than that of any other method rvvhich has been reported for this determination. LITERATURE CITED
(1) Corbridge, D. E. C., Lome, J . Chem. SOC.1954, 493. (2) Ibid., p. 4555.
E. J.,
(3) Daasch, L. W.j Smith, D. C., , 4 s a ~ . CHEX.23, 853 (1951).
(4) Miller, F. A., Wilkins, C. H., Ibid., 24, 1253 (1952). (5) Pobeguin, T., Chim. anal. 8, 203 (1954). (6) Posner, A. S., Duyckaerts, G., Ezperientia 10, 424 (1954). ( 7 ) Schiedt, u., ReinMY?in H.1 2. S&Urforsch. 7b, 270 (1952). (8)Stimson, 31. AT., O’Donnell, J. lI., J . Am. Chem. SOC.74, 1805 (1952).
(9) Trautz, 0. R., Sorelco Reptr. 2, 19 (1955). (10) UnderTT-ood, A . L., Toribara, T. Y., Ken-man, W.F., J . Am. Chenz. SOC. 77, 317 (1955). RECEIVED for revien. August 20, 1956. Accepted Xoveinber 23, 1956. Study supported by the SIedical Research and Development Board, Office of the Surgeon General, Department of the Army, under contract D.1-49-007-1ID-238.
Determination of Milligram Quantities of Thiosulfate J.
B.
by
a Clock Reaction
RISK and J. D. H. STRICKLAND’
British Columbia Research Council, Vancouver 8, B. C., Can.
b The oxidation o f p-phenylenediamine by ferric chloride (with the formation of Lauth’s violet in the presence o f sulfide) i s inhibited by the presence o f thiosulfate. A well-marked induction period results which i s proportional to the amount of thiosulfate present. This clock reaction i s almost specific for thiosulfate and can be used as a rapid and precise analytical method for small quantities o f thiosulfate in the presence of other sulfur acids. An example i s given of its application to the analysis of black liquor from kraft sulfate pulp.
T
HE FORMATION of Lauth’s violet or methylene blue b y treating sulfide solutions in acid with ferric chloride and p-phenylenediamine, or the h-,S’-dimethyl derivative thereof, is the basis of an established method for the absorptiometric determination of traces of sulfide ( I - S ) . K h e n a ferric salt is added to a mixture of p-phenylenediamine and sulfide, a blue color appears a t once or, if sulfide is absent, the solution rapidly becomes orange-yellow. Experiments in these laboratories showed that the presence of even traces of thiosulfate in the solution delayed the formation of color, either orange or blue, and that the color finally appeared with great rapidity after a n induction period of as much as 0.5 hour or more, thus giving a beautiful clock reaction with a well-defined end point. As the induction period n-as a function of the thiosulfate concentration, the use of the reaction as a n analy-
Present address, Pacific Oceanographic Group, Fisheries Research Board of Canada, P.O. Drawer 6 , Sanaimo, B. C., Can. 434
ANALYTICAL CHEMISTRY
tical procedure became apparent. Kormally such a technique would have little to recommend it, but in view of the formidable analytical difficulties encountered in the precise and rapid determination of small quantities of thiosulfate in the presence of most other substances, the method was further investigated. In the course of a study, which is not to be reported here in detail, it became clear that the oxidation of p-phenylenediamine by ferric iron proceeds by a chain reaction mechanism for which the thiosulfate acts as a chain breaker. The comparatively slow direct reaction of p-phenylenediamine with ferric ions takes place in the presence of thiosulfuric acid until the free radical first formed has used up the thiosulfate, after which the chain reaction sets in and all
01 0
1
I
I
2
the remaining amine is oxidized in a comparatively few seconds. Hydrogen sulfide is without effect on the course of this reaction, but, if present, the deep blue Lauth’s violet dye appears as an cnd product and greatly improves the detection of the onset of the chain reaction. This can then be observed with a precision of better than 1 second after a n induction period of many minutes. The induction period is a complex function of reagent concentrations and is sensitive to temperature. A wide variety of conditions can be used as the basis of a n analytical method; the general illustration of the effect of variables given here enables the analyst to choose conditions suitable for any particular problem. A full description is given of one possible procrdure which is considered about the optimum for a
I
3
4
M I L L I G R A M S NaZS2O3
Figure 1.
Calibration curve for sodium thiosulfate determination
the solution in the beaker. By blowing from a calibrated pipet, 1.00 ml. of ferric chloride solution was rapidly added to the beaker. The solutions n-ere mixed and a stop watch was started. The induction period, I t seconds, was then recorded as the time from adding the ferric solution to the moment when the blue color rapidly appeared. This time was corrected to 25.0' C.by the formula 125
Figure 2. Change of apparent rate with reagent concentration 1M CI-, 0 . 1 7 5 M H+, 4 X 10-'M thiosulfate Reagent not being varied held at 4.55 X i o - 3 ~
0 p-Phenylendiamine, slope 1 .O 0 Ferric iron, slope 1.3
general analytical method and which has been applied to the analysis of black liquor from kraft sulfate pulp.
= 1t[1 - 0
14 ( 2 5 - t ) ]
which applied accurately for temperaturw, t o C., not more than 0.5" to 1" C. removed from 26.0" C. The amount of thiosulfate \\-as then read from a calibration curve (such as Figure 1) prepared by using standard sodium thiosulfate solutions. T o eliminate the necessity for exact reproducibility of reagent and base electrolyte solution concentrations, i t is best to recalibrate each time new solutions are prepared. This need not be done a t all thiosulfate levels, hon ever, as the percentage change in induction time found a t one level of thiosulfate concentration applies a t all others. Once this is determined at a suitable level the whole curve may be reconstructed by calculation. EFFECT OF VARIABLES
Amine a n d Ferric Ion Concentrations. Figure 2 is a plot of t h e apSUGGESTED METHOD parent rate of thiosulfate destruction us. the molarity of p-phenylenediamine, Reagents. BASE ELECTROLYTE. with all other variables held constant a t Dissolve 40 grams of sodium chloride the values s l i o m . If the thiosulfate in water, add 30.0 ml. of concentrated were used up a t a uniform speed, the hydrochloric acid, (specific gravity, rate would be given b y the concentra1.18), and dilute to 1 liter. ~-PHESYLEXLDIAIIIXE SOLUTION. tion of thiosulfate ion divided by the This chemical should be recrystallized induction time. There is reason to beonce from ethyl alcohol if dark colored. lieve, however, that the reaction does Dilute 2.5 grams of amine and 6.0 ml. not necessarily proceed a t a uniform of concentrated hydrochloric acid to 100 velocity throughout the induction penil. with water. FERRIC CHLORIDESOLUTION. Prepare as a 4.07,solution in water, using a good quality of anhydrous ferric chloride. HYDROGEN SULFIDESOLUTION.Prepare a saturated aqueous solution fresh daily. Procedure. T h e bottles containing t h e reagents were allowed t o come t o temperature in a water bath thermostated at 25" h0.5" C. The clock reaction was carried out in a 100-ml. beaker which was placed on a white tile in the water bath. Base electrolyte, 25.0 ml., was added to the beaker followed by the sample dissolved in 2 ml. of water. The p H of the sample solution, unless initially neutral and unbuffered, was first adjusted to about pH 3.3 to 3.6 using xylene cyano1 screened methyl orange (gray tint). S e x t 1.OO ml. of p-phenylenediamine I 1.01 I I \ solution was added fiom a pipet, fol1.0 0 .I 0.25 0.5 M O L A R IT Y lowed by (23--2) nil. of water and 1 to 2 drops of hydrogen sulfide solution. The Figure 3. Change of apparent rate solution was stirred and allorred to with acidity reach a constant temperature between 24" and 26" C., which was noted to 1M CI-, 4 X 1 0 T 6 M thiosulfate, ferric iron 0.1" C. b y a thermometer graduated and p-phenylenediamine each 4.55 X 10-3M in 0.1" C., with the bulb immersed in Slope, - 1.8
riod, so the term apparent rate is used. The data in Figure 2, plotted on a loglog scale, show that the rate varies with the first power of the p-phenylenediamine concentration. Figure 2 also shows the effect of ferric ion concentration when the amine concentration is held constant. Here the rate is proportional to the oxidant concentration raised to a power significantly greater than unity, An increase in either amine or ferric ion concentration decreases the iiiduction period observed with a given amount of thiosulfate. Acid Concentration. Figure 3 shows t h a t t h e rate is inversely proportional to a power of the hydrogen ion concentation-Le., the induction period increases with a n increase of acidity. The exponent, - 1.8, is significantly greater than -2, illustrating the comple\ity of the reaction, but a t least part of the acid effect is thought to be the result of a suppression of the concentration of free, unionized amine, which is probably the reacting species. Temperature. This effect is very marked. T h e llrlhenius plot of log (apparent rate) against Ilk' is shonn in Figure 4. The energy of activation of about 28 kcal. probably 1-arks very little with reagent concentration and othrr Tariables. Temperature control to better than 0.1' C. is necessary for a 1% precision in the induction period. It is more convenient to note the temperature during measuremrnt and correct the induction period than to attempt exact thermostatic control. Chloride Concentration. T h e rate of reaction is very slow in t h e complete absence of chloride, indicating that a ferrichloride complex is probably the oxidant. The effect of chloride is shown by the family of curves in Figure 5. For the experiments illustrated by curTes 1, 2 , and 3 the reagents wcrc added as perchlorates and only chloride was varied. Kot only does the apparent rate vary iv-ith the amount of chloride present, but i t also depends on the quantity of thiosulfate. Thus, a linrar relationship betrreen induction period and thiosulfate concentration cannot be expected. HOKever, when the chloride concentration exceeds a few tenths molar, the deviation from linearity is not great over a n ide range of thiosulfate concentration, as shown by a calibration curve such as Figure 1 where the chloride is about 0.5Jf. The closest approach to linearity is obtained with about 0.2M chloride. However, this is dangerously near the region where the chloride concentration becomes critical, and a somelvhat higher concentration is recommended. With a concentration of 1-U or greater, the rate increases very rapidly with small amounts of thiosulfate and restricts the use of the method. The curves shown in Figure 5 are only intended to be inVOL. 2 9 , NO. 3, MARCH 1 9 5 7
* 435
dicative of thegeneral effect of chloride. The reproducibility with very low chloride concentrations was poor, but the anomalous position of curve 2 has been confirmed. Interferences. T h e induction time will vary someIvhat if a n y electrolyte is added in sufficient amounts, but in many cases the effect is undoubtedly one of ionic strength and is not very great. For the best results a calibration should be made in the presence of whatever electrolyte is present in the sample being
Table I.
react rapidly n ith p-phenylenediamine or ferric chloride must be absent or present only in amounts of 10% or less of the equivalent weight of the reagent which they attack. Similarly, substances reacting with thiosulfate cannot be tolerated-e.g., iodine-although the very low concentration of thiosulfate present “protects” it to some extent. For example, decomposition with acid is only noticed after 30 minutes or more in 0.15M hydrochloric acid, although it is almost immediate when concentrated thiosulfate solutions are allowed to stand in a n acid of this strength. Although the search for interfering ions has not been exhaustive, Table I indicates the tolerable limit of some common substances Kith this method. In some cases larger amounts could possibly be removed chemically without destroying the thiosulfate-for example, snlfite can be eliminated by bubbling air through a weakly acid solution. It should be possible to precipitate sulfide b y silver, although this was not tested. The limits given do not imply that no interference occurs, but that the effect on the induction period is not too great and can be overcome by a suitable calibration in the presence of the ion concerned. Where very serious interference is noted with metal ions such as copper(II), the behavior can be attriuted to complexing of the thiosulfate; however, the action is of no value as a n indirect method for analysis of the metal concerned. A brief search for other chain break-
analyzed, the sample weight being chosen so that between about 0.25 and 5 mg. of sodium thiosulfate is added per 50 ml. of solution. I n general, little or no interference is found with ions of the alkali metals or alkaline earth metals present to the estent of 1 to 2 grams in 50 ml. Similarly, nitrate and perchlorate anions do not interfere and chloride only inasmuch as the velocity is increased somewhat (Figure 5 ) . Oxidizing or reducing agents which
Interference of Foreign Ions
Limit of Detection of Sodium Thiosulfate, yo 0.01
Ion Max. Wt. in 50 Mi. Ka+, Kr, Ca++ Little interference up to 1to 2 grams Ba +, hIg +-, NH, +, Cl-, xo3-, clod-
so,-Br Sos--
S-Thionic acids IAg -
Zn7-, Xi++, M n + + Sn +l Fe++ Hg++ cu +
L
I
0.2 gram without special calibration 1 gram with calibration 0 5gram 0.005 gram direct 0.2 gram if 5-ml. sample is acidified to pH 3.5 and SOL removed by bubbling air for 10 to 15 minutes 0.02 gram At least 0.1 gram 0 025 gram 0 15 gram. Sulfide and chloride precipitate; end of induction period shown by orange color 0.5 gram 0 1 gram 0 02 gram Interferes a t ail levels Interferes at all levels
- -
6.0
0.125 0.02
0.05 D
0.125 1.25
0.05 (about) 1
0,015 0.05 0.25 1.25
I
I
2 MOLARITY
s,o;-
I
I
3
4
5
x 10‘
Figure 5. Effect of chloride ion and thiosulfate concentration on apparent rate
Figure 4.
Arrhenius plot of apparent rates
0.2M CI-, 0.2M H+, 4 X 10-5M thiosulfate, 4.8 X 1 O - W ferric iron, 3.1 X 10-3M p-phenylenediamine
436
ANALYTICAL CHEMISTRY
0.1 5M H+, ferric iron and p-phenylenediamine each 4.55 X 10% 1. No chloride 2. 0.05M chloride 3. 0.2M chloride 4. 0.5M chloride 5. 1 .OM chloride
Table 11.
Determination of Thiosulfate in Black Liquor Sodium Thiosulfate, Grams per Liter
Added to
0.5 ml. of black liquor
Found Recovered ,.. 1.6,1,9,1.i5 .4v. 1.75
Sone 2.0 3.0
3.7,3.9 4.7 5.65
1.0 G.0 8.0
7.3
!I. 5
2 .0
