In addition to total hardness, it is desirable and sometimes necessary to know the types of hardness present. Hardness is classified in two ways: (1) with respect to the metallic ion and (2) with respect to the anions associated with the metallic ions.
Calcium and magnesium cause by far the greatest portion of the hardness occurring in natural waters. In some considerations it is important to know the amounts of calcium and magnesium hardness in water. For example, it is necessary to know the magnesium hardness or the amount of Mg2+ in order to calculate lime requirements in lime-soda ash softening. The calcium and magnesium hardness may be calculated from the complete chemical analysis, as discussed in Sec. 19.4. Such information is not always available, and recourse is made to some method of analysis that allows separate measurement of calcium or magnesium hardness. If calcium hardness is determined, magnesium hardness is obtained by subtracting calcium hardness from total hardness, as follows:
Total hardness — calcium hardness = magnesium hardness (19.4)
This procedure yields reasonably reliable results because most of the hardness in natural waters is due to these two cations. Most methods for measuring calcium hardness will also include strontium hardness.
The part of the total hardness that is chemically equivalent to the bicarbonate plus carbonate alkalinities present in a water is considered to be carbonate hardness. Since alkalinity and hardness are both expressed in terms of CaC03, the carbonate hardness can be found as follows:
When alkalinity < total hardness,
Carbonate hardness (in mg/L) = alkalinity (in mg/L) (19.5)
When alkalinity a total hardness,
Carbonate hardness (in mg/L) = total hardness (in mg/L) (19.6)
Carbonate hardness is singled out for special recognition because the bicarbonate and carbonate ions with which it is associated tend to precipitate this portion of the hardness at elevated temperatures such as occur in boilers or during the softening process with lime.
Ca2+(a$) 4- 2HCOJ(fli?) -> CaC03(s) + C02(g) + H20(I) (19.7) Ca^(aq) + 2ECO3 (aq) 4- Ca(OH)2(s) ~> 2CaC03(s) 4- 2H20(f> (19.8)
It may also be considered as that part of the total hardness that originates from the action of carbonic acid on limestone, as illustrated in Fig. 19.2. Carbonate hardness was formerly called temporary hardness because it can be caused to precipitate by prolonged boiling [see Eq. (19.7)].
The amount of hardness that is in excess of the carbonate hardness is called noncarbonate hardness and can be estimated as follows:
Noncarbonate hardness (NCH) = total hardness - carbonate hardness (19.9)
Since all forms of hardness as well as alkalinity are expressed in terms of CaC03, the calculations for Eqs. (19.5), (19.6), and (19.9) can be made directly. These are excellent examples of the reason why alkalinity and hardness are normally expressed in terms of CaC03. Noncarbonate hardness was formerly called permanent hardness because it cannot be removed or precipitated by boiling. Noncarbonate hardness cations are associated with sulfate, chloride, and nitrate anions.
Sea, brackish, and other waters that contain appreciable amounts of Na+ interfere with the normal behavior of soap because of the commpn ion effect (Sec. 2.13). Sodium is not a hardness-causing cation, and so this action which it exhibits when present in high concentration is termed pseudo-hardness.
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