ANALYTICAL CHEMISTRY’
580 The tubes comprising the peaks were pooled, 5 ml. of 5 6 hydrochloric acid were added, and the acidic solutions were concentrated t o dryness in vacuo. The amine hydrochlorides were dissolved in 10 ml. of water, and the samples were applied to the paper t o give a t least 5-y of each amine. I n the case of l-methylheptylamine, it was necessary to apply a t least 507 to detect the ninhydrin spot. Ascending chromatograms were developed with the upper layer of a n-butanol-acetic acid-water (48:2:50) system. Rj values for the amines are listed below. Amine
R/
1-Methylheptyl n-Hept31 n-Hexyl ?%-Amyl n-Butyl n-Propy
0.77 0.70 0.65
Peak No.
Rj 0.76 0.72 n.67
0.56
0.59 0.46 0.38
0.44 0.38
DISCUSSIOY
The partition system used to separate the homologous series of amines gave good resolution, but the recoveries of the more volatile amines (propyl and butyl) were IOT. These amines could actually be seen bubbling from the walls of the column. I n an attempt to improve their recovery, the column was jacketed and cooled with tap water. Although the recovery of these amines
Table I.
Analytical Data for Partition Chromatography of Amines
Amine 1-Methylheptyl n-Heptyl n-Hexyl n-Amyl n-Butyl n-Propyl
Meq.
0.0486 0.0485 0.0495 0.0531 0.0461 0.0500
HC1, hleq
0,0479 0.0497 0.0479 0.0508 0.0330 0.0171
Recovery, R
98.j 102,s
96.6 93 . 5
T1.5 34.2
was raised to approximately SO%, the resolution of the octyl, heptyl, and hexyl amines was impaired under these conditions. LITERATURE CITED (1) Fuks, X.A.,and Rappoport, 11 A ,Doklady Akad. Naiik S.S.S.R., 60, 1219 (1948). (2) Goetx-Luthy, Kydia, J . Chem. Educ., 2 6 , 2 7 1 (1949) (3) James, A. T., Bioehem. J . , 5 2 , 2 4 2 (1952). (4) James, A. T., Martin, A. J. P., and Smith, G. H., Zbid., 52, 2 3 s (1952). (a) Lagervist, U., Acta Chem. Scarid., 4, 542 (1950). (6) Roberts, J. C., and Selby, Keith, J . Chem. Soc., 1949, 2785. R E C E I V Efor D review July 2 7 , 1953. -4ccepted Sovember 1 3 , 1053. Puhlished with the approval of the Director of t h e Wisconsin Agricultural Experiment Station. Foundation.
Supported in part by a grant from the Sational Science
Determination of Sulfur in Iron and Steel by Barium Chloride Method After Chromatographic Separation of Sulfuric Acid FOLKE NYDAHL Laboratory o f A n a l y t i c a l Chemistry, University o f Uppsala, Uppsala, Sweden HE
most reliable way of determining sulfur in iron and steel is
Tconsidered t o be the barium chloride method. The sample is dissolved in nitric acid with or without addition of hydrochloric acid; in this way the total sulfur is presumed to be converted into sulfuric acid, which is precipitated and weighed as barium sulfate. Because of the complications caused by interfering substances in the solution-e.g., iron, nitric acid, tungstic acid. and silica-this method is rather laborious if full advantage is taken of its possibilities to give accurate results. Most of the difficulties should; however, be eliminated if, before precipitation, the sulfuric acid were chromatographicslly separated from t,he other constituents of the solution. Such a possibility for simplifying the method has been briefly outlined by Sydalll and Gustafsson ( 4 ) . These authors have demonstrated that sulfuric acid is strongly and selectively adsorbed on aluminum oxide “for chromatographic analysis” even from strongly acidic salt solutions, thus making possible the desired separation. The present work is a detailed study of the method. I n this procedure the aluminum oxide functions as a strongly selective ion exchanger. Of the common anions in acidic solution, only fluoride and dihydrogen phosphate ion are about as strongly adsorbed as hydrogen sulfate ion. The adsorption of dichromate may be classed as moderately strong, and of chloride, nitrate, and perchlorate as weak, increasing in weakness in the order mentioned. The adsorption is reversible: --,41-0
-.41-0
\ -4I+X/
-.4l-O
+ Y-=
\
Al’Y-
--AI-0
/
+ x-
Thus hydrogen sulfate ion easily replaces chloride ion; the opposite reaction would, however, require a very large excess of chloride. -4s perchlorate ion is the most weakly adsorbed, the con-
ditions for adsorption of sulfuric acid are most favorable in n solution containing only perchlorates. IIydroxyl ion is the most strongly adsorbed of the anions; even a t a p H of 9 t o 10 its concentration is sufficiently high for complete predominance Thus the other anions are easily removed from the adsorbent by n-ashing Kith alkaline solutions such as 0,lM ammonia or sodium carbonate. The resulting hydroxyl form of the adsorbent may be converted into the desired anion form by washing with a solution of the corresponding acid. Cations other than hydrogen ions are not adsorbed in aridic solution, or are a t least easily replaced by hydrogen ion. I n acidic solution (HS04-, p H < I), the total adsorptive capacity of aluminum oxide for sulfuric acid is about 0.17 millimole per nil. volume of column; in neutral solution (SO,--) the adForptive capacity is about half that value. The advantage of aluminum oxide over an ion exchange wsin lies in the strong selectivity of the former; therefore its volume need only be adapted to the amount of the strongly adsorbed ions, in this case HS0,- and H,POa-. Thus the volume may be kept small; all interfering ions pass the column or are removed by washing; and t’he sulfuric acid is isolated in a small volume. These are all advantages IThich cannot be obtained by existing ion exchange resins because of their low selectivity. APPARATUS AYD REAGEYTS
The sample was dissolved in a short-necked, round bottomed, 1-liter flask provided with a normal ground splash bulb. The latter may be connected to a condenser when boiling away the excess of acids. Heating the unwetted sides of the flask )vm avoided by placing i t in a suitable hole of an asbestos board. Aluminum Oxide. A commercial preparation “for chromatographic adsorption according to Brockmann” (E. Merck, Darmstadt, Germany, 3) was digested for 1 hour a t room temperature with 1M hydrochloric acid and washed with water, and the larger
V O L U M E 2 6 , NO. 3, M A R C H 1 9 5 4
581
particles were allowed to settle. The fines were decanted off until the residue settled in less than 1 minute for a fall height of 10 em. The particle size is, however, essential only for the flow rate. The oxide may also be dry screened, removing the portion passing a 200-mesh screen. Adsorption Tube (Figure 1). A firmly stoppered plug of borosilicate glass n-ool was placed a t the lower end of the tube and a slurry of the washed alumina in water was poured into the tube until a column of 10 to 12 em. had formed. The tube, fitted with a rubber gasket, u m placed on a filtering flask, and suction was applied. All particles in the upper container were now nashed down and a second plug of borosilicate glass wool was introduced into the upper end of the tube and firmly pressed into contact with the hard surface of the oxide column. The glass rod used phould fit snugly in the tube. It is essential that no oxide particles be left in the upper compartment or mixed with the glass wool plug. If the column contracts, the plug must be pushed int,o vontact x i t h the oxide surface again. The flon- rate obtained, using suction, was 10 to 15 ml. per minute. The sniall air bubbles left in the column when water and air were alternately sucked through are of no consequence for the present purpose. The flonrate without, suction was 2 to 3 hours for 40 ml. of 0.1-11 ammonia. Before being used for the first time, the column was rvashed with 50 ml. of I N hydrochloric acid, 50 ml. of water, and 50 ml. of 1M ammonia. Kow 40 ml. of 0.1M ammonia were run through and the filtrate was caught in a beaker. After acidifying the filt#ratewith an excess of 0.5 ml. of hydrochloric acid (1 to 1) and adding 5 ml. of 0.5~21barium chloride no visible precipitate was obtained after 18 hours. The columns were never left to dry; if they were not to be used for 2 days, they were kept with the narrow portion immersed in water. S o change of the adsorption power has been observed even after a hundred analyses. Concentrated Acids. The acids used were sufficiently pure not to give higher blanks than 1 to 2 mg. of barium sulfate, the larger part of which was formed by the nitric and perchloric acids; the hydrochloric acid was practically free from sulfates. In certain experiments where the amount of nitric acid was varied, sulfates were removed from the acid by distillation. Traces of sulfate in 70% perchloric acid may, if desired, be removed by adsorption on alumina. For this purpose the alumina is purifid as above, acidified xith perchloric acid, washed vith water 011 a Buchner funnel, and dried a t 50" to 70" C. The dry oxide i p poured into an adsorption tube of the proper size and the perchloric acid i q run through without suction. Barium Chloride Solution, 0.5M. This solution was A prepared free from colloidal harium sulfate by adding 1 nil. of 1 J I ferric chloride to 500 ml. of solution, and previpitating the iron as hyfiroxide a t room temperature l)y addition of a small excess of concentrated ammonia. The hariuni sulfate nuclei are caught in the ferric hydroside, which n-as removed h y filtration, discarding the first IO.5 @we) portions of the filtrate. Ammonia Solutions. The qolutions used were prepared B from tank ammonia. The remaining chemicals were of reagent quality.
c
PROCEDURE
During the experimental k several procedures \%-ere t r i e d f o r d i s q o l v i n g the sample as well as for the subqcquent analyqis. T h e method finally adopted run3 as follow:
*
\\ 01
The usual precautions for working with hot concentrated perchloric acid should be obsrrved.
Figure
Adsorption
Tube
All dimensions in millimeters A . Receptacle, 50 ml. B . Aluminum oxide column, 10 to 12 ml. C. Firmly stoppered borosilicate glass wool plugs
Transfer a 5.00-gram sample into the digestion flask. Add 50 ml. of concentrated nitric acid. Measure out 10 ml. of concentrated hydrochloric acid in a small beaker and add it in small portions, with mixing, until reaction occurs. Continue the addition when the reaction subsides until all has been added. Some alloy steels require double the amount of hydrochloric acid. Heat to boiling and boil until solution is complete. Add 50 ml. of i o % perchloric acid and a few chips ( 2 to 3 mm.) of crushed quartz tube. Boil away the nitric and hydrochloric acids until fumes of perchloric acid are seen to condense in the flask. Remaining small amounts of nitric acid do not interfere; it is, however, essential to continue boiling until destruction of carbon is complete, chromium is oxidized to chromate, and silica is separated in an easily filterable form. The time required should be 25 t.o 30 minutes. Allow to cool and dissolve the salts in 50 ml. of water. Transfer the contents into a 250-ml. beaker using no more water than nrcessarj-, mix, and allow the precipitate to settle for at least 15 minutes. Filter through a paper of loose texture into a 250-nil. heaker and wash a few times with perchloric acid (1 t o 100). Filtrate and washings should make 150 to 200 ml. Place an adsorption tube with rubber gasket on a l-lit,er filtering flask. Apply suction, remove ammonia remaining from last elution by washing with about 20 ml. of water, and acidify the column with 5 to 10 ml. of hydrochloric acid (1 to 20). Suck the sample solution through a t a flow rate not exceeding 10 to 15 ml. per minute, wash with a total of about 50 ml. of hydrochloric acid (1 to 20), and finally with about 30 ml. of water. Remove the adsorption tube from the flask, rinse the outer walh of its narrow part with m t e r , and place it in a filter stand over a 100-ml. beaker, free from scratches. Elute the column with 5 ml. of 1 M ammonia, and, when this has run through, follow with 40 ml. of 0.1114 ammonia. The upper borosilicate glass wool plug will prevent back-diffusion of dissolved sulfates and therefore the 0.1,21 ammonia may be added a t one time. After elution the adsorption tube is ready for another analysis. Seutralize the eluate against methyl orange with hydrochloric acid (1 to l ) ,and add 0.5 ml. in excess. .idd 1 ml. of acetic acid and 5 drops of 30% hydrogen peroxide to reduce small amounts of chromates eventually present and to transform them into acetate complexes. A4110wthe blue perchromic acid color obtaiued to fade completely before proceeding with the analysis. Add 3 ml. of 0 . 5 M barium chloride rapidly from a pipet while stirring the solution; take care not to touch the walls of the beaker with the glass rod during this o1)eration. Let the solution stand undisturbed over night. Filter through a 7-cm. paper of close texture, transfer the piecipitate to the filter with warm (50' C . ) water with the aid of a policeman, and finally wash with warm 0.01~11ammonium nitrate solution until the filtrate gives no opalescence with silver nitrate. Ignite in a platinum crucible and weigh, prrferalsly on a semimicrobalance. Make a blank determination, following the same proceduie and using the same amounts of all ieagents. If no higher relative accuracy than 0.5% is aimed at, subtract the blank and use the factor 0.1347 for calculating the amount of sulfur in the sample (the stoichiometric factor reduced by 2.0%). If higher accuracy is desired, calibrate the sulfate determination and apply the corrections as carried out in the folloning section. VERIFICATIOl OF lIETHOD
Sulfate Determination. I t is well known that the gravimetric determination of sulfuric acid as barium sulfate belongs to the less accurate methods of analysis because of the errors caused by coprecipitation phenomcna. Stoichiometrically correct reqults may be obtained by ceitain elperimental conditions but only through compensation of negative and positive errors; these methods are, however, worked out for rather large amounts of sulfate. I t follows that other experimental conditions may be used mith the qame accuracy, if only the results are corrected by an empirically determined factor. This procedure is not often used in practice becauqe the experimental conditions, especially the concentrations, are seldom sufficiently constant to warrant its use; in the present raqe, however, the composition of the eluate is always the same. The problem is to find easily reproducible and convenient conditions for the precipitation; the sulfate concentration should be the only variable. If the barium sulfate could be made to precipitate slowly from a solution of a constant initial barium concentration, good reproducibility should be obtained However, the speed of for-
ANALYTICAL CHEMISTRY
582 mation of the precipitate and, consequently, the size of the particles are often found to vary even under apparently identical experimental conditions. Experiments showed this effect to be caused by the presence of varying amounts of crystal nucleiprobably colloidal barium sulfate-in the barium chloride solution. Barium chloride solutions a few months old, in which d l traces of barium sulfate had settled, gave coarse precipitates, while freshly prepared solutions, irrespective of the chemical used, gave fine precipitates. The colloidal barium sulfate was removed by collecting it in a precipitate of ferric hydroxide. Under the present conditions barium chloride solutions prepared in this way gave slowly appearing, coarse precipitates. Higher temperature, of course, increased the crystal size but also the tendency of the crystals to adhere to the n d l s of the beaker; precipitation a t room temperature was therefore preferred.
Table I.
Determination of Sulfate by Proposed Method
Weight of Precipitate, M g Stand. dev. lfg.BaSO4 lleana of mean BaSOA, 1Ig.b 1.22 1.13 0.025 1.23 12.34 0.027 12.24 12.22 36.58 36.58 37,lY 0.034 a Each result is mean of four determinations. no results are omitted. b Calculated as 0.12 0.9804 (weight of pre'cipitate).
HsSOa, Added,
+
In the analysis of chromium steels it is not possible to remove completely all adsorbed chromate with reasonable amounts of hydrochloric acid; most of the remainder-for a steel with 15% chromium, a t most 0.1 millimole of chromate (Cr03)--will be found in the ammoniacal eluate I n order to prevent its coprecipitation, the chromate was reduced with hydrogen peroxide: the trivalent chromium formed was complexed with acetic acid (6) to decompose the chromium sulfuric acids. The barium sulfate precipitate was finally washed with 0 O l M ammonium nitrate instead of imter, which is usually prescribed; the traces of barium adsorbed by the filter paper because of its ion exchanging properties are more easily removed in this way. I n the determination of the empirical factor for the calculation of the percentage of sulfur of the precipitate, the precipitation conditions must be identical with those of the actual ana1y.i.. This was attained in the following manner: The standard sulfuric acid solution was prepared from redistilled sulfuric acid, standardized against sodium carbonate and borax, and diluted to the proper concentrations. The error in the amount of sulfuric acid in a pipetted portion of the solution was estimated to be less than 0.1%. A solution was prepared containing per liter 19 grams of ammonium chloride, 20 ml. of hydrochloric acid (1 to l), and 40 ml. of glacial acetic acid, and was purified from traces of sulfate by running it through an aluminum oxide column. TFventy-five milliliters of this solution were transferred to a 100-ml. beaker, the proper amount of standard sulfuric acid and 5 drops of hydrogen peroxide were added, and the volume was brought to 45 ml.; the composition of this solution corresponds to that of the actual analysis. The sulfuric acid was precipitated and determined as in the actual procedure. The results (Table I ) are corrected for the weight of the filter ash, 0.09 mg. This must not be determined merely by weighing the ash of unused filters because some irreversible adsorption of barium occurs a t filtration. Consequently it is necessary to weigh the ash of a filter which has been treated with the mother liquor from a preceding precipitation, and then carefully washed. Filters with a mean ash content of 0.018 mg. (Xunktell No. 00, 7 cm.) gave, after this treatment, an ash content of 0.09 zt 0.002 mg. as a mean of four determinations. The precision obtained is good; the standard deviation of a single result has been about 0.06 mg. of barium sulfate fairly irrespective of the amount The methodic error is negative at
very low amounts, positive a t higher, indicating that it is composed of an additive solubility error and a proportional coprecipitation error. The stoichiometric amount of barium sulfate ( y mg.) may be calculated uith good accuracy, within the limits observed from the weight of the precipitate (z mg.) by uqe of the straight line y = 0.12
+ 0.9804~
This formula is obtained from the experimental results of Table I applying the method of least squares. For most practical purposes it should be sufficient to reduce the weight of the precipitate by 2.0%, and omit correction for the filter ash. Smaller amounts than 1 mg. of barium sulfate should not be determined because of incomplete precipitation. Consequently, when blanks are too low, a proper amount of sulfuric acid should be added. The eluate often contains small amounts of chromate, a t most 0.1 millimole The influence of this impurity on the sulfate determination as described was established by precipitation after addition of 0.1 ml. of 1M chromic acid and reduction by hydrogen peroxide. The results presented in Table I1 show that the presence of this amount of chromium has had no significant effect. Low results are, however, obtained if the addition of acetic acid is omitted, or by considerably higher chromium concentrations. Elution Procedure. After Lvashing with hydrochloric acid and water the aluminum oxide contains about 2 meq. of adsorbed anions. The elution is therefore ptarted with more concentrated ammonia-for instance, lM-and brought to an end with 0 131 ammonia; the concentrations are not important. In the preliminary experiments the elution was rapidly performed by suction, the whole operation lasting 2 or 3 minutes. It was, however, found that by this procedure about 1.3% of the adsorbed sulfuric acid remained in the column, irrespective of the initial amount. By slow elution without suction the remaining amount v a s decreased to at most 0.2%. N o correction m s applied for this error. If the column is kept saturated with 0 1 M ammonia over night, the remaining traces of sulfuric acid are easily removed by washing with water preceding its use for the next analysis.
Table 11. Precipitation of Sulfate in Presence of 0.1 Millimole of Chromium Sulfuric acid added, calcd. as Bas04 Weight of precipitate found Corrected weight, Bas04 Standard deviation of mean a Mean of four determinations; no results are omitted.
Ihlg. 12.24 12.36" 12.24 0.012
The chromic acid remaining after the washing with hydrochloric acid behaves in the same manner as sulfuric acid which is the cause of chromate in the eluate in the analysis of chromium steels. The completeness of the elution is further proved by the following experiments. Adsorption Procedure. The quantitative adsorption of sulfuric acid from a solution as obtained in the actual analysis after separation of silica has been demonstrated by Nydahl and Gustafsson (4). Losses by incomplete adsorption may occur through sulfuric acid complexing with other substances, or by too high concentration of other strongly adsorbable anions. I n both cases chromium is the principal interfering element, in the first case as trivalent ion, in the second case as chromate; the percentage of phosphate is too low to interfere. Chromium is present as chromate after evaporation of the sample solution with perchloric acid. It might be possible to reduce the chromate to chromic ions before the adsorption procedure in order to get a more complete separation from sulfuric
V O L U M E 2 6 , NO. 3, M A R C H 1 9 5 4 acid. Experiments proved this to be true a t percentages of chromium in the steel not exceeding 0.1%: a t higher percentages losses of wlfuric acid 1% ere obtained by incomplete adsorption. Thus 1% of chromium caused a loss of 5% of the sulfur. Probably a lover flow rate would give a quantitative yield of sulfuric acid, but such a procedure was considered impracticable, and instead the adsorption in the presence of chromate was investigated. An invcqtigation of the completeness of adsorption is in principle easy to accomplish; the difficulty is to obtain a sample solution with a known percentage of sulfate when all available samples of iron and iron compounds contain interfering amounts of sulfur or sulfates. Mere recovery of an added amount of sulfate is not conclusive, as additive errors cannot be discovered in that way. On that account the following argument was applied. -4 solution is run through a column causing a certain fraction of the sulfuric acid to be adsorbed. The procedure repeated using a frech column will then cause the adsorption of a t least the same fraction of the remaining sulfuric acid. Now a known amount of sulfuric acid is added to the last effluent and the adsorption procedure is repeated, using a frech column. Then, if the recovery i s quantitative, it must have been quantitative also in the first qtep. This procedure was applied on two steel sampler of different percentage of chromium under the conditions of the actual analy4.;. After each adsorption step the solution v a s evaporated to white fumes by boiling to remove the hydrochloric acid added a t xvashing; the known amount of sulfuric acid was added before the last evaporation. The result.; in Table I11 arc the means of four deterniinations on each sample. I n no case ~ 5 . 5 5any visible barium sulfate pre?ipitate obtained in the eluate from the second step column. indicating less than 0.2 mg. of barium sulfate. The recults obtained a t a low percentage of chromium show that no sulfuric acid was lost on evaporation, and adsorption and elution were quantitative. As much as 14% of chromium Y of 1.1% of sulfur; in a single experiment in on 2070 of chromium caused a loss of 1.0% of
583 iulfui. This methodic error may probably be remedied by the use of somewhat larger columns; however, this work was nearly a t an end when this error Tvas discovered, and it was considered inexpedient to change the technique. Dissolving the Sample. Three methods of dissolving the Eample Jvere tried: 1. Use of nitric acid alone. This method is recommended by the Bmerican Society for Testing Materials ( 1 ) for carbon steels. It is not generally applicable to alloy steels. 2. Use of a slightly aarmed 1 to 1 mixture of hydrochloric and nitric acids, passing the gases evolved through fuming nitric acid as recommended by Hammarberg (2). 3. The method described above.
Jn the preliminary evperiments the last-mentioned method gave slightly higher results than the other two. This method Tvas accordingly selected for further experiments, as there qcenis to be no conceivable c a u v for a poqitive error. .iPPLICAT1O.S S
The method described was applied to a number of standard steel samples from the National Bureau of Standards and the Slvedish Jernkontoret (Iron Masters Association). Four determinations were made on each sample (Table I V ) ; no results have been omitted. The blank amounted to 0.00168 i O . O O O O i % as a mean of four determinations. The standard deviation of the mean given in Table I V is the square root' of the sum of variances obtained from blank and sample. The standard deviation of a single result (blank and sample) is in the present case double the tabulated value. The standard deviat,ion of a single result varies within the limits 0.0002 and 0.0005%. The relative methodic error-after the sample has been dissolved-is believed to lie within the limits 0 and -l%, near the latter limit in the analysis of high chromium steels. A possible exception is perhaps the high speed steel Yo, 12, as losses of sulfuric acid map have occurred by adsorption in the large precipitate of tungstic acid. The methodic error in dissolving the sample is not estimated. .4 comparison of the results obtained with those of the Bureau Table 111. Completeness of Adsorption of Standards shows the former, the chromium steels excepted, t o 11g. be slightly low; they agree, however, well with the results of Sulfuric acid added, calcd. as BaSOd 11.78 the Jernkontoret or are insignificantly higher. A detailed study BaSOr foiind a t 3rd adsorption in solution of of the methods involved would be needed for a discussion of t h e 11 .i8u Steel KO.8 g , 0.009% C r Std. d e r . of mean 0.015 slight discrepancies found. Neither in the ASTAT method, nor Steel S o , 7 3 A . 14.09% C r 11.63a in the method commonly used in Sweden, is any correction apStd. d e r . of mean 0,050 plied for the coprecipitation Alean of f
