ANALYTICAL CHEMISTRY
298 Tlie results obtained from the analysis of a group of typical zirconium oxide samples are shown in Table I. These results were obtained over a period of several months by different analysts in the laboratory. The average deviation a t concentrations of 0.003% hafnium is about 1370 and could be improved by using a more elaborate and accurate method for background correction. However, t h a t would involve increasing the time required for each analysis, and the accuracy of this method is ordinarily satisfactory for a routine analytical method. At the average deviation the higher range of concentration (0.4y0) is about 4%.
values of the standards against their int.ensity ratios. By adding 0.004570 to each of the standards and replotting the curve with the same intensity ratios, a straight line was obtained indicating that the zirconium-base oxide contained 0.0045% hafnium. The third-order ultraviolet spectral lines are used in this method in preference to either the first or second order because a higher line to background ratio was obtained in this order. The use of the third-order ultraviolet depends upon the satisfactory suppression of the second-order cyanogen bands, which tend t o mask them to a considerable extent. Although the characteristics of the Corning 9863 filter indicate the possibility of a degree of solarization, several months’ use of this method has not indicated a significant diminishing of transmittance characteristics in t,he ultraviolet. CONCLUSIONS
A spectrographic method for determining hafnium in zirconium in concentrations of 0.003 to 0.4% incorporates the use of direct arc conditions with a current of 30 amperes a t 230 volts primary. The spectra are recorded in the third-order ultraviolet, using a Corning 9863 filter to suppress the second-order cyanogen band. A buffer of barium fluoride-graphite is mixed with the oxide sample to help maintain a steady, even arc stream and to enhance the hafnium spectra. The accuracy obtained with this method is 137, deviation a t 0.006% hafnium and 4Y0 deviation a t 0.38% hafnium. This accuracy is considered satisfactory, as the main purpose of this ’ method is to attain maximum sensitivity. LITERATURE CITED
The exact amount of residual hafnium in the zirconium-base material was determined by the graphic method shown in Figure 4. The experimental curve was obtained by plotting the nominal
(1) Birks, L. S., and Brooks, E. J., AX.LL.CHEX.,22, 1017 (1950). (2) Chandler, A. C., J . Opt. SOC.Amer.. 40,262 A (1950) (3) Fassel, V. A , , and Anderson, C. H., Ibid., 40,742 (1950). (4) Feldman, C., AXAL.CHEM., 21,1211 (19491. (5) Kroll, W. J., and Stephens, W. W., I n d . Eng. Chem., 42, 395 (1950). (6) Smith, D. D., and Spitzer, E. J., Oak Ridge National Laboratory, AECD2924 (1950). RECEIYED for review August 8, 1952. Accepted October 14, 1952. PreCHEMICAL SOCIETY sented a t the Northwest Regional Meeting, AMERICAN Corvallis, Ore., J u n e 20, 1952.
Volumetric Determination of Persulfate in the Presence of Organic Substances I. M. KOLTHOFF AND E. M. CARR School of Chemistry, University of Minnesota, Minneapolis, M i n n .
0
F THE several reductometric methods proposed in the literature for the volumetric determination of persulfate, the iodometric and ferrometric methods were found t o be most practicable from the points of view of rate of reaction, accuracy, and simplicity. Both methods were subjected to a critical investigation, both in the presence and absence of organic substances. Standard solutions were prepared and standardized by established procedures. C.P. potassium persulfate was recrystallized from water and dried in a vacuum desiccator over sulfuric acid. IODOMETRIC METHOD
The method is based on the stoichiometric reaction:
&Os--
+ 21-
= 2SO4--
+ I2
(1)
Kiss and Bruckner ( 3 ) made an extensive study of the rate of
this reaction and found that an increase of ionic strength of the medium increased the rate of reaction. Among the 23 electrolytes studied, potassium chloride was the most effective, while hydrogen ions did not have any specific catalytic effect. The effect of a strong mineral acid was found to be of the same order of magnitude as that of another strong electrolyte. According t o SchJvicker ( 8 ) , Reactlon 1 is quantitative in a convenient time in neutral medium, provided enough potassium iodide is added. These results were confirmed by Kurtenacker and Kubina (6) and also in the present study. Analytical use of the acceleration of thereaction betxeen persulfate and iodide by potassium chloride was made by Zombory (9) and King and Jette ( 2 ) . Rosin ( 7 ) recommends the following procedure which was applied in slightly modified form by Bartlett and Cotman ( 1 ) in polymerization xork. Approximately 0.5 gram of potassium persulfate is dissolved in 30 ml. of water containing 3 grams of potassium iodide, Ten milliliters of 10% sulfuric acid are added, the mixture is allowed to stand for 30 minutes in an iodine flask,
V O L U M E 25, NO. 2, F E B R U A R Y 1 9 5 3
299
In polymerization studies with persulfate as activator, persulfate must often be determined in the presence of various organic compounds. Unless the organic siibstance reduces iodine or oxidizes iodide the iodometric method is most convenient. A s?stematic study of this method has been made. Good results are obtained at pH values varying from 7 to less than 0. Organic substances affect the stoichiometrl of the reaction betwreen persulfate and ferrous iron, too little iron being used in the absence of oxygen, and in very dilute persulfate solutions too much in the presence of oxygen. The induced reactions can be eliminated bj addition of bromide to the reaction mixture.
and the iodine is titrated with standard thiosulfate. In the authors’ experience, this procedure gives from :3 to 5% Ion. results with amounts of persulfate betneen 30 and 400 mg. K h e n the reaction time was ext,ended to 45 minutes good results were obtained, provided air-Oxidation of iodide as prevented hy removal of oxygen. I n any iodometric procedure for thc determination of persulfate the time required for complete reaction with iodide depends especially upon the iodide concentration of the mixture and also on the ionic strength. The effect of potassium chloride is i l l u ~ t r a t r dby some typical data in Table I.
Table I.
EtTect of Potassium Chloride on Rate of Persulfate-Iodide Reaction
(Volume of reaction mixture, 30 ml. Reaction time, 15 minutee. temperature) Reaction Time, KI. KCI, Minutes Graiiis Grams 15 1 15 1 3 1.5 1 6 15 4 .. 5 4 5 J 4
K ~ S ~ iOr g~. , Taken 19.81 19.81 19.81 27.45 27.45 27.45
Found 14.43 18.91 19.69 27.45 26.37 27.56
Room
Error,
talned in the presence of all) 1 acetate, acetone, and formaldehyde n’ere reasonably accurate. JVith acetone present, the accuracy vas incieased by modifying the procedure. The reaction betneen acetone and iodine a i t h the formation of monoiodoacetone and hydrogen iodide is reversible. I n acid medium the reaction is shifted t o the iodine side and the interference by acetone in the persulfate determination can be eliminated by adding 2 nil. of 1 to 1 sulfuric acid to the reaction mixture after the reaction between persulfate and iodide has gone to completion. I n the determination of persulfate in a 32-mg. potassium persulfate sample in 1.8 JI acetone solution, a 3.3% low result was found by the general procedure. If the reaction mixture was acidified Rith 2 ml. of 1 to 1 sulfuric acid, the results were only 0.G% low. Methylisopropenyl ketone interferes, both in the unmodified and modified procedures because of its consumption of iodine. I n the presence of this ketone the ferrometric method is recommended.
70
-27 -4.5 -0.5 0.0 -3.9 -0.4
I n this n ork it 11 as desiied to make the potassium iodide concentration large enough to have the reaction completed in 15 minutes. One half of the amount of iodide can be taken in the procedure given beloll. if the reaction niixture is allowed to stand for 45 minutes before titration with thiosulfate. Other variations of the procedure can be made by making use of the accelerating effect of electrolytes (including acids) on the reaction hetiveen persulfate and iodide. PROCEDURE
Introduce a measured volume of the persulfate solution containing from 5 to 500 mg. of potassium persulfate into a 125-m1. iodine flask, add water (sufficient to bring the volume to 30 ml.) and 4 grams of potassium iodide. Stopper the flask until all iodide is dissolved and allow it t o stand for 15 minutes. Acidify with 1 to 2 ml. of 6 N acetic acid and titrate the iodine with 0.01 or 0.1 N thiosulfate to the starch indicator end point. .41~plyan indicator correction if necessary. See first note below. YOTES ON PROCEDURE
Large amounts of electrolyte (and especiall>+iodide) unfavorably affect the sharpness of the color change of the iodine-starch indicator. With 8 grams of potassium iodide (instead of 4 grams as given in the procedure) in the reaction mixture, an excess of 0.10 ml. of 0.01 A- thiosulfate was required to discharge the iodine-starch color; with 4 grains of iodide the excess amounted to only 0.02 ml. The indicator correction is negligibly small in titrations with 0.1 N thiosulfate. Organic substances n hich do not react with iodine or do not reduce iodide, do not interfere when present in a concentration or less. It is evident from Table I1 that methanol, of 1 ethyl alcohol, and acrylonitrile do not interfere. Results ob-
Table 11. Effects of Organic Substances, of Reaction Time, and Excess of Iodide on Iodometric Procedure Reaction Time, Minutes 15 I5 15
45 15 15 15
45 15 1.5 15 15 15 15
30 15 30 15 15 15
(Volume of reaction mixture, 30 PotaEs-ium Potassiut-n Persulfate. M g . Iodide, Present Found Grams 8 446 446 4 446 444 2 446 432 2 446 445 8 26.73 26.73 26.73 26.73 4 2 26.73 25.38 2 26.73 26.73 4 5.38 5.40 4 2c.72 26.74 4 2.38 5.40 4 22.80 22.73 4 26.79 26.72 26.79 26.41 4 4 26.79 26.41 4 26.79 26.41 4 26.79 26.49 26.17 25.91 4 4 26.17 26.12 4 5.23 5.18
ml. Room temperature) Error.
70
0.0 -0.5 -0.6 -0.2 0.0 0.0 -5.0 0.0 +0.4 -0 1 -0.4 -0.3 -0.3 -1.4 -1.41 -1.4 -1.0 -0.2 -1.0
Organic Substance
................. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .................
.................
...............
...............
.................
Allyl acetate 0.2 M 4cetone0.9 M Formaldehyde 0 25 M Formaldehyde 0 . 1 M Formaldehyde 0 . 1 M
The procedure gives good results in buffered or unbuffered solutions with p H between 7 and 0.2. I n stronger acid medium it is recommended to remove oxygen with carbon dioxide or another inert gas t o prevent air oxidation of iodide. ,4few representative figures which illustrate the accuracy, the effect of organic substances and some modifications of the general procedure (concentration of iodide, reaction time) are given in Table 11. FERROMETRIC METHOD
I n acid medium persulfate is reduced with an excess of standard ferrous sulfate, which is back-titrated with standard ceric sulfate solution. Even in very dilute persulfate solution the reduction is quantitative after a reaction time of 15 minutes (4). Organic substances interfere strongly with the reaction between persulfate and ferrous iron. Kolthoff et al. (6) interpreted this in the following way. The stoichiometric reaction between persulfate and ferrous iron:
ANALYTICAL CHEMISTRY
300 2Fe++
+ SzOa--
= 2Fe+++
+ 2s04--
pressors of the induced reactious. This was interpreted by assuming t h a t the bromide ion competes with both ethyl alcohol and ferrous iron for reaction with the free radical SO7 according to the step
(2)
presumably takes place through the steps
+ Stoa-- = F e + + + + Sod-- + so,' F e + + + SO4' = F e + + + + SO4--
Fe++
(3)
(4)
SO4'
+ Br-
=
so,-- + Br'
(9)
and then reacts with ferrous iron The following steps account for the induced reaction between persulfate and organic compounds, of which ethyl alcohol may be taken as typical
SO,'
+ CH3CHzOH
=
HSOA-
+ CHBCHOH
(5)
+ HSOa- + SOa'
(6)
+ Fe++ + H +
(7)
Br'
+ &OS--
-.f
CH3CHO
or CHaCHOH
+ Fe+++
CHICHO
-+
T h e sum of stepe 3, 5 , and 6 or 3, 5, and 7 gives the net induced oxidation of ethyl alcohol by persulfate CHZCHZOH
+ S?Os--
= CHaCHO
+ 2HS04-
(8) REAGENTS
I n the absence of oxygen t,he induced oxidation of the organic compound gives rise t o a molar reaction ratio of ferrous iron and persulfate of less than 2. The deviation from 2 depends on the concentrat,ion of the reactant,s and the mode of mixing. I n the presence of oxygen the ethanolium free radical reacts rapidly with oxygen wit'h the forniation of a peroxide free radical which is reduced by ferrous iron to a hydroperoxide. Further reaction of the hydroperoxide with ferrous iron, in the presence of an orga,nic substance and oxygen, results in a n extensive induced oxidation of the ferrous iron. \\-hen working with oxygen-saturated, very dilute ( 5 X N) persulfat,e solut,ions in the presence of ethyl alcohol or other organic substances, molar reaction ratios between iron and persulfate considerably greater than 2 were found ( 5 ) . I n more concentrated persulfate solutions reaction ratios less than 2 may be found in the presence of organic substances and oxygen because of the limited solubility of oxygen. Under such conditions the "induced oxygen oxidation" of ferrous iron is overcompensated by Reactions 3, 5, and 6 (or 7 ) . I t I\-as found ( . 5 ) t h a t halides, especially bromide, act as sup-
The iolloning reagents were used: 0.05 N ferrous sulfate in 0.5 iV sulfuric acid; 0.02 'Ir ceric sulfate in I iV sulfuric acid; 0.0005 M ferrous phenanthroline indicator; 5 S sodium bromide solution. These concentrations are conveiiiciit when working with amounts of persulfate varying between 6 and 30 Ing. With larger amounts correspondingly more concentratrd reagent solutions muqt lie used. RECOMMENDED PROCEDURE Ih PRESEYCE OF ORGAhIC COXPOUYDS
Introduce 10 ml. of persulfate solution (0.005 to 0.025 .V) into a 125-ml. Erlenmeyer flask. Add a suitable volume of distilled \\ater (see S o t e l ) , 7 ml. of 5 S indium bromide solution, and 2 ml. of 6 S sulfuric acid. Snirl the contents, add a known excess of standard ferrous solution (5.00 ml.), stopper the flask, and sm-irl the contents again. At this stage, the volume of the solution should be 35 ml. Allon the solution to stand for 20 minutes. Open the flask, add 2 ml. of ferrous phenanthroline indicator, and back-titrate the excess of ferrous iron with standard ceiic solution until the color changes from orange to pale yellow. The first rolor change corresponds to the end point (Xote 2 ) Calculations. hlilligrams of K A O s = 135.2(Vo - 1')'l' \\.here 1, = ml. of C e + + + +equivalent to the volume of standard ferrous solution. (See S o t e 3 about correction for indicator blank) T.' = ml. of C e + + - + required in back-titration S = normality of standard ceric solution
Table 111. Ferrometric Determination of Potassium Persulfate in Presence of Organic Substances (Volume of reaction mixture, 35 ml.) Kind and Concentration of Organic Substances in Reaction hlixture
Perelllfate*hlg. Present Found
(10)
instead of with the organic compound. I n the present work, use has been made of the suppressing effect of bromide on the induced reactions in the presence of organic compounds. At a bromide concentration as given in the recommended procedure, induced reactions were practically eliminated in the presence of ethyl alcohol, methanol, allyl acetate, allyl alcohol, and methyl isopropenyl ketone (see Table 111). I n the presence of this ketone the iodometric procedure gave low results and here the ferrometric method is of special advantage. I n general, the iodometric procedure is preferred in the presence of noninterfering organic substances because it is less subject to interference.
followed by either CH3CHOH
+ F e + + = Br- + F e + + +
.
NOTES ON RECOMMEh-DED PROCEDURE
Error, 70
I n Presence of Broniide (Reconimended Procedure) Allyl acetate 0.25 M 27.03 27.27 +1.0 20.2i 0.0 20.27 Allyl acetate 0 . 2 5 M 33.48 -0.5 13,51 Allyl acetate 0 . 2 5 M 6.76 6.83 +1.l Allyl acetate 0 . 2 5 M 26.92 -0.4 27.03 Allyl alcohol 0,lO M 5.48 5.41 +1.5 Allyl alcohol 0 . 1 0 M 27.16 +0.5 27.03 Acrylonitrile 0 . 2 5 M 26.90 27.03 -0.5 E t h y l alcohol 0 . 2 5 M 26.90 27 03 -0.5 XIethanol 0 . 1 0 A4 6.62 -2.1 Methanol 0.10 41 6.76 Methyl isopropenyl lie22.72 22.90 -0.8 tone" 0.064 .Vf I n Absence of Bromide 18.4 - 32 Allyl acetate 0 . 2 5 M 27 0 3 - 40 Ally1 alcehol 0 . 10 M 27 03 16.2 Acrylonitrileb 0 . 2 5 M 28.4 -6 27 03 Ethvl alcohol 0 . 2 5 M 27 03 - 35 17.6 -6 28.2 lIe
