Total hardness

The total hardness of water can be determined either by using standard soap solution or by using EDTA reagent. 

In the first method, 100 ml of water is placed in a 200 ml stoppered flask. 1 ml of standard Wanklyn s soap solution is added at a time until a lather is obtained on shaking which persists for one minute. If the titre on 100 ml of water is T ml, since 1.0 ml of standard soap solution = 1.0 g CaCO.  

In the second method, 100 ml of sample water is pipetted into a 250 ml conical flask. 2.0 ml of balanced buffer [50 ml HCl (Sp. gr. = 1.18) is added to 400 ml distilled water. 310 ml ethanolamine is slowly added with constant stirring, followed by 5.0 g of magnesium disodium EDTA. The content is diluted to 1 litre] is added, mixed and then added one or two drops of indicator solution [5.0 g 

Solochrome Black WDFA (C. I. Mordant Black 11) is dissolved in a mixture of 75 ml triethanol amine and 25 ml alcohol]. The content is titrated with EDTA solution (0.02 N) until last reddish tinge disappears. Let the titre obtained be T ml. Since 1.0 ml of 0.02 N EDTA solution = 1.0 mg CaCOj, 

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Alkalinity Reduction. Treatment by lime precipitation reduces alkalinity. However, if the raw water alkalinity exceeds the total hardness, sodium bicarbonate alkalinity is present. In such cases, it is usually necessary to reduce treated water alkalinity in order to reduce condensate system corrosion or permit increased cycles of concentration. 

The American Water Works Association (AWWA) Water QuaUty Goals recommend a maximum total hardness of 80 ppm for municipal purposes (19). Municipal softening plants, however, distribute waters containing 70—150 ppm the final quaUty is estabUshed based on such factors as pubHc demand and economics. 

The needed amounts of lime and soda ash can be calculated from the stoichiometry of the reactions. The effluent quaUty is a function of the solubihties of calcium carbonate and magnesium hydroxide and of the quantities of softening chemicals added. The acceptable level of total hardness can be decided and usually is 70—120 mg/L (265—454 mg/gal), expressed as CaC03. The sum of the solubihties of calcium carbonate and magnesium hydroxide is ca 50—70 mg/L (190—265 mg/gal), depending upon the pH. The sum of the concentrations of the carbonic species HCO/ +, ... 

PhenoHc dispersions made using natural soaps are sensitive to hard water, but under proper formulation they can tolerate water of 400 ppm total hardness and maintain complete clarity and germicidal activity. The highest activity for phenols was found when using secondary alkane sulfonates as solubilising agents (75). 

Environment Internal Treated cooling water adjusted with sulfuric acid for pH control and sodium hypochlorite added as a biocide pressure 50 psi (345 kPa), temperature 100-120°F (38-49°C), water velocity 7 ft/s (2.1 m/s), pH 8.0-8.4, sulfate 500-1000 ppm, chloride 100-450 ppm, total hardness 500 ppm External Steam and condensate... 

Environment Treated cooling water, alkaline program, pH 8.5-9.5, 50-105°F (10 0°C), total hardness 150 ppm, chlorides 70 ppm, calcium 90 ppm... 

Total Hardness. A combined concentration of calcium and magnesium in the mud water phase is defined as total hardness. These contaminants are often present in the water available for use in the drilling fluid. In addition, calcium... 

The hardness test sometimes is performed on the mud as well as the mud filtrate. The mud hardness indicates the amount of calcium suspended in the mud as well as the calcium in solution. This test usually is made on gypsum-treated muds to indicate the amount of excess CaSO present in suspension. To perform the hardness test on mud, a small sample of mud is first diluted to 50 times its original volume with distilled water so that any undissolved calcium or magnesium compounds can go into solution. The mixture then is filtered through hardened filter paper to obtain a clear filtrate. The total hardness of this filtrate then is obtained using the same procedure used for the filtrate from the low-temperature low-pressure API filter press apparatus. 

Calcium ions in seawater muds can be controlled and removed by forming insoluble precipitates accomplished by adding alkalis such as caustic soda, lime, or barium hydroxide. Soda ash or sodium bicarbonate is of no value in controlling the total hardness of sea water. 

Many analyses quote total hardness. Some give temporary hardness (or carbonate hardness) and permanent hardness (or non-carbonate hardness), usually in consistent units so that the values can be added together to give the total hardness. The total hardness is actually the quantity of calcium (Ca) - - magnesium (Mg) in the water. If the total is not given directly, the values given for these two constituents must be added, after conversion to mg/1 as CaC03 if necessary. 

The boiler-water standards for shell boilers, as given in BS2486 1978 for 2, 20 and 40 bar, respectively, indicate that some parameters are fairly insensitive to pressure, whilst others are not. Thus, sodium phosphate should fall within the range 50 to lOOmg/kg and sodium sulphite between 30 and 70 mg/kg for each of these pressures. Total alkalinity should have a maximum value of 1 200 mg/kg at 2 bar and 700 mg/kg at both 20 and 40 bar, with the corresponding caustic alkalinity at 350 300 and 200 mg/kg. Silica should be less than 40% of the caustic alkalinity in each case. Total hardness should be undetectable in all cases. Dissolved solids maxima are given as 3 500, 3 000 and 2000 mg/kg, again at 2, 20 and 40 bar. 

DETERMINATION OF THE TOTAL HARDNESS (PERMANENT AND TEMPORARY) OF WATER... 

Procedure. To a 50 mL sample of the water to be tested add 1 mL buffer solution (ammonium hydroxide/ammonium chloride, pH 10, Section 10.54) and 30-40 mg solochrome black indicator mixture. Titrate with standard EDTA solution (0.01 M) until the colour changes from red to pure blue. Should there be no magnesium present in the sample of water it is necessary to add 0.1 mL magnesium-EDTA solution (0.1 M) before adding the indicator (see Section 10.54). The total hardness is expressed in parts of CaC03 per million of water. 

Calculate the temporary hardness of the water by subtracting the permanent hardness from the total hardness. 

Where LP steam boiler MU requirements are in excess of 5% or heat-flux densities are high but the source of MU water is a naturally low-hardness lean water (as in New York City, where total hardness is often below 20 ppm as CaC03), it is still advisable to install a water... 

Trace amounts of other metals such as iron (Fe), aluminum (Al), and manganese (Mn) further contribute to the total hardness, although these are not alkaline earth metals. These metals may be present in natural water supplies as ... 

NOTE The reporting of calcium hardness, total hardness, and alkalinity as CaC03 is simply a convenience because the molecular weight for this compound is an easy-to-use 100 ... 

Total hardness is equal to the sum of the calcium hardness and the magnesium hardness. 

Permanent hardness or noncarbonate hardness is that portion of the total hardness that cannot be removed by heating water (e.g., chloride or sulfate hardness) and is the difference between total hardness and total alkalinity. 

For all LP steam operations in which the total hardness of RW exceeds, say, 10 to 20 ppm CaC03, the use of a softener is preferable to control solely via internal precipitation treatment. Oxygen scavenging and other internal treatment programs should also be provided. Some MPHW/HPHW and lower pressure steam boiler designs may require additional purification beyond simple water softening. 

The total hardness of the treated water may be as low as 35 ppm or so (as CaC03). The pH level normally should be approximately 9.4 to 9.6 to obtain minimum calcium solubility. If it is much above this level, it indicates an excess of lime the treated water will not be stable, and some secondary precipitation can be expected. 

Polymaleic acid (PMA). The use of chemicals based on PMA and some derivatives has become standard practice for very brackish waters and seawater distillation processes around the world, where the TDS may reach 50,000 ppm TDS, or where total hardness levels exceed 500 to 1,000 ppm CaC03. Its use in RO systems is growing. However, PMA has limited dispersing properties and may need to be formulated with a dispersant chemical to provide satisfactory performance with some RO designs. It is claimed that PMA is also a successful silica deposit control agent and therefore may be incorporated into formulations where this is a problem. 

FWTH = FW total hardness, in mg/1 (ppm) CaC03 FWMA — FW methyl orange/total alkalinity, in mg/1 CaC03 RBWMA — Required BW methyl orange alkalinity, in mg/1 CaC03 COC — Cycles of FW concentration present or desired in the BW... 

This formulation is designed as a single-drum treatment for boilers operating at up to 300 psig and with a total hardness (TH) in the FW of up to 30 ppm CaC03. Initial feed is 15 ppm product per 1 ppm TH (as CaC03) in the FW. Control is based on maintaining 200 to 300 ppm hydroxide (OH) alkalinity and a maximum of 3,000 ppm total dissolved solids (TDS) in the BW. 

Only low total hardness levels can be tolerated (the ceiling is 0.5-3.0 ppm CaC03) or the program becomes too expensive. Chelant programs work particularly well with good quality FW. 

NOTE As an example, 90% of water supplied to New York City has a total hardness below 15 to 20 ppm CaCOy Pretreatment softeners are not the norm and consequently there are many thousands of nondomestic LP steam boilers (in addition to greater numbers of domestic LP steam boilers) in daily operation, where the FW hardness is 2 to 5 ppm or more as CaCOy The BSI 2486 1978 table for shell boilers remains a valid standard for these commercial and institutional boilers. 

The FW total hardness should be maintained below 3 ppm as CaC03. 

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