Phosphate ions, sequestration

Sequestration of Phosphate Ions Polymeric Drugs for Chronic Renal Failure... 

Complex Ion Formation. Phosphates form water-soluble complex ions with metallic cations, a phenomenon commonly called sequestration. In contrast to many complexing agents, polyphosphates are nonspecific and form soluble, charged complexes with virtually all metallic cations. Alkali metals are weakly complexed, but alkaline-earth and transition metals form more strongly associated complexes (eg, eq. 16). Quaternary ammonium ions are complexed Htde if at all because of their low charge density. The amount of metal ion that can be sequestered by polyphosphates generally increases... 

Some dyes contain a coordinated transition metal as an essential part of their chromogenic structure and this must be left undisturbed by any sequestrant used to complex extraneous metal ions in the system. Hence a balance of properties is needed, phosphates and hydroxycarboxylates being useful. It is claimed that polycarboxylates can be molecularly engineered to give the required balance of properties. 

Phosphate in combination with NaCl has a beneficial effect on the waterbinding capacity of processed meat products for a detailed description, see Schmidt.227 The effect of phosphates is suggested to be alterations in pH or ionic strength, sequestration of metal ions, dissociation of actomyosin and depolymerisation of myosin.103,104,228,229 However, before action, added phosphates must be hydrolysed by muscle phosphatases or non-enzymatically. Belton et al.230 studied the hydrolysis of pyrophosphate and tripolyphosphate in comminuted chicken meat using 31P NMR spectroscopy, and found that the rate of hydrolysis was dependent on the length of ageing period of the muscle as well as the presence of NaCl. Li et al.231 studied the hydrolysis of various types of phosphates in intact chicken muscle with a similar approach by 31P NMR spectroscopy and thereby demonstrated differences in rate of hydrolysis of various phosphates. The findings of these studies... 

Phosphates have the ability to combine with metal ions, such as calcium, magnesium, iron, and copper, and so render the metals nonactive. Calcium and magnesium arc primarily responsible for the hardness of water. The addition of tripolyphosphate or hexametaphosphale will hind these elements and produce soft water. In a similar manner, sequestration is used to soften the skins of fruits and vegetables for faster cooking, and to increase the extraction and recovery of pectin in fruit. Calcium pectinates, which are... 

In sequestration (chelation) the hardness ions are bound to the builder in the form of soluble complexes. Phosphates, citrates, and nitrilotriacetic acid (NTA) are examples of this class of builder compound. Table 8.3 lists the calcium binding capacities of various builders. Other strongly chelating compounds exist, such as phosphonates and EDTA, but they are generally not extensively used in HDLDs. The most efficient builder is sodium tripolyphosphate (STPP). Unfortunately, tripolyphosphate has been identified as a possible cause of eutrophication in lakes and rivers. It is severely controlled and even banned in several countries. As a result, most countries in North America and Europe have converted to nonphosphate formulations. Other regions are also gradually imposing restrictions on the use of phosphates. 

After phosphates, silicates are the most ubiquitous builders used in LADD formulations. Like tripolyphosphates, silicates are multifunctional. They are, however, better sequestrants of magnesium ions. A combination of phosphate and silicate is therefore generally used in ADD formulations. In addition to their sequestering properties, silicates provide alkalinity, soil suspension, and anticorrosion properties. A detailed treatment of the synthesis, chemistry, and applications of silicates has been undertaken by Iler [21], Other reviews are also available [22,23],... 

A builder that has been studied as a possible phosphate replacement is sodium citrate. The structure of the citrate trianion is shown in Figure 9.4. Several properties of sodium citrate restrict its use in LADD formulations. First, it is incompatible with hypochlorite, precluding its use in most LADD compositions. It is also inferior to STPP in its sequestration efficiency for calcium ions. Finally, it is almost three times more expensive than STPP. This combination renders citrate unsuitable as a replacement for STPP. In Europe, where the different wash conditions allow for the use of milder peroxygen bleaches or enzymes and the water is presoftened, citrate-built products are possible. 

With somewhat higher concentrations of polyphosphate chains in hard water, sequestration takes place. This involves the formation of readily soluble complexes with Ca + and Mg + ions, and consequent water softening with the prevention of insoluble Ca or Mg soaps being formed (the removal of these cations with orthophosphates involves precipitation (5.35), (5.36)). Precipitation methods are also used when it is desired to reduce the phosphate anion content of eutrophied lakes (Chapter 2.3). The sequestration of Cu +, Zn and Fe cations has important industrial applications since these cations may poison, discolour or have other undesirable effects on manufactured products. 

The ability of the polyphosphates to form water-soluble complexes with calcium has been known for a long time. The phenomenon of binding alkaline earth ions in soluble complexes, and thus preventing the formation of undesirable precipitates, is generally called sequestration. This was the subject of earlier patent literature. The role of these calcium-polyphosphate complexes in softening of water was described in Chapter 23 of the book edited by Bailar. The complexing of a number of metal ions with the various phosphates was reviewed in detail in 1958. ... 

Sequestrants (chelating agents) form complexes with metal ions, thus preventing oxidation, undesirable discoloration and turbidity (calcium disodium ethylenediaminetetraacetate, E386 and its salts and phosphates, E450-E452). 

Sequestration is an important property of condensed phosphates and their ability to form soluble complexes with multivalent cations.Many studies have been made of this property. Condensed phosphates, primarily polyphosphates, differ from orthophosphates in the ways they interact with metal ions in aqueous solutions. Orthophosphates remove metal ions from solution by forming insoluble precipitates. They have been used for many years to soften water by precipitating calcium, magnesium, iron, etc. from solutions, but leaving a powdered precipitate that is usually undesirable. 

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