excess, and neutralise with HCl, using litmus paper as an indicator, a small piece of the paper being floated in the liquid. Add an excess of 10 c.cs. 10E. HCl and dilute with distilled water to 250 c.cs. The solution is now titrated with the ferrocyanide from a burette, the solution being run in rapidly, at first a few c.cs. at a time, until nearing the finishing point. A drop is tested, after each addition, by a drop of uranium acetate on the porcelain slab. Finish carefully, and as soon as the drop of uranium solution turns brown read the burette. Treat the duplicate in the same way, and calculate the value of one c.c. of the ferrocyanide solution. The neutralisation and addition of acid are for the purpose of having the conditions of the solution on standardisation similar to those in the actual analysis. 3 The Analysis.-Weigh out 5 gm. of the sampled and finely powdered ore. Transfer to a porcelain dish or casserole. Add 15 c.cs. aqua regia. Cover, and when dissolved rinse the cover and evaporate to dryness. If particles of the ore remain undecomposed add gradually 15 c.cs. of the KClO3,HNO solution. Cover, and warm gently until greenish fumes cease to be given off. Then boil just to dryness on the hot plate. Cool. Add 7 gms. solid NHCl, 15 c.cs. 20E. NH,HO, and 25 c.cs. hot water. Boil for a minute, stirring with a rod to break up any lumps or clots. Filter into a 250 c.c. E. flask and wash several times with hot NHẠC. If a considerable precipitate has been formed it should be gently scraped off the paper with the assistance of the wash bottle and transferred to a porcelain dish, the excess of water evaporated and 15 c.cs. HNO„KCIOŽ. Treat as before with the NH HO,NH4Cl and water. Filter and combine the first and second filtrates. This treatment of the first precipitate is for the purpose of removing any traces of zinc hydrate that may have been precipitated with the iron. If the combined filtrate is blue in colour, copper is present and must be removed as follows:-Neutralise with HCl and then add 10 c.cs. 10E. HCl. Heat to about 70° C. Now add a number of the aluminium strips. Shake till the copper is all precipitated. If, however, the copper is absent, neutralise the combined filtrates with HC), add then 10 c.cs. 10F. HCl, and the solution, if cool, is ready for titrating. Titrate as before, and note the volume used. Repeat the determination on a fresh sample of the ore. Duplicates should agree within 2%. Report the results in percentage of zinc. Note. The student should note the importance, when dealing with complex ores, of standardising the solution under similar conditions to those existing in the solution to be analysed. (f) THE VOLUMETRIC ESTIMATION OF ARSENIC (Pearce's Method). Apparatus, Reagents. The usual apparatus. For precipitation E. AgNO3 may be used. For the standard solution NH CNS is used, and as an indicator a saturated solution of iron alum. For the solution of the silver arseniate, pure HNO, is required. 3 Method, Reactions.-The ore containing the arsenic is fused with a mixture of KNO, and NaCO3, the arsenic being thus oxidised to sodium arseniate and potassium arseniate. fully neutralising the solution and On dissolving this arseniate and careadding neutral AgNO, in slight excess a brick red precipitate of Ag AsO is obtained, 4 Na,HASO, +3AgNO1 = Ag2AsO, +2NaNO3 + HNO3 4 or NaH2AsO+3AgNO1 = AgAsO + NaNO3 + 2HNO3 4 To be complete, this precipitation must take place in a neutral solution; and as some HNO is liberated by the reactions, a second neutralisation is necessary if the amount of arsenic is large. The solution of Ag AsO4 is dissolved in HNO, a little iron alum added, and a standard solution of NHCNS run in till by excess a brownish red is obtained. NH,CNS+AgNO3=NH,NO3 + AgCNS From the silver found and the formula Ag AsO, the quantity of arsenic is calculated. As a large amount of AgAsO, is formed from a small amount of As, the sample for analysis may be taken as 5 gm. ore when poor in arsenic and 2 gm. ore when rich in arsenic. Arsenical pyrites may be taken for analysis. Preparation of the Standard Solution.-As the salt is deliquescent it cannot be weighed very accurately, therefore weigh out roughly about 8 gms. NH CNS. Dissolve in distilled water and make up to 1 litre at N. about 16° C. Roughly this is an solution. 10 Checking the Standard.-Weigh out in duplicate about 2 gm. test silver (see Assaying). Dissolve in a flask in a small quantity of pure 5E. HNO3. When dissolved make up to about 150 c.cs. Add 5 c.cs. of the iron-alum solution and titrate with the NH CNS till the brown ferric thiocyanate just appears. N. 10 Calculate the value of 1 c.c. in terms of silver, and from the formula AgAsO calculate the value of the solution in terms of arsenic. 4 N. 10 = x Assume, for example, that 1 C.C. NH,CNS gms. Ag, then The Analysis.-Weigh out 2 gm. (or suitable quantity) of the sampled and very finely powdered ore. Transfer to a square of glazed paper, and carefully mix with about 3 gms. of a mixture of finely powdered Na,CO, and KNO. Transfer to a small porcelain crucible. Gradually heat to fusion and fuse for 4 or 5 minutes. Allow to cool. Add a little warm water; set in a 10 cm. porcelain basin containing a little water. Gently warm till the mass is dissolved. Remove from the heat. Wash the upper portion of the crucible (which may have fallen over on its side) with the wash jet. Remove the crucible on a glass rod, washing it down with the jet. The crucible may now be taken in the fingers and its interior examined, and any traces of the fusion removed by further treatment with water. Acidify the filtrate with nitric acid and boil to expel any nitrous fumes and carbonic acid. Cool and neutralise as follows:-Float a small piece of litmus paper in the liquid, and then add drop by drop 5E. NH HŌ until the litmus paper turns blue. Make slightly acid with one or two drops of 16E. HNO3. Drop in a fresh piece of litmus paper and gradually add E. NH, HO, so that on standing for half a minute the paper just turns blue. If a considerable precipitate is thus formed, filter and wash well. Now add a neutral solution of AgNO, in slight excess. Stir, and note the colour of the litmus paper. If red, again neutralise. Filter off the red precipitate of arseniate of silver and wash well with cold water. Place under the funnel a 300 c.c. flask, and dissolve the precipitate with pure 5E. HNO3. Wash the paper with distilled water acidified with HNO, bringing the total volume of the washings up to about 200 c.cs. Add 5 c.cs. of the iron-alum solution. N. Titrate as before with NH CNS. Repeat the estimation on a fresh. portion of the ore. From the value of 1 c.c. calculate the percentage of arsenic present. With some practice this method will be found both accurate and rapid, therefore well suited for the work of the metallurgical chemist. (9) THE VOLUMETRIC ESTIMATION OF MANGANESE BY POTASSIUM PERMANGANATE. (Volhard's Method). Aparatus, Reagents.-The usual apparatus. For the standard solution KMnO is required. Zinc oxide free from Mn is used for neutralisation. For analysis the student may take a ferro-manganese ore (containing Fe and Mn). 8 Method, Reactions. If a dilute, neutral or faintly acid solution of a manganese salt be heated to boiling, and a solution of K,MnO added, a brownish red precipitate of hydrated MnO, is formed KMn2O + 3MnSO4 + 2H2O 8 =K,SO, +5MnO + 2H2SO4 and as soon as the permanganate is in excess the characteristic rose colour appears. The reaction is exact in a neutral solution containing no iron or chromium, therefore excess of acid is to be avoided, and any iron and chromium present must be removed before titration. This condition is obtained by adding an emulsion of zinc oxide (pure) in slight excess, the acid being neutralised and ferric hydroxide precipitated. N. used for iron will serve again if any remain. If not prepare a ganate solution as before. Checking the Standard. If the solution previously used is available it should again be checked by titration against metallic iron (wire). If the solution be freshly prepared it will also be checked in this way- 1 c.c. N. 10 KMnOg should equal 00165 gm. Mn. The value in terms of Mn may be obtained by multiplying the value in terms of Fe by 2946. The Analysis.—Weigh out 5 gm. of the sampled and finely powdered ore. Transfer to a casserole or porcelain dish. Add 2 c.cs. 10E. HCl, 4 c.cs. 16E. HNO3, and 6 c.cs. 10E. H.SO. Heat on the iron plate till dense white fumes are evolved, and continue at this heat about three minutes. Transfer the contents of the porcelain to a 500 c.c. flask. If any particles of the ore appear undissolved (of the original colour) repeat the acid treatment and combine the solutions. Wash out the casserole or dish with hot water, and to the contents of the flask add an emulsion of zinc oxide till the acid is neutralised and the iron is precipitated as Fe, (HO) 6. agitation hastens the precipitation of the iron. Test the progress of the precipitation by removing a drop of the solution and adding to it in a small test tube a little of the zinc oxide emulsion. Shake, and notice if a precipitate forms. Pour back the test into the main solution. When the precipitation is complete any appreciable excess of zinc oxide. must be avoided. Dilute with distilled water to the mark. settled remove with a pipette 100 c.cs. of the Shake and let settle. When clear liquid to a 300 c.c. flask. N. Heat to boiling over a flame, and titrate with KMnOg, shaking after each 10 addition until a permanent pink is obtained, visible at the edges of the clear liquid by holding a sheet of white paper behind the beaker. If this titration has taken some time the solutions should again be heated to boiling. If the pink tint disappears a little more K,Mn,Og is required. If the percentage of manganese is low it will be advisable to take 200 c.cs. or more for titration. When titrating hot liquids the student must be careful that the steam does not appreciably heat the burette. As a rule, if the titration be performed quickly there is little danger of thus introducing error, but if prolonged the student had better make a connection leading sideways from the burette jet, so that the burette is not vertically above the rising steam. The results of this analysis are calculated in percentages as usual, and should agree to 1%. Conclusion to Chapter VII. Of the examples given of precipitation-volumetric methods the majority are in daily use by metallurgical chemists, and many of these methods, when employed intelligently, conform closely to the ideal typical method of technical analysis, the two leading demands of which are accuracy and speed. The student who has carefully and intelligently followed out the descriptions given should now be able to avail himself of the full store of information in the works mentioned throughout the text, and on studying these works he will obtain a fuller idea of the practical importance of this branch of volumetric analysis. CHAPTER IX. IV. COLORIMETRIC ANALYSIS. THIS method of analysis is generally applied to the determination of small percentages of an element or compound, and in this chapter the following will be considered : (a) The Colorimetric Estimation of Copper. (b) The Colorimetric Estimation of Carbon in Steel. And though not coming under this head, (c) The Volumetric Estimation of Copper by Potassium Cyanide. This last method is of considerable importance to the metallurgical chemist, and, strictly speaking, cannot be introduced under any of the five heads laid down; therefore, for convenience, it is introduced here. (a) THE COLORIMETRIC ESTIMATION OF COPPER. Apparatus, Reagents. The usual apparatus, including two burettes or Nessler tubes. For preparing the comparison standard pure electrolytic copper is required. For analysis the student may obtain a sample of copper slag. Method, Reactions.-A certain amount of pure copper is dissolved in a definite quantity of HNO3, and a known amount of NH HO added. After dilution to a given volume, the solution is transferred to a burette. The slag is so treated that the copper is separated out, preferably in the metallic form, and this separated copper is treated exactly the same as the pure copper, and is transferred to another burette or tube without dilution. Distilled water is added to the assay solution till the colours in the two tubes are exactly the same. Note the volume of the assay solution; and as the value of 1 c.c. of the comparison standard is known, the value of the assay solution is easily calculated. This method was originated by Heine. For other colorimetric methods for copper consult the works of Beringer, Furman, and Sutton. The Comparison Standard. In a flask with a small funnel in its neck dissolve 1 gm. of pure electrolytic copper in the smallest possible quantity of nitric acid. When dissolved, dilute and transfer to the 1000 c.c. flask, and make up to the mark at 16° C. Ten c.cs. of this solution contain '01 gm. copper. add 1 c.c. 16E. HNO3 and 4 c.cs. 20E. NH HO. the Nessler tube, rinsing the beaker out with distilled To 10 c.cs. of this solution Transfer this solution to water. If a burette is used, the student must ascertain the volume of the burette from the 50 c.c. mark to the upper edge of the glass stopper. This may be |