Iron ammonium sulphate

Mohr s salt, (NH4)2S04 FeS04,6H20. Iron ammonium sulphate. 

Fe(SO,). (NH)jSO,.6Hp, light-green, water-soluble crystals used in medicine, analytical chemistry, and metallurgy. Also known as iron ammonium sulphate Mohr s salt. 

Dissolve 100 g. of iron alum (ferric ammonium sulphate) in 300 ml. of water at 65°, Pour the solution, with stirring, into a solution of 25 g. of hydroquinone in 100 ml, of water contained in a 600 ml. beaker. The quinhydrone is precipitated in fine needles. Cool the mixture in ice, filter with suction, and wash three or four times with cold water. Dry in the air between filter paper. The yield of quinhydrone, m.p, 172°, is 15 g. It contains a trace of iron, but this has no influence upon the e.m.f, of the quinhydrone electrode provided that the washing of the crude material has been thorough. The quinhydrone should be stored in a tightly-Btoppered bottle. 

An exception to the lone pair or donor electron requirement of organic inhibitors is provided by the quaternary ammonium compounds. Meakins reports the effectiveness of tetra-alkyl ammonium bromides with the alkyl group having C 10. Comparative laboratory tests of commercial inhibitors of this type have been described . The inhibiting action of tetra-butyl ammonium sulphate for iron in H S-saturated sulphuric acid has been described, better results being achieved than with mono-, di- or tri-butylamines . 

Procedure. Weigh out 0.0226 g of hydrated ammonium iron(III) sulphate and dissolve it in 1 L of water in a graduated flask 50 mL of this solution contain 100 g of iron. Place 50.0 mL of the solution in a 100 mL separatory funnel, add 10 mL of a 1 per cent oxine (analytical grade) solution in chloroform and shake for 1 minute. Separate the chloroform layer. Transfer a portion of the latter to a 1.0 cm absorption cell. Determine the absorbance at 470 nm in a spectrophotometer, using the solvent as a blank or reference. Repeat the extraction with a further 10 mL of 1 per cent oxine solution in chloroform, and measure the absorbance to confirm that all the iron was extracted. 

Ammonium iron(II) sulphate solution. Dissolve 10 g of the salt in 100 mL of very dilute sulphuric acid. 

The iron(III) indicator solution consists of a cold, saturated solution of ammonium iron(III) sulphate in water (about 40 per cent) to which a few drops of 6M nitric acid have been added. One millilitre of this solution is employed for each titration. 

Bromides can also be determined by the Volhard method, but as silver bromide is less soluble than silver thiocyanate it is not necessary to filter off the silver bromide (compare chloride). The bromide solution is acidified with dilute nitric acid, an excess of standard 0.1M silver nitrate added, the mixture thoroughly shaken, and the residual silver nitrate determined with standard 0.1 M ammonium or potassium thiocyanate, using ammonium iron(III) sulphate as indicator. 

More accurate results may be secured by adding the nitrite to an acidified solution in which permanganate is present in excess (the tip of the pipette containing the nitrite solution should be below the surface of the liquid during the addition), and back-titrating the excess potassium permanganate with a solution of ammonium iron(II) sulphate which has recently been compared with the permanganate solutioa... 

Procedure A. Prepare an approximately 0.1 JVf solution of ammonium iron(II) sulphate by dissolving about 9.8 g of the solid in 200 mL of sulphuric acid (0.5M) in a 250 mL graduated flask, and then making up to the mark with freshly boiled and cooled distilled water. Standardise the solution by titrating 25 mL portions with standard potassium permanganate solution (0.02M) after the addition of 25 mL sulphuric acid (0.5JVf). 

Procedure. Weigh out accurately an amount of the salt which will contain about 0.25 g of chromium, and dissolve it in 50 mL distilled water. Add 20 mL of ca 0.1 M silver nitrate solution, followed by 50 mL of a 10 per cent solution of ammonium or potassium persulphate. Boil the liquid gently for 20 minutes. Cool, and dilute to 250 mL in a graduated flask. Remove 50 mL of the solution with a pipette, add 50 mL of a 0.1 M ammonium iron(II) sulphate solution (Section 10.97, Procedure A), 200 mL of 1M sulphuric acid, and 0.5 mL of /V-phenylanthranilic acid indicator. Titrate the excess of the iron(II) salt with standard 0.02M potassium dichromate until the colour changes from green to violet-red. 

Standardise the ammonium iron(II) sulphate solution against the 0.02/Vf potassium dichromate, using /V-phenylanthranilic add as indicator. Calculate the volume of the iron(II) solution which was oxidised by the dichromate originating from the chromium salt, and from this the percentage of chromium in the sample. 

Note. Lead or barium can be determined by precipitating the sparingly soluble chromate, dissolving the washed precipitate in dilute sulphuric acid, adding a known excess of ammonium iron(II) sulphate solution, and titrating the excess of Fe2 + ion with 0.02M potassium dichromate in the usual way. 

Procedure. To obtain experience in the method, the purity of analytical-grade potassium chlorate may be determined. Prepare a 0.02M potassium chlorate solution. Into a 250 mL conical flask, place 25.0 mL of the potassium chlorate solution, 25.0mL of 0.2M ammonium iron(II) sulphate solution in 2M sulphuric acid and add cautiously 12 mL concentrated sulphuric acid. Heat the mixture to boiling (in order to ensure completion of the reduction), and cool to room temperature by placing the flask in running tap water. Add 20 mL 1 1 water/phosphoric(V) acid, followed by 0.5 mL sodium diphenyl-amine-sulphonate indicator. Titrate the excess Fe2+ ion with standard 0.02M potassium dichromate to a first tinge of purple coloration which remains on stirring. 

Standardise the ammonium iron(II) sulphate solution by repeating the procedure but using 25 mL distilled water in place of the chlorate solution. The difference in titres is equivalent to the amount of potassium chlorate added. 

Dissolve 4.9 g of ammonium iron(II) sulphate heptahydrate in 150 mL of water and add 2.5 mL of concentrated sulphuric acid. Dilute the solution to 500 mL in a graduated flask. 

Procedure. Prepare an approximately 0.1 M solution of ammonium iron(II) sulphate in dilute sulphuric acid and titrate with the cerium(IV) sulphate solution using ferroin indicator. 

Discussion. Copper(II) ions are quantitatively reduced in 2M hydrochloric acid solution by means of the silver reductor (Section 10.140) to the copper(I) state. The solution, after reduction, is collected in a solution of ammonium iron(III) sulphate, and the Fe2+ ion formed is titrated with standard cerium(IV) sulphate solution using ferroin or AT-phenylanthranilic acid as indicator. 

Procedure (copper in crystallised copper sulphate). Weigh out accurately about 3.1 g of copper sulphate crystals, dissolve in water, and make up to 250 mL in a graduated flask. Shake well. Pipette 50 mL of this solution into a small beaker, add an equal volume of ca AM hydrochloric acid. Pass this solution through a silver reductor at the rate of 25 mL min i, and collect the filtrate in a 500 mL conical flask charged with 20 mL 0.5M iron(III) ammonium sulphate solution (prepared by dissolving the appropriate quantity of the analytical grade iron(III) salt in 0.5M sulphuric acid). Wash the reductor column with six 25 mL portions of 2M hydrochloric acid. Add 1 drop of ferroin indicator or 0.5 mL N-phenylanthranilic acid, and titrate with 0.1 M cerium(IV) sulphate solution. The end point is sharp, and the colour imparted by the Cu2+ ions does not interfere with the detection of the equivalence point. 

Procedure (copper in copper(I) chloride). Prepare an ammonium iron(III) sulphate solution by dissolving 10.0 g of the salt in about 80 mL of 3 M sulphuric acid and dilute to 100 mL with acid of the same strength. Weigh out accurately about 0.3 g of the sample of copper(I) chloride into a dry 250 mL conical flask and add 25.0 mL of the iron(III) solution. Swirl the contents of the flask until the copper(I) chloride dissolves, add a drop or two of ferroin indicator, and titrate with standard 0.1 M cerium(IV) sulphate. 

Procedure. Weigh out accurately about 1.5 g of sodium nitrite and dissolve it in 500 mL of boiled-out water in a graduated flask. Shake thoroughly. Place 50 mL of standard 0.1 M cerium(IV) sulphate in a conical flask, and add 10 mL of 2M sulphuric acid. Transfer 25 mL of the nitrite solution to this flask by means of a pipette, and keep the tip of the pipette below the surface of the liquid during the addition. Allow to stand for 5 minutes, and titrate the excess of cerium(IV) sulphate with standard 0.1 M ammonium iron(II) sulphate, using... 

To 25.0 mL of 0.01-0.015 M persulphate solution in a 150 mL conical flask, add 7 mL of 5 M sodium bromide solution and 2 mL of 3 M sulphuric acid. Stopper the flask. Swirl the contents, then add excess of 0.05M ammonium iron(II) sulphate (15.0mL), and allow to stand for 20 minutes. Add 1 mL of 0.001 M ferroin indicator, and titrate the excess of Fe2+ ion with 0.02 M cerium(IV) sulphate in 0.5 M sulphuric acid to the first colour change from orange to yellow. 

With the exception of iron(II) and uranium(IV), the reduced solutions are extremely unstable and readily re-oxidise upon exposure to air. They are best stabilised in a five-fold excess of a solution of 150g of ammonium iron(III) sulphate and 150 mL of concentrated sulphuric acid per litre [approximately 0.3M with respect to iron] contained in the filter flask. The iron(II) formed is then titrated with a standard solution of a suitable oxidising agent. Titanium and chromium are completely oxidised and produce an equivalent amount of iron(II) sulphate molybdenum is re-oxidised to the Mo(V) (red) stage, which is fairly stable in air, and complete oxidation is effected by the permanganate, but the net result is the same, viz. Mo(III)- Mo(VI) vanadium is re-oxidised to the V(IV), condition, which is stable in air, and the final oxidation is completed by slow titration with potassium permanganate solution or with cerium(IV) sulphate solution. 

Determination of chlorate as silver chloride Discussion. The chlorate is reduced to chloride, and the latter is determined as silver chloride, AgCl. The reduction may be performed with iron(II) sulphate solution, sulphur dioxide, or by zinc powder and acetic (ethanoic) acid. Alkali chlorates may be quantitatively converted into chlorides by three evaporations with concentrated hydrochloric acid, or by evaporation with three times the weight of ammonium chloride. 

Determination of perchlorate as silver chloride Discussion. Perchlorates are not reduced by iron (II) sulphate solution, sulphurous acid, or by repeated evaporation with concentrated hydrochloric acid reduction occurs, however, with titanium(III) sulphate solution. Ignition of perchlorates with ammonium... 

The procedure may be illustrated by the actual results obtained for the potentiometric titration of 25.0 mL of ca 0.1 M ammonium iron(II) sulphate with standard (0.1095M) cerium(IV) sulphate solution using platinum and saturated calomel electrodes ... 

Prepare 250 mL of 0.02 M potassium dichromate solution and an equal volume of ca 0.1 M ammonium iron(II) sulphate solution the latter must contain sufficient dilute sulphuric acid to produce a clear solution, and the exact weight of ammonium iron(II) sulphate employed should be noted. Place 25 mL of the ammonium iron(II) sulphate solution in the beaker, add 25 mL of ca 2.5M sulphuric acid and 50 mL of water. Charge the burette with the 0.02 M potassium dichromate solution, and add a capillary extension tube. Use a bright platinum electrode as indicator electrode and an S.C.E. reference electrode. Set the stirrer in motion. Proceed with the titration as directed in Experiment 1. After each addition of the dichromate solution measure the e.m.f. of the cell. Determine the end point (1) from the potential-volume curve and (2) by the derivative method. Calculate the molarity of the ammonium iron(II) sulphate solution, and compare this with the value calculated from the actual weight of solid employed in preparing the solution. 

Repeat the experiment using another 25 mL of the ammonium iron(II) sulphate solution but with a pair of polarised platinum electrodes. Set up... 

The chromium in the substance is converted into chromate or dichromate by any of the usual methods. A platinum indicator electrode and a saturated calomel electrode are used. Place a known volume of the dichromate solution in the titration beaker, add 10 mL of 10 per cent sulphuric acid or hydrochloric acid per 100 mL of the final volume of the solution and also 2.5 mL of 10 per cent phosphorus) V) acid. Insert the electrodes, stir, and after adding 1 mL of a standard ammonium iron)II) sulphate solution, the e.m.f. is measured. Continue to add the iron solution, reading the e.m.f. after each addition, then plot the titration curve and determine the end point. 

Discussion. Dead-stop end point titrimetry may be applied to the determination of nitrate ion by titration with ammonium iron( II) sulphate solution in a strong sulphuric acid medium ... 

Two platinum electrodes are immersed in sulphuric acid of suitable concentration containing the nitrate ion to be determined and a potential of about 100 millivolts is applied. Upon titration with 0.4M ammonium iron(II) sulphate solution there is an initial rise in current followed by a gradual fall, with a marked increase at the end point the latter is easily determined from a plot of current against volume of iron solution added. The concentration of water should not be allowed to rise above 25 per cent (w/w). The temperature of the solution should not exceed 40 °C. 

Ammonium iron(II) sulphate solution, ca 0.4M. Dissolve about 15.6 g, accurately weighed, of ammonium iron(II) sulphate in 100 mL of Solution A. Potassium nitrate solution, ca 0.3 M. Dissolve about 3.0 g, accurately weighed,... 

Chromium in steel Discussion. The chromium in the steel is oxidised by perchloric acid to the dichromate ion, the colour of which is intensified by iron (III) perchlorate which is itself colourless. The coloured solution is compared with a blank in which the dichromate is reduced with ammonium iron(II) sulphate. The method is not subject to interference by iron or by moderate amounts of alloying elements usually present in steel. 

Standard solution of iron(III). Use method (a), (b) or (c). (a) Dissolve 0.7022g ammonium iron(II) sulphate in 100mL water, add 5mL of 1 5 sulphuric acid, and run in cautiously a dilute solution of potassium permanganate (2 g L 1) until a slight pink coloration remains after stirring well. Dilute to 1 L and mix thoroughly. lmL = 0.1mg of Fe. (6) Dissolve 0.864 g ammonium iron(III) sulphate in water, add 10 mL concentrated hydrochloric acid and dilute... 

Iron (III) solution, 0.05M. Dissolve about 12.0 g, accurately weighed, of ammonium iron(III) sulphate in water to which a little dilute sulphuric acid is added, and dilute the resulting solution to 500 mL in a graduated flask. Standardise the solution with standard EDTA using variamine blue B as indicator (Section 10.56). 

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