Ferrous chromate

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]. 

Ferrous chromate does not appear to be capable of existing. When solutions of ferrous sulphate and chromic acid, or its alkali salts, are mixed at low temperatures, the iron is oxidised, basic ferric sulphates resulting, the chromium being correspondingly reduced. If the ferrous sulphate is in excess, ferroso-ferric sulphate results. 

The mechanism and rate of hydrogen peroxide decomposition depend on many factors, including temperature, pH, presence or absence of a catalyst (7—10), such as metal ions, oxides, and hydroxides etc. Some common metal ions that actively support homogeneous catalysis of the decomposition include ferrous, ferric, cuprous, cupric, chromate, dichromate, molybdate, tungstate, and vanadate. For combinations, such as iron and... 

The most suitable oxidizing agent is potassium ferricyanide, but ferric chloride, hydrogen peroxide ia the presence of ferrous salts, ammonium persulfate, lead dioxide, lead tetraacetate or chromate, or silver and cupric salts may be useful. Water mixed, eg, with methanol, dimethylformamide, or glycol ethers, is employed as reaction medium. 

Atmospheric corrosion can be prevented by using volatile inhibitors which need not be applied directly to the surfaces to be protected. Most such inhibitors are amine nitrites, benzoates, chromates, etc. They are mainly used with ferrous metals. There is still some disagreement as to the mechanism of action. Clearly, any moisture that condenses must be converted to an inhibitive solution. There is no doubt that the widely used volatile inhibitors are effective in aqueous solutions containing moderate... 

Chemical that reduces the tendency of iron to oxidize (rust) to ferrous ion, such as chromate which suppresses that part of the electrolytic corrosion process occurring at the anodic sites on a metal surface. 

Potassium chromate may be used as an indicator producing a red colour with excess Ag ion. More widely applicable is the method of back titration. Excess AgNOj is added to the sample containing chloride or bromide ions. The excess AgNOj is then titrated with ammonium thiocyanate and ammonium ferrous sulphate is used as an indicator of excess SCN. ... 

Save the filter paper with residue for Zr detn. Fill the volumetric flask to the mark and pipet a 50-ml aliquot into a 400-ml beaker for the chromate detn. Dilute to 250 ml, and add 10 ml of dilute sulfuric acid (1 4) and 10 ml of phosphoric acid (1 1). Add a measured excess consisting of ca 30 ml of standardized 0.05N ferrous ammonium sulfate soln and 6 to 8 drops of Na diphenylamine sulfonate indicator soln (0.2 g in 100 ml of w). Titrate the excess ferrous iron with 0.05N std K dichromate soln, adding the dichromate slowly with stirring, until the pure grn color changes to gray-grn. Then add the dichromate one drop at a time until the first tinge of purple or violet-blue appears... 

Additional Exercises Lead chromate, PbC rO, calcium phosphate, Ca3(PC>4)2 ferrous oxalate, Fe(C2C>4) sodium chloride, NaCl, precipitated from a saturated solution by HC1 gas, Richards and Wells, Revision of the atomic weights of sodium and chlorine, J. Am. Chem. Soc., 27, 459 (1905). 

In aqueous solution, manganous salts are oxidised to manganese dioxide,6 and if silver nitrate is present as catalyst, to permanganate 0 the latter change constitutes Marshall s reaction. Chromium solutions in a similar manner give rise to chromate,7 even without a catalyst. Ferrous and cerous salts are converted into ferric and ceric salts, respectively, and phosphites are oxidised to phosphates. 

Ferric salts are reduced to the ferrous condition and chromates are reduced to chromic salts. Salts of gold, silver, copper, antimony, bismuth and mercury are reduced to the free metals, which are frequently obtained as colloidal solutions if the original solutions are weak with the exception of the first-named there is a tendency for the liberated metal to be accompanied by sulphide, especially if excess of hydrosulphite is used.7 Chloroplatinic acid is reduced to red chloroplatinous acid solution. Tellurous and telluric acids, selenious acid and arsenic compounds, are reduced to the free elements.8... 

Selenious acid readily decomposes potassium permanganate, but analytical results are untrustworthy in the presence of more than a limited quantity of sulphuric acid.1 The oxidation should be carried out at 50° C., a known quantity of 0-lN potassium permanganate being used and the excess determined either by means of standard oxalic acid solution or by electrometric titration with ferrous sulphate. In the presence of tellurium, the latter is also oxidised and should be determined in a separate sample by oxidation with potassium di-chromate, which does not oxidise the selenium, and the necessary deduction can then be made.2... 

Chromium occurs chiefly as chromite (ferrous chromite). Fe(CrOy)-. in Zimbabwe, the Republic of South Africa, the former USS.R, New Caledonia. India. Philippine Islands. Japan, Turkey. Greece. Cuba, and California. (I I Healing chromite in the elcclric furnace with carbon yields ferrochrume for alloys, and (2) when chromite is heated with sodium earhonale and nilralc. sodium chromate is formed, which is then extracted with HyO. This is the substance from which chromium compounds are obtained. See also Chromite. 

Acid leaks result in the deactivation of some chemical treatment programs, for example, P-PO4 is hydrolyzed to O-PO4, and chromate (in the presence of ferrous ions) is reduced to trivalent chromium. Also, depassivation of the inhibitor film and corrosion occurs. The correct action is to first stop the leak, then to increase the pH by bleeding off the low pH water and making up, to the maximum extent of the system. If the leak is so severe that the pH drops below 4.3, and free mineral acidity (FMA) occurs, soda ash should be used to restore the pH to 4.3 to 4.5, then discontinued. Caustic soda should not be added, as this action will almost certainly result in heavy fouling from precipitated ferric hydroxide. Dispersants or antifoulants may be required. When the system is stabilized, it should be repassivated and the reason for the acid leak resolved (faulty pH meter ). 

Chromium is used in the manufacture of stainless steel and other specialist steels, and non-ferrous alloys. Chromate salts are used as tanning agents, pigments, catalysts, corrosion inhibitors and in electroplating solutions. Although stainless steel is inert, the dissolution of chromium from this steel is likely to be the major source of chromium in food. 

The general sequence of surface preparation for ferrous surfaces such as iron, steel, and stainless steel consists of the following methods degreasing, acid etch or alkaline clean, rinse, dry, chemical surface treatment, and priming. The chemical surface treatment step is not considered a standard procedure, but it is sometimes used when optimum quality joints are required. It consists of the formation of a corrosion-preventing film of controlled chemical composition and thickness. These films are a complex mixture of phosphates, fluorides, chromates, sulfates, nitrates, etc. The composition of the film may be the important factor that controls the strength of the bonded joint. 

Occluded hydrogen is more reactive chemically than the normal gas. Hydrogenated palladium precipitates mercury and mercurous chloride from an aqueous solution of the dichloride, without any evolution of hydrogen. It reduces ferric salts to ferrous potassium ferricyanide to ferrocyanide chlorine water to hydrochloric add iodine water to hydriodic acid 2 chromates to chromic salts ceric to cerous salts whilst cupric, stannic, arsenic, manganic, vanadic, and molybdic compounds are also partially reduced.3... 

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