thumbnail /ketchel EDTA-Type Chelating Agents in Everyday Consumer Products: Some Food, Cleaning, and photographic Applications J. Roger Hart
.
W. R. Grace S . Co Lexington. MA 02173
The first part of this article (J. CHEM. EDUC., 61, 1060 (1984)) concentrated on giving some background on the chemistry of chelating agents and introduced their practical usefulness in medical and personal care products. Mayonnaise and Salad Dressings
EDTA, usually as the disodium salt of its calcium complex (Na2CaEDTA),is added in trace amounts to salad dressings to prevent the formation of rancid tastes and odors. The presence of traces of copper, iron, and manganese in the water, vinegar, oils, and other ingredients causes the formation of peroxides that lead to rancidity and spoilage. This effect is far more pronounced when the oil is greatly dispersed with an aqueous phase as in an emulsion. Rancid flavors and odors soon develop when the oil phase is subject to the high surface areas in such a system. By displacing the more weakly held calcium and converting these metal ions to anionic EDTA complexes, which lowers the oxidation potential, the salad dressings are stabilized against early spoilage and loss. Canned Legumes
Kidney beans, blackeyed peas, and chick peas or garbanzos all form off colors and textures when cooked in the presence of traces of iron and other heavy metals. These metal ions come from the cooking water and from the legume itself. Without this additive, kidney beans will he a muddy red-gray color and somewhat mushy in texture. A more appealing and appetizing finished product results when a trace of EDTA is used. Plant Foods
Household fertilizers often contain added trace elements such as iron, zinc, copper, and manganese, which are required by the plant for its full growth potential. Plants have major requirements for nitrogen, potassium, and phosphorus. The latter element is usually in the form of an orthophosphate (P043-). If trace elements are added to a fertilizer containing orthophosphate, these trace metals are immediately precipitated as their insoluble phosphate compounds. This will prevent the movement of the trace element through the soil to the plant roots where nutrient uptake occurs. If these trace elements are added as a chelated form, they remain soluble as the negatively charged anionic complex. They are not precipitated by the phosphate content of the fertilizer and are able to move freely to the root zone. Even if these trace elements could be added to the soil alone, being cations, they would immediately become adsorbed in the anionic negatively charged clays and he immobilized and unavailable for rapid uptake by the plant. Being anions, the chelated trace metals can move freely through the similarly charged clays and arrive a t the root zone intact for efficient plant nutriton. Converting these cationic metal ions to anionic metal-chelate forms allows them to remain mobile and effective as plant nutrients.
MARY
edited by VIRGINIA ORNA, 0.S.U
College of New Rochelle
New Rochelle. NY 10801
Liquid Dishwashlng Detergents
Raw materials used to make dishwashing liquids often contain trace amounts of calcium and iron, which cause turbidity and slowly precipiate to the bottom of the container. In order to maintain clarity and freedom from sediment, a small amount of chelating agent, usually HEEDTA, is added. Toilet Soaos
Bar soaps are subject to rancidity and discoloration caused by traces of heavy metal ions present in the fats, alkalies, and water used in their manufacture. Small amounts of EDTA or HEEDTA will extend the useful shelf life of these products. Certain antibacterial agents used in deodorant soaps form dark comoounds with traces of iron and will discolor the soao bar unless a chelating agent is added. Floor Wax Removers
Modern liquid floor wax products are polymers that cross-link upon drying. The cross-linking catalyst is usually ionic zinc, which is held in an inactive state as a zinc complex with the ammonia present in the product. As the applied coating dries, the ammonia evaporates and allows the zinc to cross-link the polymer into a higher-molecular-weight, water-resistant polymer film. In order to effectively remove this cross-linked polymer, wax remover products contain a chelating agent like EDTA t o pull the zinc away from the oolvmer and. thus. to break . . do& the rtructure .into a water-dispersable'form.~hechelatinr! arent also aids in t h e oenetration of the ~roductinto the wax-polymer film and inthe rinsing of the disintegrated wax residue. Hard Surface Cleaners
Spray-on cleaners use EDTA to aid in the removal of inorganic and fatty soils. The chelating agent helps to build the cleaning power of these products while improving rinsability. Spray-on, wipe-off-type formulations often leave an undesirable streak or film after use. The use of EDTA as the principal huilder significantly reduces streaking compared to formulations using ortho- or polyphosphates. Carpet Cleaning
The so-called "steam" method of rug cleaning has brought about widespread use of EDTA as the huilder in these lowfoam anionic systems. EDTA is stable to hydrolysis, and prevents scale and precipitation from occurring in the machine and carpet while it boosts the overall cleaning effectiveness. The chelating agent, unlike phosphates, minimizes penetration and wetting of the carpet through to the hacking. Thus, faster drying is obtained, and mildew growth is prevented. Bathtub and Tile Cleaners
Bathtub cleaning products rely on the rapid and effective action of EDTA in removing hardwater soap films and scums from bathroom surfaces. Here, the chelating agent chemically removes the calcium from the soap film while converting it to the soluble sodium soap form, which is easily rinsed away. Germicidal action is also improved by the same mechanism described for ophthalmic products. Volume 62
Number 1 January 1985
75
Photography EDTA-type chelatingagents have been used for many years to prevent precipitation of hard-water salts in photographic orocessine~solutionsand to stabilize develoner solutions against aiFoxidation. More recently, the iron &late of EDTA ferricvanide salts as bleachinn- agents has raoidlv . .reolaced . - in the development system f& color film. Fe(1II) EDTA oxidizes the dark. reduced metallic silver image to silver ions. These ions are then dissolved away from the film hs a bath of hvoo (thiosulfate). For many years, ferricyanidekalts have heen used as the bleaching agent. Ferricyanides, however, may not he pre-mixed with hypo as they will decompose when mixed. It was found, however, that FeEDTA and hypo could be pre-mixed in a stable solution to yield a bleach-fix (hlix) solution and, therefore, eliminate a separate step. A continu& and regenerative process ha* been developed around this idea for the blraching and fixing of color film. In rolor prints, all of the silver (exposed as image and unexposed as silver halide) is removed from the film during processing. This mav he done bv a sinele-stare bleach-fix in which the iron-EDFA oxidizes ihe ex&ed metallic silver image to ionic silver. This is in turn removed bv the hwo. which also dissolves (fixes)and removes rhe uneiposed?lver halide. During this process, wme of the ferric-KIYI'A is reduced to ferrous-
76
Journal of Chemical Education
EDTA and the silver ions are picked up by the solution. This silver may be recovered by percolation through a steel wool cartridge. Silver metal is deposited and an equivalent amount of iron is dissolved and added to the hleach-fix solution. The ferrous-EDTA is then simply oxidized to the ferric form by aeration of the solution. Thus, with proper care, the bleach fix solution may be regenerated for reuse many times, whereas the older ferricyanide bleach system was not readily regenerated and required frequent replacement. The ability to combine the iron-EDTA bleach with the thiosulfate fixer bath to form a convenient bleach-fix or "blix" bath and its ease of regeneration and recycling has brought about a rapid adoption of this system by the photographic industry. Conclusion Metal ions like iron, calcium, zinc, etc., are greatly altered chemically when they are chelated by an aminopolycarboxylic chelating agent like EDTA. These cations are converted to anions i n J their chemical properties are dramatically changed. New chemical possiblities exist when metal ions are chelakd, and these new chemical pmsihilities lead to new and improved consumer products. Chelating agents perform important functions in the products we use everyday. Their unusual properties will lead to new uses in tomorrow's consumer products.
