INDUSTRIAL AND ENGINEERING CHEMISTRY
604
Reagents Required Hydrochloric acid, 1 part to 1 part of water. Crystalline potassium hydrogen sulfate. Ammonium hydroxide, specific ravity 0.90. Hydrochloric acid wash, 10 ml. of concentrated ydrochloric acid er liter of water. Ammonium hydroxide wash, 333 ml. dilute! to 1 liter with water. p-Dimethylaminoazohenylarsonic acid, 0.250 gram of dye, 10 ml. of concentrated ydrochloric acid, diluted to 250 ml. with ethyl alcohol.
%
E
Method Dissolve 1 gram of sample in 50 ml. of 1 to 1 hydrochloric acid. When solution is complete, dilute to 75 ml. with water and filter. Save the filtrate. Wash the residue 3 times with hot water, 3 times with hot 1 to 1 hydrochloric acid, and 3 times with hot water. Ignite paper and residue in a platinum crucible, fuse ash with as little potassium hydrogen sulfate as needed, dissolve cooled melt by heating the crucible, and melt in a beaker containing 100 ml. of water and a few drops of sulfuric acid. Now remove the crucible, washing well with hot water, and to the solution of dissolved melt add 2 drops of methyl orange indicator and a slight excess of ammonium hydroxide; boil a minute, filter, and wash the precipitate with cold water. Put paper and recipitate back into the beaker, add 50 ml. of 1 to 1 hydrochgric acid, break paper into pulp, and heat to boiling. Boil a minute and filter into original filtrate saved from first filtration. Wash well with hot water, transfer to a 500-ml. volumetric flask, cool, and dilute to the mark with water. Mix well and ipet 50 ml. of the combined filtrates into a 250-ml. beaker. bring the measured 50 ml. of solution to the boiling point, add 15 ml. of dye solution, cover with a watch glass, boil 1 minute, and remove from plate. Let stand at room temperature for a t least 30 minutes. Filter through 3 close-texture filter pa ers, wash excess dye from papers, and precipitate with hydrochforic acid wash solution. After washings are absolutely colorless, set the funnel containing washed paper and precipitate into the neck of a 100-ml. volumetric flask. Now remove dye combined with zirconium by washing with ammonium hydroxide wash, about 3 to 4 washes. Dilute to the mark with water, mix well, and fill absorption cell by filtering through cotton. Photoelectric reading is read in milligrams of zirconium on plotted curve.
Vol. 13, No. 9
In this case, weight in grams X 10 X 100 = per cent zirconium. The solution of dye which is colorimetered is yellow. A blue filter is used in the photelometer.
In the authors’ laboratory this determination is made in less than 2 hours. The results do not deviate by more than 0.005 per cent from those obtained by the selenious acid method. This method is subject t o the same necessity for fusion of the insoluble zirconium with potassium pyrosulfate that is met in all other methods. It is possible, however, t o eliminate the usual sodium carbonate fusion when working with steel containing less than 0.05 per cent of phosphorus. The authors have added phosphorus t o the original solution of the steel in amounts equivalent to 0.170 per cent without producing any significant change in the results for zirconium. However, with some steels a sodium carbonate fusion of the residue should be made before fusion with potassium pyrosulfate. Titanium interferes with this method by reacting in the same manner as zirconium, but this can be prevented by adding 3 drops of 3 per cent hydrogen peroxide before the precipitation, if the titanium is not more than 10 times the airconium content. The p-dimethylaminoazophenylarsonic acid obtained from the Paragon Testing Laboratories, Orange, N. J., has been found very satisfactory. The authors were handicapped in their work by the fact that there is no Bureau of Standards steel sample of certified zirconium content.
Literature Cited (1) Feigl, F., Krumhols, P., and
Rajman, E., Mikrochemie, 9, 395
(1931). (2)
Nasarenko, V. A., J. Applied Chem. (U.S. S. R.),10, 1696-9 (1937).
A Study of the Ferric Thiocyanate Reaction CHARLES A. PETERS AND CHESTER L. FRENCH, Massachusetts State College, Amherst, Mass.
T
HE thiocyanate method for iron is much used. However, it has some disadvantages. Kitrates generally in-
tensify the color, although small amounts may be tolerated with some concentrations of thiocyanate (14, 16). Phosphates change and decrease the color (4, 6) and salts may interfere (1,3, 6,9,13). I n view of these facts a study of the optimum conditions for the development of the red ferric thiocyanate color and the effect of salts on the color was undertaken. It has been assumed by Schlesinger and VanValkenburgh (12) that the red substance is the complex ion, Fe(CNS)G---; however, the work of Bray and Hershey (2) and Lamb and Jacques (6), indicating the existence of FeOH++ and FeCl++, leaves the authors ready t o accept evidence that the complex may contain less than six thiocyanate ions.
Procedure and Reagents Dilutions of an iron wire solution, oxidized with hvdrogen peroxide, were prepared daily and had a pH of 2.5. The reagents were added in the following order: thiocyanate, dilution water, acid, hydrogen peroxide, iron, and water to the mark. Hydrogen peroxide, when used, was added in quantity to make the solution either 0.0032 or 0.0064 molar. Inasmuch as the formula weights of CNS and Fe are nearly the same, the figures agp. p. m. may be read as molar quantities with only slight loss in accuracy.
Interfering Substances Nitric acid intensifies the color with thiocyanate in t h e presence of iron, Experiments in the absence of iron showed that solutions 0.1 N with nitric acid also developed a color when the thiocyanate concentration exceeded 0.1 N and, if the acid were changed to hydrochloric, solutions 1.26 N with this acid developed a color when the thiocyanate exceeded 0.05 N ; when the hydrochloric acid was weaker, 0.1 N , the thiocyanate had to be 0.4 N t o develop color. I n a previous article (11) the amounts of hydrogen peroxide necessary to make a colored substance from thiocyanate were given, but no limit for a safe maximum was set. Further work showed that the peroxide could be increased to 0.0064 M without significant error, but greater concentrations intensified the yellow color produced.
Thiocyanate-Iron Influence on Color From the formula Fe(CNS)G--- one would expect to obtain a colored solution when the molar ratio of CNS/Fe equaled or exceeded 6. In order to find out if the amount of thiocyanate necessary t o produce a color bore any relation t o this ratio, several experiments were performed in which various amounts of thiocyanate were added t o iron in 5 0 4 . Nessler tubes, the acidity being 0.01 N . The least amount of
605
ANALYTICAL EDITION
September 15, 1941
thiocyanate necessary to develop a color was measured. The results are used to make Figure 1. The experiment was repeated in the presence of hydrogen peroxide, but the findings were not significantly different. It appears that when the ferric ion is 0.1 p. p. m. the thiocyanate must be 60 p. p. m. to
60
50
40 *
(30 2
B i,LO IO
0.5
I.0 IWC
I.5
Ion, P. P. I.
2.0
FIGURE1. AMOUSTS OF THIOCYANATE AND IHONNECESSARY TO SHOW COLOR show a color; when the iron is 1.0 p. p. m. only 4.5 p. p. m. of thiocyanate are necessary. Apparently, with the higher concentration of iron, only part is active in producing color or else there is a colored substance with less thiocyanate than the postulated Fe(CKS)G---. It is evident that the presence of six ions of thiocyanate to one of iron does not necessarily mean that a colored substance is formed.
Effect of Hydrochloric Acid In the next series of experiments the effect of varying amounts of hydrochloric acid on the production of the red color was observed. In the first set the iron was 0.5 p. p. m., the thiocyanate 3.0 p. p. m. (ratio 6), and the acid concentration varied from 0.002 to 0.010 N in ten tubes; no color was evident. In the second set the iron and thiocyanate were not changed, but the acid concentration was varied from 0.02 to 0.10 N in nine tubes; a slight color was present in all experiments, but no difference could be detected between the individual tubes. In the third set the thiocyanate was increased ten times to 30 p. p. m. (ratio 60), and the acid concentration varied from 0.002 to 0.40 N. A slight color appeared in all tubes, the strongest being at an acidity of 0.01 N . It is evident that most color appears when the acid concentration is 0.005 to 0.010 N and that the color decreases when the acid is either less than 0.005 N or greater than 0.01 N . The data also show that when the CNS/Fe ratio was 6 color first appeared when the hydrochloric acid was 0.02 N, while when the CXS/Fe ratio was 60 color first appeared when the hydrochloric acid was 0.002-0.005 and the maximum color appeared a t 0.01 N . Thus at higher thiocyanate concentrations less acid is required to bring out the color. It is possible that a change in the amount of iron would cause a change in the acidity requirements. Accordingly the iron was increased to 5.0 p. p. m., and the thiocyanate to 150 p. p. m. (ratio 30). Again most color appeared a t an acid
concentration of 0.01 N , decreasing as the acidity decreased or increased from this figure. The acidity 0.01 N thus appears to be the optimum for iron determinations when hydrochloric acid is present. This concentration seems just about enough acid to prevent hydrolysis of the ferric ion; however, the slowness of this reaction, as noted by Lamb and Jacques (B), would have the same effect as the presence of more acid. Calculation of the amount of FeOH++ existing a t a pH of 2, using 3 x l o + as the equilibrium constant (6, p. 1219), shows that an appreciable amount exists at that pH under equilibrium conditions. EFFECTOF ACID COUNTERACTED BY THIOCYANATE. The previous results showed that the red color was less intense as the acid concentration was increased beyond 0.01 N when the CNS/Fe ratio was 60. To see if this relationship held at higher concentrations of thiocyanate, varying amounts of thiocyanate were added to Nessler tubes containing different amounts of acid at two iron levels and the color intensities were compared. The time from mixing to reading was about 7 minutes. Table I gives the data. It seems that at high thiocyanate concentrations the influence of the acid in decreasing the red color is lessened-for example, changing the acid from 0.01 to 0.60 N , when the concentration of thiocyanate was 240 p. p. m. and the CNS/Fe ratio was 2400, resulted in less color in the more acid solution. This held also for a thiocyanate concentration of 1200 p. p. m., CNS/Fe ratio 12,000, but when the concentration of thiocyanate reached 2400 p. p. m. no difference in color was noticed with increased acid. In other words, with a small amount of iron (0.1 p. p. m.), a thiocyanate concentration of 0.040 N (2400 p. p. m.) produces the same color whether the acid is 0.01 or 0.60 N . When the amount of iron is increased five times to 0.5 p. p. m. it is necessary to increase the thiocyanate concentration to over 11,000 p. p. m., CNS/Fe ratio 22,400, before the effect of the stronger acid is overcome. However, a t the two different iron levels a t the point where the effect of acid is overcome by thiocyanate, the CNS/Fe ratios are essentially the same. The usual analytical practice is t o use a thiocyanate concentration of approximately 0.3 N which is, according to this work, high enough to minimize the effect of the acid in lessening color. TABLD I. EFFECTON RED COLOROF VARYINGTHIOCYANATE CONCENTRATION AT DIFFERENT ACIDITIES Tube No. 1
2 3 4 5 6 7 8 9 10
11 12 13 14
HCI CNS p. p . m. N P. p . m.
c1
Fe
0.1
0.1 0.5
0.5
CNS
.t4
240 2.4 1,200 0.48 2,400 0.24 0.60 240 150 1,200 29 2,400 15 0.01 561 1.0 1,122 0.52 0.10 5,610 0.05 11,220 62 0.60 561 1,122 31 6.2 5,610 3.1 11,220 0.01
E
S
Fe p . P. m. p . P. 1.5 0.30 0.15 91
1s
9.1 0.65 0.33 0.06 0.03 3.9 2.0 0.39 0.20
2.400 12,000 24,000 2,400 12,000 24,000 1,122 2.244 11,220 22,440 1,122 2,244 11,220 22,440
Intensity Color
4 5 Same a8 6
