Carbonates, Alkalinity and pH in Corrosion Control Introduction Carbonates Alkalinity Table Water Distribution System Properties Chart 1: Alkalinity/pH curves Chart 2: pH titration curves Introduction With the advent of the EPA Rule on lead and copper control in drinking water, more emphasis must be placed on the pH and the alkalinity of a water system. In many cases, the distinction between these characteristics and the differing roles they play in safe drinking water are not well understood. This bulletin clarifies the functions and roles of these criteria and explains the value of carbonates in this application. It will also help in the selection of the proper alkali to achieve satisfactory corrosion resistance in a water distribution system. Carbonates, pH and Alkalinity Carbonates, pH and alkalinity are three of the more important tools for corrosion control in drinking water systems. This bulletin explains the relationship between pH and alkalinity, and in a simple way, their application to corrosion control. It also explains the manner in which carbonates can contribute to the management of these characteristics in drinking water production and the distribution system. Another important corrosion control tool, not discussed here, is the use of inhibitors such as phosphates. For information on the use of phosphates, please read our Technical Information Bulletin entitled "Inhibitors" (TIR-18). Bases (alkalies) such as soda ash or lime react with acids such as sulfuric acid to form neutral salts like sodium or calcium sulfate. As they react together, the pH and the alkalinity of the water or solution changes. Alkalinity expresses the acid neutralizing capacity of a water. In water, alkalinity is usually represented by carbonates, bicarbonates and hydroxides, but may also include other anions, such as silicates and phosphates, which also act as bases. In water treatment, alkalinity is usually expressed as milligrams per liter of calcium carbonate equivalent. Alkalinity is useful in the water distribution system to stabilize the pH. Generally, the higher the alkalinity, the more stable is the pH. Also, when calcium is present, carbonate and bicarbonate alkalinity will contribute to the ability to form films of insoluble calcium salts such as calcium bicarbonate, a reaction which is believed to contribute to reduced corrosion activity. pH is a measure of the concentration of the hydrogen ion [H+] in the water. It is expressed as the negative logarithm of the hydrogen ion concentration. In pure water, the hydrogen ion concentration is 1 x 10-7, so that the pH of pure water is 7. The higher the concentration of hydrogen ions, the lower is the pH number (see Table 1.) The hydrogen ion is one of the main causes of corrosion in water systems because it readily accepts the electrons which metals give up during corrosion. At lower pH’s (below pH 5), lead, copper and iron corrode rapidly. Even in the range 6.5 to 8.5 corrosion continues, but at a reduced rate. In the range 8.5 to 9.5, this kind of corrosion is largely arrested, but can still produce problems. Because of this, pH is often controlled at pH 8.3 and higher in a water distribution system.     pH Hydrogen Ion Concentration (Moles/Liter) Concentration of Hydrogen Ions Relative to that of H2O Acid Range       0 1 2 3 4 5 6 1x10-0 1x10-1 1x10-2 1x10-3 1x10-4 1x10-5 1x10-6 10,000,000 1,000,000 100,000 10,000 1,000 100 10 Pure Water 7 1x10-7 1.0 Alkaline Range 8 9 10 11 12 13 14 1x10-8 1x10-9 1x10-10 1x10-11 1x10-12 1x10-13 1x10-14 0.1 0.01 0.001 0.0001 0.00001 0.000001 0.0000001    Water Distribution System Properties Changes occur in water as it flows through the distribution system of a utility to a point of use. One of these changes, often due to bacterial action, is a drop in pH that leads to increasingly aggressive water and corrosion. This drop in pH may be readily dampened if buffering capacity is present in the water. Buffering is the name given to the effects of weak acids and bases in solution, when they are able to resist changes in pH to one degree or another. In natural waters the most commonly found buffering agents are the carbonate or bicarbonate ions. Although not the same as buffering capacity, alkalinity is the most convenient measure available to a utility of a water’s ability to resist acidification. Generally, the higher the alkalinity, the greater is the resistance to acid and the resulting pH change. High alkalinity should not be confused with being highly alkaline. It is possible to have high alkalinity without a high pH, whereas being highly alkaline infers a high pH. Chart 1 demonstrates this difference quite clearly. In order to reduce corrosion and its effects through pH changes, it is good practice to ensure that all waters being delivered to a distribution system contain some alkalinity (at least 20 to 40 mb/L CaCO3 equivalent) and preferable much more. Other changes which take place in the distribution system, especially at a higher pH (8.3 and over) include loss of disinfection and increased trihalomethane (THM) formation. The prevention of these changes demands precise pH management in the distributed water and so places an additional premium on the need for sufficient and controlled alkalinity in the water. If this alkalinity is not naturally present in the system, it may be added by using alkalies such as soda ash and sodium bicarbonate. The accompanying charts illustrate how they may be used to do this.  Charts 1 and 2 show the effects of treating well water with sodium bicarbonate, soda ash and caustic soda respectively. Chart 1 plots the dose of alkali in terms of its calcium carbonate equivalent dose, and Chart 2 plots the dose of each base in parts per million against the resultant pH. In this way, Chart 1 tells us the alkalinity added to the water against the pH change, while chart 2 shows us how much of each alkali we must add to produce a given pH in this water.   Chart 1. Alkalinity/pH Curves: The titration of drinking water with various alkalies Looking at the results, you can see that caustic soda produces a rapid pH change, but contributes little to the alkalinity. Sodium bicarbonate, on the other hand, contributes a great deal of alkalinity with very little pH change. Soda ash falls in between.   Chart 2. pH Titration Curves: The titration of drinking water with various alkalies. Making practical use of this, in waters where we want to change the pH, we should use a strong alkali such as caustic soda. Where we need to add alkalinity to the water, without much pH change, we should use sodium bicarbonate. Where we need to do both, we should select soda ash, or alternatively, particularly for small water systems, blend sodium bicarbonate, soda ash and/or caustic soda to produce a given alkalinity/pH combination as required. This piece of straight forward technology finds application in meeting the EPA Lead and Copper Rule (1) as well as in routine corrosion control. Sufficient buffering and alkalinity is important in order to be able to control the distribution water pH accurately throughout the system. This is especially desirable for those utilities opting to elevate the pH of their distribution water to levels above 8.2 in order to meet the EPA Lead and Copper Rule action levels. Such control will help to reduce metal corrosion, while limiting trihalomethane (THM) formation and loss of disinfection. These principles are also important for those systems which can not raise their pH above pH 8, but desire to increase the alkalinity of their water to secure better protection against lead and copper contamination. Soda ash and sodium bicarbonate are available from Innophos. They are widely used in water treatment. Soda ash and sodium bicarbonate have been certified as meeting NSF Standard 60. They are available in bulk, bags and supersacks. Ref. USEPA, Office of R&D, Seminar Publication, Control of Lead and Copper in Drinking water. TIR-24 February 1994 See your Innophos Sales Representative or call our Order and Sales Customer Service Department at 1-609-495-2495 for more information about products discussed in this Technical Information Report. Innophos believes all information given in this report is accurate. It is offered in good faith, but supplied without consideration or guarantee. Innophos assumes no obligation or liability for the accuracy or sufficiency of the information given or the results obtained, all such information being given or accepted at user’s risk. The use(s) referred are listed for purposes of illustration only and the user is urged to investigate and establish the suitability of application of such use(s) in every case. Nothing herein contained is to be construed as a recommendation for uses which infringe valid patents or as extending a license under valid patents or as advising or authorizing practice of any patents or patent applications owned by Innophos or others.