CERTIFICATION ACTION LINE: Answers
Answers for
Certification Action Line
Questions appear on page 44.
Answers
1. c. The halogen family comprises group VII A of the periodic table of elements and
consists of fluorine, chlorine, bromine, iodine and astatine, listed in order of their
activity. Each atom of these elements has seven electrons in its outermost shell. They
each, therefore, behave chemically very similar and are among the most active of all
electronegative or oxidizing elements, fluorine being the most active of all chemical
elements.
2. a. Chlorine is considered to have more diverse uses than any chemical known —
from the manufacture of food products to antifreeze fluids, plastics and synthetic
fibers. As an oxidizing agent, chlorine also is extremely effective as a disinfectant
and microbiological control agent.
3. c and d. At the pH values encountered in domestic water supplies, chlorine compounds react to form hypochlorous acid (HOCl) and hypochlorite ion (OCl-). Hypochlorite
ion becomes the dominant chlorine form at pH greater than 8. At pH less than 7, HOCl is
the dominant form; a transition region occurs between pH 7 and 8.
4. a. The relative proportions of HOCl and OCl- are very important with respect to chlorine biocide efficacy. HOCl is a stronger oxidant than OCl-. HOCl is also uncharged and
is, therefore, capable of penetrating and damaging microbial cells much more quickly
than OCl-. Consequently, the level of disinfection that is accomplished in five minutes
at pH 6 may take 30 minutes or more to accomplish at pH 8 with the same amounts of
available chlorine.
5. c. In breakpoint chlorination, chlorine is added until all free ammonia is used up.
Then additional chlorine is added to produce a residual of free available chlorine.
6. f. Chlorine will readily oxidize certain inorganic constituents, such as ferrous iron
(Fe++), manganese (Mn++), sulfides (S=) and non-microbiological organic materials,
such as hydrocarbons, humic substances and pollen. These reactions can significantly increase the chlorine demand of the water and consequently the quantity of
chlorine required for microbiological control.
7. a. Chlorine dioxide is more effective, particularly as a bacterial sporicide and viri-cide. Theoretically, chlorine dioxide has 2. 6 times the oxidizing capacity of chlorine.
8. b. Due to the instability of gaseous chlorine dioxide, it is not produced and transported in a gaseous state. Rather, it is generated at the application site by the reaction of chlorine with a precursor material such as a solution of sodium chlorite.
9. b and d. Research work sponsored by the US Environmental Protection Agency
(EPA) has revealed that chlorine dioxide in the absence of free chlorine does not produce chloroform (or other trihalomethanes) as does chlorine. Also, chlorine dioxide
does not react with ammonia nitrogen. It does however produce significant by-products of chlorite and chlorate ions. The EPA has listed an MCL health limit of one milligram per liter for chlorite in drinking water.
10. c. Unlike chlorine, chlorine dioxide (ClO2) does not react with water; it is a true dissolved gas. It therefore remains unaffected by the pH of water, and is a practical
alternative for high pH conditions.
