Common Chelants

The most common chelants employed for boiler deposit control, solubilization, and cleaning duty include ... 

Phosphonates exhibit all the properties of polyphosphates, such as threshold effect, crystal distortion, and sequestration, but are superior in their effectiveness. They provide good chelates for calcium, magnesium, iron, and copper and are commonly used where iron fouling is a problem. Their sequestering properties are generally superior to other common chelants, such as EDTA and NTA. 

Common chelants can be divided into three main classes aminocarboxylates, phosphonates, and carboxylates. 

Stability constants for common chelants and some key metal ions are given in Table 10.3. Based on this table, using Fe3+ as an example, the order of binding... 

The most common chelants include EDTA (ethylenediaminetetraacetic acid) and its salts, NTA (nitrilotriacetic acid) and its salts, gluconic acid, sodium gluconate, sodium glucoheptonate (sodium heptonate), as well as the various phosphonates, polyphosphates, lignin sulfonates, and citric acid. 

Chelants at concentrations of 0.1 to 0.2% improve the oxidative stabiUty through the complexation of the trace metal ions, eg, iron, which cataly2e the oxidative processes. Examples of the chelants commonly used are pentasodium diethylenetriarninepentaacetic acid (DTPA), tetrasodium ethylenediarninetetraacetic acid (EDTA), sodium etidronate (EHDP), and citric acid. Magnesium siUcate, formed in wet soap through the reaction of magnesium and siUcate ions, is another chelant commonly used in simple soap bars. 

Phosphate—Chelant—Polymer Combinations. Combinations of polymer, phosphate, and chelant are commonly used to produce results comparable to chelant—polymer treatment in boilers operating at 4137 x 10 Pa or less. Boiler cleanliness is improved over phosphate treatment, and the presence of phosphate provides an easy means of testing to confirm the presence of treatment in the boiler water. 

Table 1 Hsts a number of chelating agents, grouped according to recognized stmctural classes. Because systematic nomenclature of chelating agents is frequently cumbersome, chelants are commonly referred to by common names and abbreviations. For the macrocyclic complexing agents, special systems of abbreviated nomenclature have been devised and are widely used. Some of the donor atoms involved ia chelation and the many forms ia which they can occur have been reviewed (5).

Ghelants and Precipitation Inhibitors vs Dispersants. Dispersants can inhibit crystal growth, but chelants, such as ethylenediaminetetraacetic acid [60-00-4] (EDTA), and pure precipitation inhibitors such as nitrilotris(methylene)tris-phosphonic acid [6419-19-8], commonly known as amino trismethylene phosphonic acid (ATMP), can be more effective under certain circumstances. Chelants can prevent scale by forming stoichiometric ring stmctures with polyvalent cations (such as calcium) to prevent interaction with anions (such as carbonate). Chelants interact... 

Organic chelant compounds, such as the sodium salts of ethylenedi-aminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) are commonly used in BW deposit control treatments, often in combination with phosphates. 

Where chelant attack of boiler surfaces does take place, the corrosion results in a thinning wastage of the metal rather than pitting. The metal surface most commonly is uniformly dark, smooth, and featureless, and the corrosion often is not immediately discemable to the naked eye. 

Chelant programs These programs are commonly prescribed for both FT and WT boilers and are employed either as replacements for or used in combination with phosphate precipitation programs. 

Ligands bite at one or more points. Chelants bite at two or more points, so all ligands are not necessarily chelants. Chelants forming water-soluble complexes with metal ions are called sequestrants (but not all sequestrants are chelants). The most commonly employed BW chelant, ethylenediaminetetraacetic acid (EDTA) produces coordination complexes with four points of attachment and is termed a tetraden-tate ligand. 

Aminotri(methylenephosphonic acid) [ATMP or AMP] is the least expensive phosphonate. It is a good, general-purpose, cost-effective scale inhibitor an effective chelant and the most thermally stable of all the common phosphonates. It is satisfactory up to at least 700 psia. However, if fed as a concentrate AIMP may easily form insoluble calcium phosphonate and it may also affect copper. ATMP has a sequestration value of 870 mg CaC03/g product at a pH level of 11 and for iron, a sequestration value of 150 mg Fe/g product at a pH level of 10. The pentasodium salt has a MW of 409. Examples include Dequest 2000/2006, Mayoquest 1230, Phos -2, Briquest 301-50A, Unihib 305, and Codex 8503. 

The most common and best known chelant is ethylenediaminetetraacetic acid (EDTA). The related compounds diethylenetriaminepentaacetic acid (DTPA) and nitrilotriacetic acid (NTA) are also well known (Figure 10.2). EDTA is a powerful chelant that complexes strongly with most metal ions to form six-coordinate complexes. It has therefore become the first choice in most applications. Indeed, a search of the bathroom cabinet will spot EDTA on the ingredient list of many personal care formulations. DTPA is also a powerful chelant, but tends to be used more often in industrial settings. NTA has only four binding sites and is used more often where hardness ions require control, such as in cleaning, for example, hard surface cleaners, dishwashing, and the dairy industry. 

The three most commonly used phosphonates are diethylenetriaminepenta (methylenephosphonic acid) (DTPMP), ethylenediaminetetra(methylenephospho-nic acid) (EDTMP), and hydroxyethyldiphosphonic acid or etidronic acid (HEDP) (Figure 10.3). DTPMP and EDTMP are also powerful chelants that complex strongly with most metal ions to form six-coordinate complexes, and for this reason are often... 

However, this is not the whole story, as in most applications there is a mixture of hardness ions (50-200 ppm) that compete for chelant with a small amount of transition metal ions (0.1-5 ppm). This is where selectivity of chelants becomes important. A simple way to analyze this is shown in Table 10.4, where the selectivity for different metal ions in the presence of the most common hardness metal ion Ca2 + has been calculated. Selectivity is calculated as the ratio of the stability constant for a metal ion divided by the stability constant for Ca2+ (Equation 10.2) ... 

Chelant-based cleaners are also used, however, for many situations, and it is now becoming more common to use a slow and steady on-line cleaning protocol by the use of polymer/phosphonate-based, multiblend formulations. These programs employ relatively high doses of active product and can be expensive to use in large volume systems, but are unlikely to cause blockages, corrosion, or other damage to the system itself or any sensitive components. 

Chelants are used to ensure that no precipitation occurs on aging. The most common problem is iron, which is introduced as an impurity from surfactants... 

To form a dispersion polymer, a starting solution that typically consists of monomers, water, acids and/or bases for pH adjustment, buffering agents, multivalent salts (ammonium sulfate and sodium sulfate are common), chain transfer agents, the prescribed polymeric dispersant, and chelant is added to the reactor. Polymerization is accomplished with azo and/or redox initiators. As the reaction begins, the solution becomes viscous. After a short period of time, a milky dispersion is formed. Near the end of the reaction, residual monomer levels are reduced to values below the specified limits by increasing the reaction temperature and/or initiator concentration. When the polymerization reaction is complete, additional stabilizing additives may be added to the mixture. 

Soaps, fatty acids, and perfumes are susceptible to oxidation during aging (25). The oxidation process is quite complex but typically results from the reaction of the unsaturated bonds in these components with oxygen in the air, resulting in the formation of shorter chain-length acids, aldehydes, and ketones which are extremely odoriferous. In the case of perfume components, oxidation can produce a change in product odor character and cause discoloration of the bar. To minimize the oxidation of the base soap and other minor ingredients in soap bars, both chelants and antioxidants are commonly used. 

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