1、BRITISH STANDARD BS 6200-3.10.4: 1985 ISO 4140:1979 Sampling and analysis of iron, steel and other ferrous metals Part 3: Methods of analysis Section 3.10 Determination of chromium Subsection 3.10.4 Ferrochromium and ferrosilicochromium: potentiometric method UDC 669.1:543.257.1:546.76BS6200-3.10.4:
2、1985 This British Standard, having been prepared under the directionof the Iron and Steel Standards Committee, was published under the authority ofthe Board of BSI and comes intoeffect on 28February1985 BSI 10-1999 The following BSI references relate to the work on this standard: Committee reference
3、 ISM/18 Draft for comment 78/70865 DC ISBN 0 580 14265 5 National foreword This Subsection of BS 6200 has been prepared under the direction of the Iron and Steel Standards Committee. It is identical with ISO4140:1979 “Ferrochromium and ferrosilicochromium Determination of chromium content Potentiome
4、tric method” published by the International Organization for Standardization (ISO). Terminology and conventions. The text of the International Standard has been approved as suitable for publication as a British Standard without deviation. Some terminology and certain conventions are not identical wi
5、th those used in British Standards; attention is drawn especially to the following. The comma has been used as a decimal marker. In British Standards it is current practice to use a full point on the baseline as the decimal marker. Wherever the words “International Standard” appear, referring to thi
6、s standard, they should be read as “British Standard”. Cross-reference. At present there is no corresponding British Standard for ISO3713 which is still at draft stage. Appropriate procedures from ISO3713 will be incorporated in BS6200-2 “Methods of sampling and sample preparation”, which will be pu
7、blished in due course. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages T
8、his document comprises a front cover, an inside front cover, pagesi andii, pages1 to4, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. Amendme
9、nts issued since publication Amd. No. Date of issue CommentsBS6200-3.10.4:1985 BSI 10-1999 i Contents Page National foreword Inside front cover 1 Scope and field of application 1 2 Reference 1 3 Principle 1 4 Reagents 1 5 Apparatus 2 6 Sample 2 7 Procedure 2 8 Expression of results 3 9 Reproducibili
10、ty 3 10 Test report 3 Publication referred to Inside back coverii blankBS6200-3.10.4:1985 BSI 10-1999 1 1 Scope and field of application This International Standard specifies a potentiometric method for the determination of the chromium content of ferrochromium and ferrosilicochromium. The method is
11、 applicable to all grades of these ferroalloys. 2 Reference ISO 3713, Ferroalloys Sampling and preparation of samples General rules 1) . 3 Principle Fusion of a test portion with sodium peroxide, dissolution of the melt in water, and acidification of the aqueous solution with sulphuric acid; alterna
12、tively, for some ferroalloys, dissolution of a test portion by direct acid attack. Oxidation of chromium with ammonium persulphate in the presence of silver ions as catalyst. Determination of the sum of chromium and vanadium by potentiometric titration with ammonium iron(II) sulphate. Reoxidation of
13、 vanadium with potassium permanganate; destruction of the excess of permanganate by nitrite, and destruction of the excess of nitrite by urea. Determination of vanadium by potentiometric titration with ammonium iron(II) sulphate, and determination of the chromium content by difference. The reactions
14、 are the following: 4 Reagents During the analysis, use only reagents of recognized analytical grade, and only distilled water or water of equivalent purity. 4.1 Sodium peroxide 4.2 Sodium carbonate 4.3 Ammonium persulphate (NH 4 ) 2 S 2 O 8 . 4.4 Urea 4.5 Phosphoric acid, 1,7g/ml. 4.6 Nitric acid,
15、1,42g/ml. 4.7 Hydrofluoric acid, 1,14g/ml. 4.8 Sulphuric acid,50% (V/V) solution. To 400ml of water, add cautiously500ml of sulphuric acid ( 1,84g/ml), mix, cool, dilute to1000ml and mix. 4.9 Hydrochloric acid, 40% (V/V) solution. To 600ml of water, add400ml of hydrochloric acid ( 1,19g/ml), and mix
16、. 4.10 Silver nitrate solution Dissolve 5g of silver nitrate in water and dilute to1000ml. 4.11 Potassium permanganate solution Dissolve 2,5g of potassium permanganate in water and dilute to100ml. 4.12 Potassium nitrite solution Dissolve 1g of potassium nitrite in water and dilute to100ml. 4.13 Pota
17、ssium dichromate Dissolve pure potassium dichromate in water, recrystallize, dry the crystals at150 C and melt them at415 C. Grind the product to about8 mesh and store in a glass bottle with a ground glass stopper. NOTEThe potassium dichromate reagent may be purchased as high-grade material for volu
18、metric analysis and dried at105 C before use. 4.14 Ammonium iron(II) sulphate, approximately0,17N standard volumetric solution. 4.14.1 Preparation Dissolve 65g of ammonium iron(II) sulphate in500ml of water in a1000ml volumetric flask. When dissolution is complete, add100ml of the sulphuric acid sol
19、ution (4.8), cool, make up to the mark and mix. 4.14.2 Standardization Transfer 1,000 0,0002g of the potassium dichromate (4.13) to a600ml beaker. Dissolve in100ml of water, add30ml of the sulphuric acid solution (4.8) and dilute to300ml. Proceed as described in7.2.3 and7.2.4. The chromium equivalen
20、t, T, of the ammonium iron (II) sulphate solution, expressed in grams of chromium corresponding to1ml of solution, is given by the formula where V 1is the volume, in millilitres, of the ammonium iron(II) sulphate solution added. 1) At present at the stage of draft. T 0,353 5 V 1 - =BS6200-3.10.4:198
21、5 2 BSI 10-1999 5 Apparatus Usual laboratory equipment and in particular. 5.1 Crucible, of capacity40ml, of iron, nickel, zirconium, alumina or pocelain. 5.2 Beaker, of capacity600ml. 5.3 Potentiometer Any high-impedance voltmeter of the type used for pH measurements is suitable. 5.4 Calomel/platinu
22、m electrodes 5.5 Motor-driven stirrer, or a magnetic stirrer. 6 Sample Use a sample prepared in accordance with ISO3713. 7 Procedure 7.1 Test portion Take a test portion of 0,5 0,0002g. 7.2 Determination 7.2.1 Place the test portion (7.1) in the crucible (5.1), containing10g of the sodium peroxide (
23、4.1), or7g of the sodium peroxide (4.1) and3g of the sodium carbonate (4.2), and mix carefully. Holding the crucible in tongs, swirl it over a flame, heating gently at first, then more strongly until fusion is complete and particles of metal disappear from the bottom of the crucible (approximately6m
24、in). Allow the crucible to cool. Place the crucible in the beaker (5.2). Add carefully approximately150ml of water and cover with a watch glass. Heat carefully, and, after effervescence has ceased, allow to cool. Add40ml of the sulphuric acid solution (4.8). When the solution is completely clear, ri
25、nse the watch glass with a jet from a wash-bottle and remove the crucible after having thoroughly washed it with water. Dilute the solution to approximately300ml. 7.2.1.1 Dissolution of low carbon ferrochromium(0,5% C max.) soluble in acids Transfer the test portion (7.1) to the beaker (5.2). Add25m
26、l of the sulphuric acid solution (4.8) and heat very gently, avoiding reduction in volume, for20min or until solvent action has apparently ceased. 2)Add5ml of the phosphoric acid (4.5) and evaporate just to fuming. Cool, add40ml of cold water, heat to boiling, oxidize with the nitric acid (4.6) and
27、boil for2min. Dilute to400ml with hot water. 7.2.1.2 Dissolution of ferrosilicochromium (1% C max.) soluble in acids Place the test portion (7.1) in a platinum dish, add10 to15ml of the hydrofluoric acid (4.7) and, carefully, drop by drop,15ml of the nitric acid (4.6). Heat the contents of the dish
28、until the test portion is completely decomposed. Add20ml of the sulphuric acid solution (4.8) and5ml of the phosphoric acid solution (4.5) and evaporate until sulphuric acid fumes appear. Cool the dish, rinse the walls with water and again evaporate until sulphuric acid fumes appear. Cool the dish,
29、dissolve the salts in40to60ml of water and transfer the solution into a600ml beaker. Rinse the dish with water and collect the washings in the beaker. Dilute to300ml and heat until dissolution of salts is complete. 7.2.2 Add 20 ml of the silver nitrate solution (4.10), 3 drops of the potassium perma
30、nganate solution (4.11), and8g of the ammonium persulphate (4.3) and boil for10min. 3)Add0,5ml of the hydrochloric acid solution (4.9) to reduce excess permanganate and continue to boil for5min after the permanganate colour has disappeared. 4)Allow the solution to cool to room temperature. 7.2.3 Add
31、10ml of the phosphoric acid (4.5) in the case of dissolution as in7.2.1, and5ml of phosphoric acid in the case of dissolution as in7.2.1.1 and7.2.1.2. Set up the stirrer (5.5). Place the electrodes (5.4) in the solution and connect them to the potentiometer (5.3). 7.2.4 Titrate with ammonium iron(II
32、) sulphate solution (4.14), with continuous stirring, to a potential deflection of about200mV. 5)The amount of ammonium iron(II) sulphate solution used corresponds to chromium + vanadium. 2) It is important to ensure that the whole of the test portion is covered by the acid and that no particles adh
33、ere to the wall of the beaker during transfer of the test portion or following shaking of the beaker during dissolution. 3) If the colour of permanganic acid does not develop after boiling for5min, make0,5g additions of ammonium persulphate to the hot solution until the colour is obtained. 4) If the
34、 colour of permanganic acid is not discharged, make further0,5ml additions of the hydrochloric acid solution, boiling after each addition, until the colour disappears. 5) If the chromium content of the sample exceeds31% (m/m), start by adding50ml of the ammonium iron(II) sulphate solution from a pip
35、ette; if it exceeds68%. (m/m), start by adding100ml.BS6200-3.10.4:1985 BSI 10-1999 3 7.2.5 Add the potassium permanganate solution (4.11) drop by drop until a distinct pink colour which lasts for more than1min is obtained 6) . Add the potassium nitrite solution (4.12) drop by drop until the permanga
36、nate colour disappears and a change in potential is observed, then2drops in excess. Add approximately0,5g of the urea (4.4) to destroy excess nitrite. Titrate with the ammonium iron(II) sulphate solution to a potential deflection. This titration corresponds to the vanadium content. 8 Expression of r
37、esults The chromium content, expressed as a percentage by mass, is given by the formula where 9 Reproducibility Experience has shown that the95% confidence limits for an experienced operator are 0,20% (m/m) for chromium contents in the neighbourhood of70% (m/m). 10 Test report The test report shall
38、include the following particulars: a) the reference of the method used; b) the results and the method of expression used; c) any unusual features noted during the determination; d) any operation not included in this International Standard or regarded as optional. 6) The appearance of the pink colour
39、ation indicates the reoxidation of vanadium. This may also be recognized by the corresponding increase in potential. T is the chromium equivalent of the ammonium iron(II) sulphate solution (4.14), expressed in grams of chromium corresponding to1ml of solution; V 2 is the volume, in millilitres, of t
40、he ammonium iron(II) sulphate solution (4.14) used in the first titration (chromium + vanadium); V 3 is the volume, in millilitres, of the ammonium iron(II) sulphate solution (4.14) used in the second titration (vanadium); m is the mass, in grams, of the test portion (7.1). 100TV 2 V 3 () m -4 blank
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