25. colloidal ferric

Uranium coprecipitated with aluminium phosphate, precipitate dissolved in nitric acid Adsorption onto colloidal ferric hydroxide... 

To the filtered seawater (500 ml about 1.5 xg U) is added 0.05 M ferric chloride (3 ml), the pH is adjusted to 6.7 0.1 and the uranium present as (U02(C03)3)4- is adsorbed on the colloidal ferric hydroxide which is floated to the surface as a stable froth by the addition of 0.05% ethanolic sodium dodecyl sulfate (2 ml) with an air-flow (about 10 ml min-1) through the mixture for 5 min. The froth is removed and dissolved in 12 M hydrochloric acid-16 M nitric acid (4 1) and the uranium is salted out with a solution of calcium nitrate containing EDTA, and determined spectrophotometrically at 555 nm by a modification of a Rhodamine B method. The average recovery of uranium is 82% co-adsorbed WO4- and M0O4- do not interfere. 

Kuma, K. and Matsunaga, K. (1995). Availability of colloidal ferric oxides to coastal marine phytoplankton, Mar. Biol., 122, 1-11. 

Fox, L.F. (1988) The solubility of colloidal ferric hydroxide and its relevance to iron concentrations in river water. Geochim. Cosmochim. Acta 52 771-777... 

When water pH is <6, iron corrosion and the formation of corrosion products such as colloidal ferric hydroxide can result. Colloidal ferric hydroxide, however, is difficult to detect and difficult to remove through filtration. Fuel containing these particles appears bright and clear. Only about 1 micron in diameter, colloidal ferric hydroxide compounds can pass through fuel filters and deposit onto fuel system components. Further system corrosion can follow. 

Ferrous hydroxide can continue to react with water and oxygen to form colloidal ferric hydroxide ... 

Decarlo, E. H. Thomas, D. M. 1985. Removal of arsenic from geothermal fluids by adsorptive bubble flotation with colloidal ferric hydroxide. Environmental Science Technology, 19, 538-544. 

When a solution of ferric chloride is poured into a relatively large volume of boiling water, colloidal ferric hydruxide is furmed The ferric hydroxide sol does nol react with hydrogen sulfide nor with potassium hcxacyanoferralc(ll), and like all colloidal substances does nol pass readily through animal membranes or parchment. 

This electrolyzer has a U-shaped concrete frame on the open sides diaphragms A are fastened these are made of asbestos cloth covered with a paste of iron oxide, asbestos powder and colloidal ferric oxide. Cathodes B made of perforated MS sheet are placed tightly against the diaphragms. 

Excessively dilute solutions of ferric chloride9 give no coloration with potassium ferrocyanide, the salt being completely hydrolysed and converted into colloidal ferric hydroxide (see p. 125). 

Colloidal ferric hydroxide.—Ferric hydroxide may be obtained in colloidal solution10 by adding 5 c.c. of 33 per cent, ferric chloride to a litre of boiling water and removing the chloride remaining, together with the hydrochloric acid, by dialysis. 

Another method consists in boiling a solution of ferric nitrate with copper filings or zinc dust. The ferric nitrate need not be specially isolated for the purpose, but may be made merely as an intermediate product during the course of the reaction—if, for example, iron filings containing copper are treated with concentrated nitric acid. After dilution and filtration, the solution is dialysed, whereby a deep red liquid is obtained, containing colloidal ferric hydroxide.1... 

When a 10 per cent, solution of ferric chloride is poured into excess of ammonia, the colloidal ferric hydroxide initially produced is coagulated by the ammonium chloride. On evaporating to dryness and washing with water, the ammonium salt washes out, and then the ferric hydroxide deflocculates, passing into subsequent wash waters as a red colloidal solution.4... 

The colloid, as usually prepared, is electro-positive in character, and may be precipitated from solution by electrolysis, by the addition of small quantities of electrolytes, or by the action of an oppositely charged colloid, such, for example, as (negative) arsemous sulphide, whereby the two electrical charges neutralise each other.7 The smallest quantities of a few electrolytes required to precipitate colloidal ferric hydroxide from solution are given in the following table —8... 

It is possible also to prepare colloidal ferric hydroxide with a negative charge. This may be done by adding slowly 100 c.c. of 0 01-normal ferric chloride solution to 150 c.c. of 0-01-normal sodium hydroxide, the mixture being continuously shaken during the process.2... 

Negative colloidal ferric hydroxide may be converted into the positive colloid by adding it to a very dilute solution of sodium hydroxide (0 005-normal) with constant shaking. In order to account for this amphi-electrical behaviour, it is suggested that the potential difference at the surface of colloidal particles is due to adsorption of ions from the solution. Hence the sign depends upon whether cations or anions are in excess in the layers nearest the particles.5... 

Attempts to determine the molecular weight of colloidal ferric hydroxide lead to very high values. Thus, a colloidal solution prepared by addition of ammonium carbonate to ferric chloride solution was purified by dialysis, and the freezing-point determined of that portion which would not pass through a collodion membrane. The point was only slightly lower than that of the filtrate, indicative of a molecular weight of 3120 for the colloid.2... 

For particulars of further researches on colloidal ferric hydroxide the reader is referred to the subjoined references.6... 

The heat of coagulation of colloidal ferric hydroxide with potassium oxalate has been studied by Doerinckel.7... 

A study of the electric conductivities of aqueous solutions of the salt indicates that the hydrolysis proceeds in two stages, embodying (1) a rapid change unaccompanied by precipitation, and (2) a slower change, progressing at a measurable rate, and accompanied by the production of a so-called basic salt.8 Colloidal ferric hydroxide does not appear to be formed during hydrolysis,9 the salt thus differing from ferric chloride and nitrate. 

In dilute solution ferric nitrate is hydrolysed, yielding colloidal ferric hydroxide and free nitric acid. Such solution gives no coloration with potassium ferrocyanide. In less dilute solutions, to which potassium ferrocyanide has already been added, the blue colour gradually intensifies owing to the continued re-formation of ferric nitrate, as the equilibrium represented by the equation... 

Colloidal ferric phosphate is obtained by dissolving ferric phosphate in ammoniacal di-ammonium hydrogen phosphate and dialysing the solution until all electrolytes have been washed away. The colloidal solution thus obtained is tasteless and without action on litmus. Addition of electrolytes, such as alkali chlorides, effects its gelatmisation. 

Colloidal ferric arsenate is prepared by the action of ammonium hydroxide on the insoluble salt.10... 

In very dilute solution the intensity of colour produced is not quite proportional to the amount of iron present—indeed, the more concentrated solution becomes decolorised on dilution, as also by addition of oxalates, tartrates, etc. The decoloration on dilution is usually explained on the assumption that the water hydrolyses the red undissociated salt into yellow colloidal ferric hydroxide and free thio-cyanic acid —... 

Iron and Oxygen—Ferrous Oxide and Hydroxide—Magnetic Oxide—Ferric Oxide—Polymorphism—Hydrated Ferric Oxide—Ferrous Acid—Colloidal Ferric Hydroxide—Ferrites—Ferrates—Perferrates. 

In oxidized surface waters and sediments, dissolved iron is mobile below about pH 3 to 4 as Fe and Fe(lII) inorganic complexes. Fe(III) is also mobile in many soils, and in surface and ground-waters as ferric-organic (humic-fulvic) complexes up to about pH 5 to 6 and as colloidal ferric oxyhydroxides between about pH 3 to 8. Under reducing conditions iron is soluble and mobile as Fe(II) below about pH 7 to 8, when it occurs, usually as uncomplexed Fe ion. However, where sulfur is present and conditions are sufficiently anaerobic to cause sulfate reduction, Fe(H) precipitates almost quantitatively as sulfides. Discussion and explanation of these observations is given below. Thermodynamic data for iron aqueous species and solids at 25°C considered in this chapter are given in Table A12.1. Stability constants and A//° values computed from these data are considered more reliable than their values in the MINTEQA2 data base for the same species and solids. 

Somm, C.H., The preparation of chloride free colloidal ferric oxide from ferric chloride, J. Am. Chem. Soc., 50, 1263, 1928. 

The origin of the surface charges, and of the double layers is probably different for different materials. Thus colloidal ferric hydroxide... 

Fox (43) proposes that the colloidal ferric oxyhydroxide first precipitated in the laboratory and present in river waters has a Fe(III)/OH ratio of about 1/2.35. His suggested stoichiometry is based on the slope of a -log[Fc ] versus pH plot between pH 1.7 and 6.6. The published measurements which he argues support his stoichiometry involve a narrow pH range from 1.7 to 3.6. Taken separately, the published data are as well fit by a line of 1/3 slope as by one of 1/2.35 slope. Only when Fox s own data, measured in 0.05 M nitrate solutions, are included is the 1/2.35 stoichiometry suggested. Fox s conclusions are discussed below in comparison to experimental results presented here for both added solid oxyhydroxide phases and those precipitated from chloride (or mixed chloride-nitrate) solutions and aged less than 200 hours. 

Another possibility is that colloidal ferric oxyhydroxide species can provide the necessary exchange current directly from the aqueous phase. The resultant potential would not be nernstian with respect to the [Fe ]/[Fe ] couple, but rather to a couple involving Fe(II) ions and the solid phase. This potential should be different for each solid phase, although such differences may not be resolvable in these experiments. 

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