T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERINO CHEMISTRY
Nov., 1921
1053
The Determination of Chromium in Ferrochromium by Electrometric Titration' By G. L. Kelley and J. A. Wiley CHEMICALLABORATORY, MIDVALL?STEEL& ORDNANCGCo., NICETOWN PLANT,PHILADELPHIA, PA.
A paper on the determination of vanadium and chromium in steel was published by Kelley and Conant,2 and more recently a paper by Kelley and other collaborators3 describes the determination of vanadium and chromium in ferrovanadium. I n these cases the end-point was determined electrometrically. The present paper describes a method for the determination of chromium in ferrochromium in which the end-point in the titration is taken as the point of greatest change in the oxidation-reduction potential during the titration of the chromic acid with ferrous sulfate. The description of apparatus suited to this determination has been given by Kelley, Adams and Wiley.4 STANDARD SOLUTIONS POTASSIUM DICHROMATE SOLUTION-5.6586 g. Of the recrystallized and fused salt in enough water to make one liter. FERROUS AMMONIUM SULFATE SOLUTION^^ g. of ferrous ammonium sulfate and 100 cc. of sulfuric acid fsp. gr. 1.58) in enough water to make one liter. This should be standardized daily by comparison with the dichromate solution which is used as the standard of reference. HYDROCHLORIC ACID soLuTroN-One part of concentrated acid and three parts of water. SILVER NITRATE SOLUTION-2.5 g. of silver nitrate in enough water to make one liter. A M M O NIUM PERSULBATE SOLUTIO N-1 00 g. arnmonium persulfate and water enough to make one liter.
METHOD Use a nickel crucible of about 60-cc. capacity, preferably fret. from mangancse. Fuse 20 g. of sodium carbonate in the crucible, and during cooling rotate it in such fz manner as to produce a lining on the crucible. When cool, place 16 g. of sodium peroxide in the crucible. Make a hole in the center of this and place in it 1 g. of ferrochromium, ground to pass a 100-mesh sieve. Mix the ferro-alloy with the peroxide by stirring with a stiff platinum wire, taking care that the alloy does not sink to the bottom. Fuse over a blast lamp until fusion is complete, and apply heat enough to maintain the melt in a state of quiet fusion for 3min., rotating gently meanwhile. Avoid fusing the lining as this is detrimental to the crucible. During cooling, cause the molten mass to flow in thin layers on the surface of the crucible. When cold, wipe the outside of the crucible clean. and place it in 300 cc. of water contained in a 600-cc. beaker. Warm to complete the solution of the mass, and remove the crucible after rinsing. Boil the solution a t least 30 min. Cool to room temperature, and add gradually 80 cc. of sulfuric acid (sp. gr. 1.58). Boil 5 min., cool, filter through asbestos, and make the volume up to exactly 1 liter. Analyze the solution as soon as i t is prepared. Remove a 100-cc. portion, add 25 cc. of sulfuric acid, and titrate electrometrically with ferrous ammonium sulfate and potassium dichromate. A modification of the method involves the following procedure: To 100 cc. of the solution, add 25 cc. of sulfuric acid, 10 cc. of silver nitrate solution, and 40 cc. of ammonium persulfate solution. Boil 10 min. after the permanganic acid color appears. Then add 5 cc. of 1 - 3 hydrochloric acid, boil an additional 5 min., cool, and titrate electrometrically with ferrous sulfate. I
Received April 28, 1921.
e
I b i d . , 9 (1917),780.
* THISJOURNAL,8 (IQlS), 719. * I b i d , 18 (1921). 939
For the highest accuracy, the potassium dichromate should be recrystallized and fused. The dichromate solution is of such strength that each cubic centimeter corresponds to 2.00 per cent of chromium in a 0.10-g. sample. To calculate, multiply by 2 the cubic centimeters of dichromate solution to which the ferrous sulfate solution used is equivalent. This gives the result directly in per cent. Vanadium is rarely present in ferrochromium in quantities warranting its determination. When it is desirable to determine it, this may be done, after reduction of the chromate and vanadate with ferrous sulfate, by oxidizing the vanadium alone by means of nitric acid. It is then titrated electrometrically with ferrous sulfate. This procedure has already been described elsewhere' by one of us.
EXPERIME NTAI. This method has bee: tested experimentally against pure potassium dichromate and mixtures of dichromate and finely powdered pig iron in which an exactly similar procedure as to fusion and solution was followed. The results are given in the table. I n each case 1.9802 g. of potassium dichromate were used and one-tenth of this amount was titrated. The pig iron contained 0 04 per cent of chromium, for which correction was made. ANALYSISO F DICHROMATE &ONE WITH Chromium Amount Pig Iron Fused with Present SAMPLE Gram NazOz and NazCOs Gram 1 None Notfused 0.7000 2 None Fused 0,7000 3 0.25 Fused 0.7000
PIC IRON Chromium --FoundGram Gram 0.7002 0,7005 0.7002 . 0.7003 0.7008
.. .
This method was checked by oxidizing with ammonium persulfate. A portion of the third solution was treated with 25 cc. of sulfuric acid of sp. gr. 1.58, and diluted to 300 cc. After heating to boiling, 10 cc. of silver nitrate solution and 20 cc. of ammonium persulfate solution were added. After boiling 10 to 15 rnin., 5 cc. of dilute hydrochloric acid were added and the boiling was continued 5 min. Boiling with silver nitrate and ammonium persulfate served to oxidize all of the chromium and manganese, and boiling with hydrochloric acid decomposed the permanganic acid. After cooling and titrating, 0.7001 g. of chromium was found. The procedure was next applied directly to a sample of ferrochromium. After preparing the solution for titration after the fusion witch sodium peroxide, 100-cc. portions were reduced with an excess of ferrous sulfate and titrated with 0.1 N permanganate, correcting the titration with a carefully determined blank. The percentages of chromium found in this way were 63.43 and 63.65. Other portions of the solution titrated electrometrically with ferrous sulfate gave 63.48 and 63.48 per cent. One portion was boiled for 5 min. with 5 cc. of 1:3 hydrochloric acid, and on electrometric titration with ferrous sulfate after cooling gave 63.43 per cent. A portion was treated with an additional quantity of sulfuric acid and oxidized with silver nitrate and ammonium persulfate. The permanganic acid was destroyed with hydrochloric acid, and the solution was cooled and titrated electrometricnlly with ferrous sulfate. The value found was 63.46 per cent. In all of this work where dependence was placed on the oxidation of the chromium by sodium peroxide, a portion of the solution was analyzed as soon as it had been made up to volume. This was found necessary owing to the 1
THIS JOURNAL,11 (1919) 632.
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T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
tendency of such solutions to show a lowered oxidizing capacity after standing. It occurred to the writers that this might be due to a slow reaction with traces of hydrogen peroxide which had not been decomposed by boiling the alkaline solution. To test this point, the solution obtained by dissolving the fused mass from a Bample of ferrochromium was boiled 30 min., made up to volume, and on analysis found to contain 63.42 per cent chromium. After 4 days the apparent percentage of chromium was 62.10 and 62.20. A solution of another fusion was boiled 2 hrs. before acidification. The analysis of the freshly prepared solution gave
Vol. 13, No. 11
63.53 per cent chromium. After 4 days the apparent percentage of chromium was 63.05 and 63.15 per cent. It is thus clear that even boiling 2 hrs. does not give a stable solution.
SUMMARY I n this paper a method has been described for determining chromium in ferrochromium in which the chromate produced by oxidizing either with sodium peroxide or with silver nitrate and ammonium persulfate is titrated with ferrous sulfate, using an electrometric end-point. The procedure described has been in use in this laboratory for several years.
The Use of Perchloric Acid as an Aid to Digestion in the Kjeldahl Nitrogen Determination’ By Brainerd Mears and Robert E. Hussey THOMPSON CHEMICAL
LABORATORY, WILLIAMS COLLEGE, WILLIAMSTOWN,
On observing the corrosive action of 60 per cent perchloric acid on organic matter a t somewhat elevated temperatures, it was decided to carry out a series of experiments in which amall quantities of this material were used as an aid to digestion in the Kjeldahl determination of nitrogen. It was feared that the combination of sulfuric acid, perchloric acid, and organic matter might lead to serious explosions, but after running over 500 determinations with varying additions of perchloric acid without an explosion and a t most with only a rapid deflagration, i t was determined that this reaction was not particularly dangerous. I n most instances, however, the perchloric acid, sulfuric acid, and other materials were mixed cold. It was also feared that there might be considerable loss of nitrogen due to the volatilization or decomposition of the ammonium perchlorate which might be formed a t the elevated temperatures, or that other interfering factors might develop.
EXPERIMENTAL DETAILS A series of preliminary experiments proved that the addition of copper sulfate was beneficial, that it was impracticable to attempt to regulate the temperature of the digestion, and that most of the ammonia remained in the digestion flask. Accordingly, with a view to obtaining comparable dataon the use of the perchloric acid, the following experiments were carried out. One-gram samples of casein, tankage, egg albumin, gelatin, and dried blood, and about>10-g. samples of urine and milk were treated with 25 cc. of concentrated sulfuric acid and 1 g. of copper sulfate. Several determinations were run by the official Gunning method, slightly modified,2as a check on the nitrogen content of the material and to furnish an estimate of the time required for the complete digestion of the sample. At the same time, a series of determinations containing the same quantities of sulfuric acid and copper sulfate but with increasing amounts of the perchloric acid was carried on. Blank checks were run on the reagents in each instance, the average of which showed a nitrogen content of 0.0027 per cent. I n some instances, because of the lack of uniformity or low nitrogen content of the samples, the accuracy of the determinations might have been increased if larger samples had been employed, but it was considered advisable to keep the quantity of the sample constant to assist in drawing conclusions from the results obtained. The 1 g. of copper sulfate employed was probably more than was required; but inasmuch as many of the experiments were run with this quantity and no harmful effects were observed from its presence, the same quantity was employed in all the determinations. The study of the Kjeldahl method by the Association of Official *Received June 3, 1921. 2 A. 0 .A. C., “Methods of Analysis,” 1990, 7.
MAB~ACIIUSETTS
Agricultural Chemists,>partly substantiates these conclusions. ‘Table I summarizes the results obtained. N o . of Determma-
tions
METHOD
Kjeldahl Modified HCIO4:
, l CC. 2 cc. 3 cc. 4 cc. 5 cc.
3
1 3 4 2 1
Kjeldahl Modified HClOI: 0.5 CC. 1 cc. 2 CC. 3 CC. 4 cc. 5 CC.
4
Kjeldahl Modified HC104: 1 cc. 2 CC. 3 cc. 4 cc. 5 CC.
7
Kjeldahl Modified HClOa: 1 cc. 2 cc. 3 cc. 4 cc. 0 cc.
.
Kjeldahl Modified HC106: 1 cc,
2 cc. 3 cc. 4 cc. 5 cc.
Rjeldahl Modified HClOI: 1 cc. 2 cc. 3 cc. 4 cc. 5 cc. Kjeldahl Modified HClOi: 1 CC. 2 cc. 3 cc. 4 rc. 5 cc.
4 6
6 2 2 2
4 12 2 2 2 8 2 5 2 2 2
11 2 6 2 2 2 7 2 3 2 2 2 7 3 3 1 1 2
TABLE I Time of Digestion Hrs. Min. MILK 4 30
-Per Max.
cent Nitrogen-Min. Av.
0.70
0.69
0.70
2 1
30 30 25 25 10 URINE 1 45
0.68 0.69 0.69 0.69 0.64
0.68 0.67 0.69 0.67 0.64
0.68 0.68 0.69 0.68 0.64
1.02
0.92
0.99
20 20 20 20 20 20 CASEIN 4
0.99 0.89 0.81 0.74 0.66 0.66
0.96 0.84 0.73 0.67 0.61 0.56
0.98 0.87 0.77 0.70 0.64 0.61
12.44
12.14
12.29
15 12.31 12.40 20 20 12.26 20 11.41 20 10.81 TANKAGG 3 8.06
12.17 12.14 11.45 10.64 10.55
12.22 12.31 11.85 11.03 10.68
1
1
30 20 20 20 20 ALBUMIN 2 30
1
15 20 20 20 20 GRLATIX 3
7.55
7.76
7.80 7.89 7155 6.70 6.30
7.73 7.54 6.32 5.95 6.20
7.78 7.73 6.93 6.34 6.25
12.03
11.81
11.89
11.93 12.12 11.64 9.85 9.44
11.86 11.90 11.00 9.32 9.21
11.89 12.02 11.32 9.59 9.33
14.38
14.34
14.36
15 14.38 20 14.43 20 12.75 20 10.96 20 11.08 DRIEDBLOOD 4 30 13.14
14.34 14.36 10.77 9.74 9.43
14.36 14.38 11.76 10.85 10.25
13.09
13.11
13.09 18.11 11.88 11.92 11.93
13.06 13.10 11.88 11.92 11.93
13.08 18.11 11.88 11.92 11.93
1
1
30 20 20 15 15
The curves which follow were obtained by employing the above data and plotting the percentage of nitrogen against the quantities of perchloric acid used.
DISCUSSION OF RESULTS It is to be observed that, with the constant condition of a 1-g. sample, plus 1 g. of copper sulfate and 25 cc. of con1
Vol. 4,367.
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