is added to public water supplies, usually in the form of NaF, fluorosilicic acid (H2
), or its sodium salt (Na2
). The addition of a fluoride source to municipal water, called water fluoridation
, is strictly controlled to meet the standards of the World Health Organization, 0.5-1.5 mg of fluoride per liter. In some countries, including China, India, and many European states, water is not artificially fluoridated for one reason or another. In some others, such as Spain, only a fraction of the population receives fluoridated water. In Ireland, water fluoridation is mandatory. In Switzerland, a fluoride source is added to table salt. About 50-70% of all residents of the U.S.A. and Canada receive fluoridated water.
The remarkable nonstick cookware employs materials based on fluoropolymers such as Teflon®. Teflon®, a unique material, has many applications ranging from car and aircraft building to nuclear industry and computer manufacturing. Fluorine-containing organic compounds often exhibit biological activity
and are broadly used as active ingredients of pharmaceuticals and agrochemicals. Chlorine.
Table salt is NaCl. Most bleaching agents contain sodium or calcium hypochlorite as the main active ingredient. Potassium chloride is a broadly used fertilizer and a supplement to treat low blood pressure. Polyvinyl chloride (PVC) is one of the top three most broadly used polymers in the world. PVC is employed to manufacture water and sewage pipes, electric cables, and many materials for construction. Phonograph records are conventionally called "vinyl records" or just "vinyl" because they are made of PVC.
Chlorination of municipal water aims at killing bacteria to make tap water safe for drinking. To disinfect water in the chlorination process, Cl2
gas or NaClO are used in strictly controlled small amounts. Water chlorination is particularly important to prevent the spread of such serious water-borne diseases as cholera, typhoid, and dysentery.
Chlorine compounds are also active ingredients of many crop protection agents and pharmaceuticals. Two highly important cancer drugs Cisplatin and Mitotane are derivatives of chlorine. Bromine.
The main use of bromine is in the production of flame retardants. Brominated flame retardants are blended into flammable polymers and resins to make them nonflammable. The thus made materials are used to make various components for electronics (plastic covers, printed circuit boards, connectors, etc.) and kitchen appliances, as well as carpets, pillows, and upholstery. The use of bromine as well as chlorine and iodine in silver photography has reduced considerably with the spread of digital imaging techniques and devices. Iodine.
Being an essential element for life, iodine is of special importance for the thyroid gland. The thyroid gland is responsible for the production of hormones that regulate many vital body functions. Iodine deficiency in the body causes intellectual and developmental disabilities. To avoid this problem, small quantities of iodine compounds are added to table salt to make iodized salt. In some countries such as Argentina, Canada, and South Africa, all table salt for human consumption must be iodized. One of the oldest and still broadly used external antiseptics is tincture of iodine, a solution of iodine and KI or NaI in aqueous alcohol. 3.3.11. Exercises.
1. Write the electronic configuration of halogen atoms and explain why the halogens (a) are oxidizers and (b) exist in the form of diatomic molecules. [Answer: See 3.3.2]
2. The oxidizing ability of the halogens increases in the order F2
. True or false? Answer
3. All elemental halogens, F2
, and I2
are naturally occurring substances. True or false? Answer
4. How can laundry bleach be made from NaCl and water? Answer
5. Write balanced chemical equations for the reactions between (a) KBr and Cl2
; (b) NaF and Cl2
; (c) Al and Br2
; (d) NaI and Br2
; (e) CuO and HBr; (f) NaOH and Cl2
; (g) BaCl2
; (h) Ca and I2
(i) Fe and Cl2
; (j) Fe and HCl. Answer
6. If fluoridated water contains 0.84 mg of NaF per liter, what is the molar concentration of NaF in the water? Answer
7. It is often said that a more reactive halogen displaces a less reactive halogen from its salts. For example, in the reaction 2 KI + Cl2
= 2 KCl + I2
, more reactive Cl2
displaces less reactive I2
from the iodide salt. There is a reaction, however, where iodine displaces chlorine from its salt: 2 KClO3
= 2 KIO3
. Does the occurrence of this reaction make sense, or should it be viewed as a discrepancy? Answer
8. All hydrogen halides in water are strong acids. True or false? Answer
9. If no precipitate is formed on addition of AgNO3
to a water sample, the water is free from halide anions. True or false? Answer
10. Adding vinegar to laundry bleach is safe and highly recommended. True or false? Answer
11. Chlorine gas can be obtained in the reaction of HCl with (a) Br2
; (b) KClO3
; (c) K2
; (d) HF; (e) MnO2
12. Name HClO, HClO2
and their anions. Answer