Determination of water hardness

The hardness of a water sample can be determined by direct titration of a known volume with a standardised solution of a soap such as potassium oleate. Initially, this soap will precipitate, and the calcium and magnesium salts and the water will not produce any foam on shaking. Gradual addition of the potassium oleate solution is continued until the agitation of the solution produces a reasonably permanent foam. F rom Scheme 8.4, 1.00 ml of 0.02 M potassium oleate solution 2R– K+ (aq) + Ca2+ (aq)= R2Ca(s) + 2K+ (aq) R= Oleate radical is equivalent to 0.01 mmol Ca2+. Thus, if a 100.0 ml sample of water is titrated with 0.02 M potassium oleate solution to just give a permanent foam after addition of V ml of soap, equivalent to V 0.01 mmol Ca2+, the hardness willbe V 10 ppm CaCO3.Despite the simplicity of this method, it is not always possible to detect precisely the point at which a layer of foam persists. Hardness is therefore more usually determined by titration with a standardised solution of the disodium salt of ethylenediamine tetra-acetic acid (EDTA). EDTA forms very stable, soluble complexes with calcium and magnesium ions (Figure 8.1). EDTA is a tetraprotic acid and the various acid–base forms are usually abbreviated by H4Y, H3Y–, H2Y2–,HY3–,and Y4–, as the degree of dissociation progresses. Scheme 8.5 illustrates complex formation with calcium and magnesium ions. The equilibrium constants are large and ensure quantitative formation of the colourless complexes on addition of EDTA to a solution containing calcium and magnesium ions. The titration involves gradual addition of standardised EDTA solution(Na2H2Y) to the water sample, buffered at pH 10 with an ammonia–ammonium

chloride solution. The indicator is a dye that changes colour on complexing with calcium or magnesium, such as Eriochrome Black T (CI Mordant Black 11). At the beginning of the titration, the large concentrations of calcium and magnesium ions ensure that all the dye indicator is in the form of the red complexes with these metals (Ca–Dye–, Scheme 8.6). Very close to the equivalence point, the free Ca2+ and Mg2+ ion concentrations become so low that the added EDTA begins to remove the metals from their red complexes with the dye. This liberates the blue dye ion. The colour of the solution therefore changes from red (dye–metal complex) to blue (free dye anion) at the end-point.

In the titration of calcium ions alone at pH 10, the red calcium–dye complex(Ca–Dye–) is not sufficiently stable and the calcium transfers to EDTA liberating the blue dye anion (H–Dye2–) slightly before the true equivalence point. For titrations of magnesium ion, the magnesium–dye complex is more stable than that of calcium and the blue colour does not occur until all the Mg2+ ions in solution have reacted with the EDTA. Thus, the red-to-blue colour change occurs at the true equivalence point. Towards the end of the titration of hard water, as long as calcium ion is present, it will liberate magnesium ion from the magnesium–EDTA complex because the calcium–EDTA complex is more stable. The liberated magnesium thus ensures that the more stable magnesium–dye complex is always present, until all the calcium has been complexed with EDTA. The last addition of this reagent then removes the magnesium from the red magnesium–dye complex and the solution becomes blue. To ensure that a water sample contains a sufficient quantity of magnesium, it is common to add a little to the EDTA solution before standardisation with a known calcium ion solution.

Various units used for water hardness

Various units used for water hardness Unit of water hardness ppm CaCO3 1 British degree 14.3 1 American degree 17.2 1 French degree 10.0 1 German degree 17.9 1 mmol/L CaCO3 100

Definition of Hardness

Soft water is relatively free of calcium and magnesium ions. It produces a rich foamy lather with soap. This is essential for the soap to be an effective emulsifying agent for oils and dirt. With hard water, the soluble sodium salt of soap reacts with the alkaline earth metal ions and precipitates as the useless and undesirable calcium or magnesium soap (Scheme 8.2). The cleaning ability is lost. Hardness is defined as the presence of soluble calcium and magnesium salts in the water. If these are present in the form of bicarbonates, the hardness is temporary. Heating hard water containing bicarbonates eliminates dissolved carbon dioxide and the reverse of Scheme 8.1 causes precipitation of calcium carbonate. Magnesium carbonate is slightly soluble in water but heating will cause its hydrolysis into the much less soluble magnesium hydroxide (Scheme 8.3). Simply boiling and filtering the water therefore eliminates temporary hardness. In regions where water has high temporary hardness, and is used directly without treatment, it is not uncommon to see hot water rinsing and washing baths with a generous crust of chalk (CaCO3) on the inner surfaces. This type of precipitation inside a boiler is also undesirable because the scale reduces the efficiency of heat transfer. Mg(HCO3)2(aq)= MgCO3(s) + CO2(g) + H2O MgCO3(aq) + H2O =Mg(OH)2(s) + CO2(g) Permanent hardness arises when water contains soluble salts of calcium and magnesium such as chlorides and sulphates. It is unaffected by boiling the water. The total hardness of water is determined by the amount of dissolved calcium and magnesium, but expressed in the form of CaCO3. This is possible because calcium and magnesium carbonates have identical stoichiometry. Therefore, water that contains 1.0 mM calcium and 0.5 mM magnesium ions, has a total concentration of 1.5 m M . Since the molecular weight of CaCO3 is 100 g mol–1, 1.5 m M corresponds to 150 mg l–1 of CaCO3, or 150 ppm (parts per million), assuming that 1.0 l of the water has a mass of 1.0 kg. The permanent hardness is that remaining after the water has been boiled and filtered to remove the precipitated calcium and magnesium carbonates and magnesium hydroxide. The temporary hardness is the difference between the total and permanent hardness.Occasionally, hardness due to magnesium alone is of interest, again expressed as the equivalent amount of CaCO3. For the water mentioned above, 0.5m M of magnesium ions is equivalent to 50 mg l–1 or 50 ppm CaCO3. Understanding water hardness values is often a tedious exercise in manipulation of units. In the past, the hardness of water was given in units called degrees. The definition of one degree of hardness, however, varied from one country to the next (Table 8.2). It is much simpler to express hardness in mg l–1 or ppm of CaCO3.

Consequences of using hard water

Hard water using in a textile dyeing or finishing mill can have some serious consequences. Below some consequences- (1) precipitation of soaps; (2) redeposition of dirt and insoluble soaps on the fabric being washed – this can cause yellowing and lead to unlevel dyeing and a poor handle; (3) precipitation of some dyes as calcium or magnesium salts; (4) scale formation on equipment and in boilers and pipelines; (5) reduction of the activity of the enzymes used in desizing; (6) decreased solubility of sizing agents; (7) coagulation of some types of print pastes; (8) incompatibility with chemicals in finishing recipes.

Typical Dye House Water Quality

Dyeings Water Quality Component Permissible Concentration mg/L Hardness 0-25 CaCO3 Iron .02-.1 Manganese .02 Silica .5-3.0 Alkalinity to Ph 4 35-65 CaCO3 Dissolved Solids 65-150 The water may pass from the primary source into a settling pond, from which water can be drawn after any small particles have settled to the bottom, or it may be filtered by passage through a bed of fine sand. For particles smaller than about 0.02 mm, a coagulant aids sedimentation and clarification of the water. Addition of small quantities of sodium aluminate (NaAlO2) or aluminium sulphate, and adjustment to around pH 7, give a gelatinous precipitate of aluminium hydroxide that imprisons the small particles and accelerates sedimentation or filtration. Addition of a flocculant, such as a polyacrylic acid–polyacrylamide copolymer, gives better particle adhesion and a higher rate of sedimentation. This type of treatment also improves the colour of the water. Soluble impurities in the water, such as iron, pollutants, and organic matter, are a real problem. Good aeration of the water will usually convert iron into Fe3+, which precipitates as ferric hydroxide unless the water is quite acidic.

WATER QUALITY FOR THE DYEHOUSE

During the Industrial Revolution, the textile industry invariably grew in regions having an abundant supply of soft water such as in West Yorkshire. Such water is relatively free from calcium and magnesium salts. It does not precipitate soap or other chemicals during washing, or form a scale on the boiler walls when generating steam. Water for a textile plant may come from various sources. These include surface water from rivers and lakes, and subterranean water from wells. The water may be obtained directly from the source or from the local municipality. Natural and pretreated water may contain a variety of chemical species that can influence textile wet processing in general, and dyeing in particular. The various salts present in water depend on the geological formations through which the water has flowed. These salts are mainly the carbonates (CO32–), hydrogencarbonates (HCO3–, more commonly named bicarbonates), sulphates(SO42–) and chlorides (Cl–) of calcium (Ca2+), magnesium (Mg2+) and sodium(Na+). Although calcium and magnesium carbonates in limestone are relatively insoluble, the action of dissolved carbon dioxide in rain water can leach them out in the form of the more soluble bicarbonates (Scheme 8.1). Hard water contains calcium and magnesium ions and gives an immediate precipitate with soap. Soap is a water-soluble sodium salt of a high molecular weight carboxylic acid, such as stearic acid. Its calcium and magnesium salts are much less soluble and give a gummy precipitate with a soap solution in hard water (Scheme 8.2). When this happens, the cleaning efficiency of the soap decreases dramatically. The dirt in suspension, as well as the precipitated calcium and magnesium soaps, can deposit back onto the material being washed. This gives a dingy fabric with a poor handle that will be difficult to dye in a level shade. CaCO3(s) + CO2(aq) + H2O(l) = Ca(HCO3)2(aq) Scheme 8.1 2CH3(CH2)16CO2Na+ (aq) + Ca2+ (aq) = (CH3(CH2)16CO2-)2 Ca2+ (s) + 2Na(aq) Scheme 8.2 Besides dissolved salts of natural origin, water may contain a variety of other salts from human or industrial activity. These include nitrates (NO3–), phosphates(HPO42– and H2PO4–) and various metal ions. The ions of certain transition metals, such as those of iron, copper and manganese, can precipitate dyes, or form complexes with them, dulling or even changing their shade. In addition, many of these ions catalyse the decomposition of hydrogen peroxide in bleaching baths. It is not uncommon for a particle of iron rust on the surface of a cotton fabric to generate such rapid peroxide decomposition that the cellulose is totally oxidised and a hole results. These transition metal ions may be present in the natural or municipal water, or, in the case of iron, come from corrosion within pipelines. Both dissolved and colloidal silica in boiler feed water can lead to the formation of a hard resistant scale on the boiler’s internal surfaces, considerably decreasing heat transfer efficiency. Organic compounds from the decomposition of vegetable matter in the water, or from sources of pollution, can be very varied. They can have such high concentrations that the water is coloured. Table 8.1 illustrates the quality of water usually considered acceptable for textile processing and steam generation. Water entering a textile dyeing or finishing plant usually passes through a grill to eliminate floating debris. Small particles in suspension must also be removed.

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Water treatment

Water treatment In textile finishing, water consumption is far greater than the amounts of fibres processed. It is the ubiquitous solvent for the solutions of chemicals used. Rinsing and washing operations alone consume enormous amounts of water. Steam is still the major heat-transfer medium for many processes and the quality of water fed to boilers is often critical. The volume of aqueous effluent leaving a textile finishing plant is very close to the total input for the many different wet processes. The effluent consists of a relatively dilute solution of a considerable number of chemicals, including acids, alkalis, salts, dyes and other organic chemicals. Any presentation of textile dyeing would be incomplete without some discussion of the quality of water required for textile wet processing, and of the treatment of waste water so that it can be discarded with minimum environmental impact. Important topics for water treatment- Water treatment in the textile, Water treatment in the leather, Reprocessed water using in the dyeing, Total amount of water required in knit dyeing, Procedure of water treatment, The result of water treatment in dyeing.