PROHIBITED COAL TAR COLOURS I N FOODSTUFFS 423 The Determination of Vanadium in Steel.* BY A. T. ETHERIDGE. Ph.D. F.I.C. (Read at the Meeting March 4 1928.) METHODS used for this determination have been described and criticised by Cain (Reprint No. 161 Bulletin of Bureau of Standards). The author of this paper has already endeavoured to improve the ordinary volumetric method (ANALYST 1923, 48 588). I t was pointed out that the process is somewhat limited in its scope, and it is clear that a more accurate method is necessary. The process described in this paper has been in use for the last three years, and beenproved to be accurate for all kinds of steel. Briefly the method consists in removing iron and other interfering metals, leaving a solution of the vanadium which can be determined by the ordinary permanganate titration.This is carried out in two main operations viz. removal of iron as chloride by the ether extraction process followed by removal of the rest of the interfering metals by electrolysis over a mercury cathode. There are no large precipitates to deal with as in Cain’s proposed method (vide su$ra) in which excess of cadmium carbonate is used to precipitate the vanadium from a ferrous sulphate solution. Chromium is also precipitated which is a disadvantage with high chromium steels. The author has adopted Cain’s electrolytic purification of the solution modifying it in detail; in particular the special apparatus used by Cain has been discarded for an ordinary wide beaker of 800 C.C. to 1000 C.C. capacity, containing a layer of half an inch of mercury.The connection with the current is made by a platinum wire enclosed in a thin glass tube sealed at the bottom, with a small length of platinum projecting into the mercury. The beaker is covered by a notched glass. * Communication from the Research Department Royal Arsenal Woolwich 424 ETHERIDGE THE DETER?tlINATION OF VASADIUM I N STEEL There is nothing new in either of these two main operations but the combina-tion of them as a means of determining vanadium has not been published so far as the author is aware. The details of the method as used for an ordinary nickel-chromium-vanadium steel are as follows : PREPARATION OF THE SoLuTIoN.-The steel in the form of drillings (5 grms.), is dissolved in 80 C.C. of strong hydrochloric acid in a tall 800 C.C.beaker oxidised with a few C.C. of nitric acid digested to complete disappearance of black carbide, evaporated to about 15 c.c. cooled and transferred to a large separating funnel, with the minimum quantity of 50 per cent. hydrochloric acid (equal volumes of strong acid and water) from a wash bottle. Two solutions of ether and hydro-chloric acid are necessary; p i x . solution A which consists of 50 C.C. of strong acid with 75 C.C. of ether added slowly while cooling; solution B which consists of 100 C.C. of 50 per cent. hydrochloric acid with 30 C.C. of ether. According to Rothe’s directions for removing ferric chloride by ether from a solution in 50 per cent. hydrochloric acid it is necessary to add 6 C.C. of solution A per grm. of dissolved iron or 30 C.C.for 5 grms. Ether is added till the funnel is about four-fifths full (about 300 c.c.). The contents are then shaken together cautiously a t first and with frequent loosening of the stopper to ease the pressure due to ether vapour. Finally it is vigorously shaken and allowed to stand till a clear separation into two layers has occurred. The lower layer is run out into the original beaker. The ether is washed five or six times with 20 C.C. of solution B together with a few C.C. of hydrogen peroxide (as recommended by Bauer and Deiss Sampling and Analysis of Iron and Steel). Evaporation with hydrochloric acid converts vanadium from the pentavalent into the tetravalent state in which condition Bauer and Deiss state that its chloride is very slightly soluble in ether.Hydrogen peroxide oxidises it back to the pentavalent state, in which it is insoluble in ether. Each aqueous layer as it forms after shaking with solution B is run out into the beaker; besides the vanadium this liquid contains nickel chromium manganese etc.; also some iron which is not entirely removed by thc process; in fact all the alloy metals present in the steel except molybdenum which is retained in the ethereal ferric chloride. It is boiled down to a low bulk after removing the excess of ether on a water bath. A little nitric acid is added to oxidise any organic matter derived from the ether followed by 25 per cent. sulphuric acid and then the liquid is evaporated slowly a t a low temperature until fumes appear. The amount of acid used depends on the amount of metals present; 20 C.C.are sufficient for the ordinary type of vanadium steel under consideration. The ‘‘ fuming” must be done at the lowest possible temperature as otherwise it is difficult to dissolve the an-hydrous sulphates particularly chromium sulphate. It is necessary to eliminate hydrochloric and nitric acids before the next operation but this can be quite well done a t a low temperature if sufficient time is allowed. After cooling the sulphates are dissolved in 100 C.C. water which is heated till a clear solution is obtained. The liquid is passed through a small filter to remove silica neutralised with 50 per cent. This is shaken together and cooled ETHERIDGE THE DETERMISATION OF VANADIUM I N STEEL 425 ammonia (equal volunies of strong ammonia sp.gr. 0.880 and water) and made acid with 15 drops of 25 per cent. siilphuric acid. SEPARATION OF INTERFERING METALS.-It is now ready for the operation of purification from interfering metals. The liquid is transferred to the electrolytic beaker 1 grm. of hydrazine sulphate added and electrolysis carried out a t 4 ampsres the anode being a rotating platinum gauze cylinder such as is used as cathode for copper estimations. The beaker is kept covered as already stated. Hydrazine sulphate serves to reduce ferric to ferrous iron and chromic acid (formed at the anode) to chromium salt and so hastens deposition by saving current which would otherwise be expended on this a t the cathode region. As a matter of fact, hydrazine sulphate is essential when chromium is present.In its absence there is formed chromic acid a t the anode followed by a black precipitate (this has not been investigated) which not only obscures the liquid and prevents the removal of chromium from being followed visually but also hinders the removal of chromium. Iron and nickel (or cobalt) are quickly removed in the amounts usually dealt with. Manganese may go into the mercury to some extent (?) but chiefly gathers on the anode. It is quickly removed when more hydrazine sulphate is added. Manganese is immaterial since it has no influence in the subsequent procedure. It has been found that 0.75 grm. is the largest amount that can be conveniently removed in one day (= 15 per cent. of chromium). During the electrolysis it is necessary to neutralise with 50 per cent.ammonia the free acid formed from time to time. The liquid must not be allowed to become ammoniacal for any appreciable time. The formation of a discoloration indicates too much ammonia which is immediately neutralised by a few drops of 25 per cent. sulphuric acid. Electrolysis is continued for 6 hours after which the liquid is dxwn off through a siphon into a large beaker. When the level of liquid has fallen nearly to the end of the siphon tube about 200 C.C. of hot 5 per cent. am-monium sulphate slightly acidified with sulphuric acid are added carefully so that no mercury is thrown up into the siphon. The liquid is allowed to run out as before and the washing process repeated four times. The current is kept on during the washing and is constant at 4 amp6res by the use of ammonium sulphate solution instead of water.Formation of ammonium amalgam is avoided by having the solution hot and slightly acid. TITRATION OF THE VANADIUM.-The liquid is filtered into another large beaker to remove the small amount of flocculent mercury which usually accompanies it. It is then boiled down to about 200 c.c. the acidity increased by adding about 20 C.C. of 25 per cent. sulphuric acid cooled somewhat and saturated with hydrogen sulphide gas. This removes traces of soluble mercury (also arsenic if present), which quickly coagulates and is filtered off the filtrate being collected in a large flask. After boiling off the hydrogen sulphide gas excess of a saturated solution of potassium permanganate is added to the boiling liquid from a dropping bottle until a permanent precipitate of manganese dioxide results.The liquid is cooled somewhat and after addition of an excess of sulphur dioxide saturated solution, Chromium is the most difficult metal to remove 426 ETHERIDGE THE DETERMINATION OF VANADIUM I N STEEL is boiled vigorously for half-an-hour cooled to 70" C. and titrated with N j l O permanganate solution 1 C.C. N/10 permanganate = 0.0051 grm. of vanadium. Therefore one drop = 0.005 per cent. on 5 grms. of steel. = 0.102 per cent. on 5 grms. of steel. Experience has shown that this titration is accurate to a t least one drop so that the error from this source is less than 0.01 per cent. The following table shows results obtained with vanadium percentages from 0.10 to 1-00 by the use of electrolytic iron and standardised ammonium vanadate solution of which 1 C.C.= 0.001 grm. of vanadium. The ammonium vanadate solution was standardised by the permanganate titration as previously described. Five grms. of electrolytic iron were used in all cases. TABLE I. Vanadium added per cent. 0.10 0.20 0.30 0.40 0.50 0.60 0.70 Vanadium found per cent. 0.10 0-20 0.31 0.40 0.495 0.61 0.70 0.80 0.90 1.00 0-805 0-895 1.01 The following table gives results with nickel and chromium added: TABLE 11. Vanadium added. Per Cent. 0.10 Nickel 3. Chromium 1 0-20 0.30 0.75 Vanadium found. Per Cent. 0.10 0-20 0.30 0.74 0.25 Nickel 5. Chromium 5. 0.26 0.50 0.50 0.75 0.76 British Chemical Standard V steel.0.30 repeatedly. = Nickel nil. Chromium 0-86. It is necessary to consider some special steels. SPECIAL STEELS (1) High Silicon Steels.-Silicon may be present up to 2 per cent. After dissolving as usual and evaporating water is added silica filtered off burnt and heated to fuming with hydrofluoric and sulphuric acids, burnt again and the residue dissolved in hydrochloric acid and added to the main solution which is again evaporated. The rest of the procedure is the same as usual. Vanadium is not co-precipitated with silica but as the latter is gelatinous and difficult to wash it is safer to carry out the process described above. (2) Tungsten Steels.-Hydrochloric acid throws out tungsten as a black residue. According to the nature of the steel and its heat treatment the tungsten is accompanied by vanadium phosphorus iron nickel etc.On oxidising and digesting for some time tungstic oxide is formed and vanadium etc. are occluded ETHERIDGE THE DETERMINATION OF VANADIUM I N STEEL 427 Evaporation is continued till a pasty condition is reached; this is followed by treatment with hot 5 per cent. hydrochloric acid and filtration. The filtrate is evaporated and treated as described for the usual procedure. Recovery of Vanadium fyom Tun& Oxide.-The simplest method is that given by S. G. Clarke (ANALYST 1927 52 466) in which the vanadium is pre-cipitated with cupferron. Prior to this a more laborious but quite accurate method was used which need not be given here. The cupferron precipitate is burnt at a low temperature to vanadium pentoxide and dissolved in the sulphuric acid used in the sulphating process prior to electrolysis.It cannot be held over till the end, because it contains traces of iron etc. derived from the impure tungstic oxide. I t is important that tungstic oxide should be completely removed from the steel as described. It has a tendency to become colloidal which is overcome during evaporation. If it is not all removed a t this stage it will gradually pre-cipitate later on and it has been found that suspended tungstic oxide prevents complete removal of chromium by electrolysis (probably by lowering the over-voltage a t the mercury surface; suspended pulp in a fine state of division has the same effect). In considering the cause of occlusion of vanadium etc. by tungstic oxide it follows that these effects could not be imitated by adding tungstic oxide to elec-trolytic iron; consequently no tests could be made in this way.British Chemical Standard Steel W (16.2 tungsten 3.01 chromium 4.76 cobalt 0.44 nickel) has given on repeated testing a vanadium content of 0-775 to 0.780 per cent, High Chromium Steels.-Up to the present time vanadium has not been found in these steels (stainless steels). Therefore it is better to make a qualitative test first. Owing to the intense green colour of these steels when dissolved it is impossible to carry out the hydrogen peroxide test directly as can be easily done in other steels. The chromium must be removed. This is done by dissolving 1 grm. in hydrochloric acid oxidising with nitric acid and then adding 30 C.C.of 25 per cent. sulphuric acid and evaporating to fumes at a low temperature. After cooling dissolving in hot water (digesting till clear) and filtering from silica, the free acid is neutralised 15 drops of 25 per cent. sulphuric acid are added and the liquid is electrolysed over mercury as described. After a few hours the metals are deposited in the mercury as shown by the disappearance of colour; the electrolyte is then drawn off without washing concentrated by boiling and tested with hydrogen peroxide care being taken to observe the acidity conditions, etc. as laid down by Meyer and Pawletta (2. anal. Clzem. 69 19). It has been mentioned before that not more than 0.75 grm. of chromium can be conveniently removed in one day by electrolysis.Should vanadium be found and the chromium be greater than 15 per cent. there are several ways of over-coming the difficulty: (a) Less than 5 grms. of steel can be taken to bring down the amount of chromium to 0.75 grm. This reduces the accuracy somewhat as com-pared with the ordinary procedure of working on 5 grms. (3 ETHERIDGE THE DETERMINATION OF VANADIUM I N STEEL The solution for electrolysis can be divided into two (or more) parts, electrolysed separately and subsequently re-united. This is the best plan if there is apparatus to spare. Cupferron can be used to precipitate vanadium after the ether extraction process. It has the advantage that chromium is not precipitated but iron (and copper if present) is precipitated as well. It is therefore neces-sary to make a double ether extraction.The aqueous extract from the first ether treatment is returned to the separating funnel and the process repeated with fresh ether. Washing with solution B is carried out as before. The ether is evaporated off, the acidity reduced to about 5 per cent. and 3 grms. of cupferron dissolved in water are added. After standing a short time the precipitate is filtered off. As it is bulky and plastic it is not easy to wash it free from chromium, etc. It is best washed by decantation first the precipitate being pressed with a glass rod to squeeze out most of the liquid. It is not necessary to wash it exhaustively as it must be purified from iron in the subsequent work during which chromium etc. are also separated. The precipitate is burnt at a low temperature dissolved in 20 C.C.of 25 per cent. sulphuric acid the acidity adjusted and electrolysis carried out as described. It is important to burn off at a low temperature as otherwise chromium oxide does not dissolve readily. The rest of the procedure is as usual. Tests made on electrolytic iron with varying amounts of vanadium from 0-10 to 1-00 per cent. and also containing 20 per cent. of chromium have given vanadium figures correct to within 0.01 per cent. This reduces the iron considerably. The results given by method (a) are not quite so good but are correct to 0.02 per cent. with steel containing 1-00 per cent. of vanadium and better than this with 0-10 to 0.50 of vanadium. With method (b) the figures are also correct to within 0.01 per cent. Molybdenum Steels.-As already mentioned molybdenum is removed in the ether and ferric chloride. Any small amount carried forward in the aqueous extract is removed over mercury by electrolysis. High Manganese Steels.-During electrolysis large amounts of manganese dioxide are formed partly on the anode and partly suspended in the liquid. Hydrazine sulphate suppresses it so that the course of the deposition can be followed visually a t any moment. Apparently manganese does not enter the mercury to any extent but this is of no consequence as (like aluminium) it has no influence on the final permanganate titration