Ferric ions, precipitation

Phytate analysis of muffins, diet composites and stools was performed by a modification of the ferric ion precipitation method for HS-I (5) and by an ion exchange method for HS-II and -III (6). Calcium was determined by atomic absorption spectrophotometry. 

The Fe(II)/Fe(III)-H202 system has its maximum catalytic activity at a pH of 2.8-3.0. Any increase or decrease in the pH sharply reduces the catalytic activity of the metal ion. At high pH, the ferric ion precipitates as ferric hydroxide, whilst at low pH, the complexation of Fe(III) with hydrogen peroxide is inhibited. To overcome this problem, Sun and co-workers have used Fe(III) chelates in place of Fe(II)/Fe(III).21 Sun has shown that a variety of herbicides and pesticides can be transformed and practically mineralized by Fe(II) chelates at neutral pH. 

As discussed in Chapter 5, ferric and ferrous ions have very different solubility properties. Ferric ions precipitate in neutral to alkaline environments as ferric hydroxide, and under anaerobic conditions they may be reduced to the more soluble ferrous form. However, under certain anaerobic conditions enough H2S may be present so as to precipitate iron as ferrous sulfide. 

Quantitative. Classically, silver concentration ia solution has been determined by titration with a standard solution of thiocyanate. Ferric ion is the iadicator. The deep red ferric thiocyanate color appears only when the silver is completely titrated. GravimetricaHy, silver is determined by precipitation with chloride, sulfide, or 1,2,3-benzotriazole. Silver can be precipitated as the metal by electro deposition or chemical reduciag agents. A colored silver diethjldithiocarbamate complex, extractable by organic solvents, is used for the spectrophotometric determination of silver complexes. 

NOTE Although the addition ofHF to HCl can be beneficial in helping to control the corrosion of steel (because fluoride ions form very stable self-limiting complexes with ferric ions), HF should not be used where significant hardness scales are present because calcium and magnesium fluorides (CaF2, MgF2) may be precipitated. 

Silverman and Dodson made the first detailed isotopic study of this exchange system using the separation afforded by the addition of 2,2 -dipyridyl at pH 5, followed by the precipitation of the ferric iron with either ammonia or 8-hydro-xyquinoline. Dodson , using this separation method, had previously obtained an overall rate coefficient of 16 l.mole" sec at 23 °C for 0.4 M perchloric acid media. The exchange in perchlorate and perchlorate-chloride media was found to conform to a rate law, first order with respect to both total ferrous and ferric ion concentrations, with an observed rate constant (k bs) dependent on the hydrogen-ion concentration, viz. 

Except at very low pH values, the hexa-aqua ferric ion, Fe(H20)63+, would then undergo a series of hydrolysis and polymerization reactions, leading progressively to more and more insoluble ferric polynuclears which would precipitate to give the geologic evidence of atmospheric oxygenation by the presence around the... 

Iron is an essential cofactor of numerous enzymes, involved in, for instance, electron transfer and oxygen metabolism. It seems counterintuitive that the fourth most abundant element in the biosphere is in many instances the least bioavailable bioelement and therefore the limiting growth factor. The reason for this lies in the extremely low solubility of ferric iron (Fe3+) the prevailing form of iron under oxic conditions. Iron is precipitated as Fe(OH)3 with a solubility product of 10 39, which limits the aqueous concentration of ferric ion... 

It is therefore believed that at pH 6 and greater the corrosion process is localised and large local concentrations of ferrous iron are achieved. At pH 6 the oxidation to ferric iron is very rapid ( ) and precipitation of Fe(0H)j occurs to exhibit localised corrosion or "flash-rust" spots. At pH 5 and below a small but finite uniform dissolution of the iron substrate occurs. However, in this pH range the oxidation of the ferrous dissolution product to ferric ion is considerably slower, by almost 1000 times, and hence "flash rusting" is not observed. 

The T relaxation agent increased the sensitivity of the NMR instrument by decreasing several of the mono- and diester phosphate relaxation times by factors of 2-5. In this way the delay time between scans was decreased (43). This change permits an increase in the number of scans observed per unit time. Although the presence of ferric ions creates the potential for precipitation of phosphorus-iron complexes, the addition of a large molar excess of ethylenediaminetetraacetate (EDTA) relative to ferric ions prevented precipitation, even over a large pH region (44). 

Other typical reagents generated for coulometric titrations are hydrogen and hydroxyl ions, redox reagents such as ceric, cuprous, ferrous, chromate, ferric, manganic, stannous, and titanous ions, precipitation reagents such as silver, mercurous, mercuric, and sulfate ions, and complex-formation reagents such as cyanide ion and EDTA [8-10]. 

A third chemical weathering mechanism that is of importance is oxidation/ reduction that involves mainly the elements carbon, iron, manganese and, of course, oxygen. An equilibrium reaction between dissolved C02 and bicarbonate ions can lead to the precipitation of ferrous iron, giving a hematite (ferric oxide) precipitate ... 

In the determination of sulfate, 2 to 5 g of the analysis sample is mixed with HC1 (2 volumes concentrated HC1 + 3 volumes of water), and the mixture is gently boiled for 30 minutes. After filtering and washing, the undissolved coal may be retained for the determination of pyrite sulfur, or it may be discarded and a fresh sample used for pyrite sulfur. Saturated bromine water is added to the filtrate to oxidize all sulfur forms to sulfate ions and ferrous ions to ferric ions. After boiling to remove excess bromine, the iron is precipitated with excess ammonia and filtered. This precipitate must be retained for the determination of nonpyrite iron if a fresh sample of coal was used for the determination of the pyrite iron. The sulfate is then precipitated with ISaCE, and the BaSC>4 is determined gravimetrically. 

The method can be used for the direct determination of the chloride ion content in precipitation samples within the 0.05-5 mg/L range. Chloride ions will replace the thiocyanate ions in undissociated mercury thiocyanate. The thiocyanate ions thus released react with ferric ions to form a dark red iron-thiocyanate complex. 

HEDTA was developed specifically for chelating ferric ions in alkaline systems. It chelates with maximum effectiveness in a pH range of 6 to 12. This prevents precipitation of iron compounds (e.g., ferric hydroxide and oxide) in this alkaline range. It is not as effective as EDTA for calcium and magnesium ions. It also tends to be less stable250. 

An example of removing multiple interfering elements is strontium purification in the presence of fission products. Ferric ion is added as a holdback carrier for the rare earths (and other radionuclides) and then precipitated as Fe(OH)3, the scavenger that carries these radioactive impurities. This or any other step can be repeated for enhanced removal of impurities. 

Cupric ion is said to give highly insoluble precipitates and this has been suggested as a test for the —CON(OH)— bond (146). In the case of iron it is apparent that the number of hydroxamate anions coordinated to the ferric ion is mainly a function of pH, and this in turn has a profound effect on the wavelength maximum and the intensity of absorption. In general, at about pH 1 to 2 the 1 1 complex is favored, the color is purple (maximum 5100 A) and the amM is about 1.0 (Table 2). (35, 116, 142). 

Under real circumstances equilibrium conditions are difficult to attain, and kinetic criteria (which are often hard to predict) play a key role. To complicate matters, the stoichiometries of some of these reactions do not obey their theoretical chemical equations. In addition, the formation of a solid phase can be the result of successive reactions with varied rates. See, for example, the simplified precipitation pathway of ferric ions with hydroxide ions, depicted in Figure 5.7. 

The chemical reactions of the ferrous and the ferric ions were already discussed in a previous chapter. From the topic in the preceding section, iron is more efficiently removed as ferric hydroxide. The natural iron is in the form of Fe(II), so this ferrous must therefore oxidize to the ferric form in order to precipitate as the ferric hydroxide, if, in fact, the iron is to be removed in the ferric form. In Chapter 12, this was done using the dissolved oxygen that is relatively abundant in natural waters. It must be... 

These two A s produce comparable concentrations of the ferric ion to precipitate either Fe(OH)3 or FeP04. Thus, at high pH conditions, the phosphate ion would have a big competitor in the form of the hydroxide ion. An Fe available in solution is grabbed by the OH ion to form the ferric hydroxide, leaving less amount of Fe " to precipitate ferric phosphate. The ferric salts are, therefore, a poor performer for removing phosphorus. Also, to be effective requires adjusting the pH to the pH of almost mineral acidity of less than 3. 

Ferrous carbonate, FeCOg, occurs in nature as a mineral, and can be obtained as a white precipitate by the action of carbonate ion on ferrous ion in the absence of dissolved oxygen. Like calcium carbonate, ferrous carbonate is soluble in acidic waters, the bicarbonate, Fe(HC03)2, being a soluble salt. Hard waters often contain ferrous or ferric ion. 

Ferric hydroxide, Fe(OH)3, is formed as a brown precipitate when alkali is added to a solution of ferric ion. When it is strongly heated ferric hydroxide is converted into ferric oxide, Fe203, which, as a fine powder, is called rouge and, as a pigment, Venetian red. 

Complex Cyanides of Iron. Cyanide ion added to a solution of ferrous or ferric ion forms precipitates, which dissolve in excess cyanide to produce the complexes. Yellow crystals of potassium ferrocyanide, K4Fe(CN)(./3H20, are made by heating organic material, such as dried blood, with iron filings and potassium carbonate. The mass produced by the heating is extracted with warm water, and the crystals are made by evaporation of the solution. Potassium ferricyanide, K3Fe(CN), is made as red crystals by oxidation of ferrocyanide. 

The pigments Prussian blue and TurnbulFs blue are made by addition of ferrous ion to a ferricyanide solution or ferric ion to a ferrocyanide solution. The pigments which precipitate have the approximate composition KFeFe(CN)g HoO. They have a brilliant blue color. Ferrous ion and ferrocyanide ion produce a white precipitate of KoFeFe(CN)6, whereas ferric ion and ferricyanide ion do not form a precipitate, but only a brown solution,... 

Exainple 2. To what extent would the oxidation of ferrocyanide ion by an equivalent amount of ferric ion proceed (Neglect the effect of the precipitation of Prussian blue.)... 

With safety glasses securely in place, add a teaspoon (5 milliliters) of hydrogen peroxide to a quarter cup (60 milliliters) of iron acetate solution (see the Shopping List and Solutions ). The solution should turn a nice reddish brown as the ferrous ion becomes the ferric ion. Slowly add a half cup (120 milliliters) of cold, brewed brown tea. You should get a black, mushy precipitate that will eventually settle to the bottom. Historically, this material was used as black ink, and this precipitation reaction was used to produce it. 

In addition to the surfactant, a white oil was a component of the microemulsion. This oil was added in the minimum amount required to solubilize enough xanthan polymer to produce the target viscosity. In the absence of the white oil, the polymer could not be solubilized. The xanthan polymer itself was required for mobility control. To prevent biodegradation of the polymer, formaldehyde was added. Citric acid was also a component of the microemulsion, added to prevent the oxidation of ferrous ion present in the brine to ferric ion. The presence of ferric ion would lead to precipitation of iron compounds as well as cross-linking of the biopolymer. 

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