Photometric Determination of Potassium with Dipicrylamine ELIAS A>IDUR, Lniversity of RIinnesota, JIinneapolis, RIinn.
K
filter in increasing the sensitivity is best determined by experiment, although examination of t'lie transmission curves of various concentrations of potassium dipicrylaminate will be helpful in a preliminary selection of filt'ers.
OLTHOFF and Bendix ( I ) describe the use of dipicrylamine in the gravimetric, volumetric, and colorimetric determination of potassium. Several problems were encountered by the author in the adaptation of the colorimetric procedure to the determination of potassium in tissues and blood. It was found that information regarding the selection of a light filter for this determination was not available, and that modification of the procedure to reduce the amount of necessary manipulation and to allow working a t room temperature x a s desirable. A rexision of the procedure of Kolthoff and Bendix and a new indirect method are described below.
il Cenco-Sheard-Sanford photelometer lyas used for measuring transmission percentages. In this instrument, the light passes horizontally through a light filter, then through a cell having parallel glass walls 1 em. apart, and finally falls on a photoelectric cell which actuates a microammeter. A slide allows any of the three identical cells t o be placed between the filter and the photoelectric cell. One cell is filled with the pure solvent, and this acts as the standard with a transmission percentage of 100. The transmission percentages of the other solutions are determined by adjusting the light source so that the instrument reads 100 with the standard cell in place, and then substituting the cell containing the solution of unknovm transmittancy. With a green glass filter (supplied with the photelometer for use in hemoglobin determinations) this instrument proved to be insensitive, when compared to the results obtained by Kolthoff and Bendix with a Lange colorimeter. Following their suggestion, blue filters were substituted for the green filter. Cobalt blue glass and blue cellophane were tried, but did not give improved results.
Selection of Light Filter I n the colorimetric procedure of Kolthoff and Bendix, and in the work described below, the concentrations of known solutions of potassium dipicrylaminate are plotted against the light transmission percentage (I;Io x 100) and unknown samples are compared with the curve obtained. A group of such curves, using different light filters, is shown in Figure 2 . The sensitivity of the determination to changes in the potassium dipicrylaminate concentration varies greatly with the light filter used and the thickness of solution through which the light passes. The latter factor can be rendered unimportant by using identical cells to contain solutions to be snbjected to transmission determination. The effect of a light
As the first step in finding a better light filter, the absorption spectra of several solut,ionsof potassium dipicrylaminate were determined with the Hilger visual spectrophotometer. Figure 1 shows three of these curves. While they are rough, it is apparent that the filter should have a sharp cutoff a t about 5300 -%.,and allow as little as possible of the red or infrared to come through. The cobalt glass and the blue cellophane failed because both have substantial transparency to red light, whereas the absorption of solutions of potassium dipicrylaminate of widely differing concentrations is almost the same. At the suggestion of the Corning Glass Works, filters of Corning glasses KO. 556 (signal blue) and S o . 429 (blue green) were obtained. I n Figure 2 are plotted the logarithms and the values of the transmission percentages of dipicrylaminate solutions as a function of the concentration, using several 0 different light filters. The values of Kolt0 hoff and Bendix are not strictly comparable X to the others, as the solution thickness in 0 the cells used by them was much greater Y 2 i than the 1 cm. of the photelometer cells. Several important conclusions may be drawn from the curves. The equipment used by Koltlioff and Bendix gives superior results up to a concentration of 50 micrograms of potassium as potassium dipicrylaminate per 100 ml. The use of a 1-cm. cell and a combination of Corning glasses S o s . 556 and 429 vi11 give better results over a larger range of concentrations. If a single filter must be used, Yo 556 is preferable. It can be seen from curve I (Figure 2 ) that dipicrylamine solutions follow Beer's lax closely if the combination of filters is used. This makes possible the use of a Duhoacq colorimeter if a photoelectric instrum e n t i s n o t a v a i l a b l e . Care must be FIGURE 1. TR.~XSMISSIOY CURVES O F POTASSIW>I DIPICRTL.43fINBTE S O L U T I O S s 731
732
INDUSTRIAL AND ENGINEERING CHEMISTRI-
VOL. 12, KO. 12
microgram samples gave results of fl and +2 per cent, respectively.
Indirect Procedure for Potassium An indirect empirical method for potassium was developed which has certain advantages over the procedures described above. It is more r a p i d , r e q u i r e s l e s s manipulation and apparatus, uses less of bhe reagent per determination, and gives more accurate results. The reagent used is a 0.6 per cent solution of lithium dipicrylaminate in water, saturated at 25" C. with potassium dipicrylaminate. A measured quantity of this reagent is added to the dried sample, and the mixture is allowed to stand for 2 hours. An aliquot portion of the supernatant liquid is then withdrawn and diluted to 100 ml., and the transmission percentage is determined. The value obtained is compared with a standard curve obtained by treating a graduated series of standard samples in the same manner. A standard curve of this type is shown in Figure 3. MATERIALSUSED. Dipicrylamine is obtainable from the Eastman Kodak Company at $2 per 10 grams. APPARATUS. If the laboratory is subject to great temperature variation, a crude thermostat controllable to 1 2 ' C. is required. LITHIUMDIPICRYLAMISBTE I I E b G E X T , 0.8 per cent. A solution of 0.55 gram of lithium carbonate in 100 ml. of water is prepared, and 3 grams of dipicrylamine are added. This MICROGRAMS OF POTASSIUM aolution is heated to 50" C., allowed to stand for 24 hours, then filtered into a 500-ml. flask FIGURE 2. TRANSMISSIOX PERCESTAGE PLOTTED .4S d FUXCTIOK O F COSCENand made up t o about BOO ml. This diluted TRATIOX OF POTASSIUM DIPICRYLAMINB FOR \rARIOIJS COMBIX.iTIOSS OF rpagent is heated to 50" C. and moist pot,asLIGHTFILTERS sium dipicrylaminate is added until no more I. 5.5 mm. of Corning glass No. 556 and 3 . 8 mm. of Corning glass S o . 429 dissolves. The saturated solution is allox-ed 11. 5.5 mm. of Corning glass No. 556 to cool to room temperature and is not fil111. 3.8 mm. of Cornin glass No. 429 tered from the deposited crystals. The potti.;IV. Green glass light ffter V. Values of Kolthoff and Bendix sium salt used for saturation is prepared by adding a few milliliters of 3 per cent reagent to the calculated quantity of potassium cliloride solution, and washing the precipitate with distilled water. taken, however, that the concentration and thickness of soluST.4SD.kRD POTASSIUM SOLUTION. A solution of potassium sulfa'' within the range curve I approximates a fate or &loride is prepared, containing 0.1 mg. of potassium per ml. .~. straight line. Kine samples of the standard potassium soluPROCEDURE, tion are run into conical centrifuge tubes (15 ml.) so that the first Revision of Kolthoff-Bendix Procedure contains 1 ml., the second, 2 ml., etc. The nine tubes are then placed in a drying oven and evaporated t o dryness. In their colorimetric method, Kolthoff and Bendix evaporate a The dried samples are allowed t o cool to room temperature, and liquid to dryness in a porcelain crucible, place the cooled crucible 1 ml. of freshly filtered reagent is added t o each, using the same in ice water to a depth of 1 cm., and then add a few drops of 3 pipet. The tubes are well mixed by rotating betveen the palms per cent magnesium dipicrylaminate. They allow the mixture t o of the hands, and are allowed to stand at a convenient temperastand for 10 minutes, and then draw off the excess reagent through ture for about 2 hours. A 0.4-ml. sample is then withdrawn from a filter stick. They wash the precipitate successively with ice each, using a blood pipet, the tip of Tvhich is covered with a small water, a solution of potassium dipicrylaminate (saturated at piece of filter paper held in place with a rubber band. The filter 0" C.), and finally with ice water, dissolve the precipitate in a few paper is removed, the sample is adjusted to the mark, and the drops of acetone, and dilute with water to 100 ml. They then deentire contents are rinsed into a 100-ml. volumetric flask, and termine the transmission percentage, and compare with a curve made up to the mark with distilled water. The light transmission such as those of Figure 2. percentage is then determined. The values obtained for the nine standard samples and for 0.4 ml. of the reagent diluted to 100 This procedure may be used a t room temperature if both the reagent and the wash liquid are saturated a t t h a t temperature with potassium dipicrylaminate. The washings with ice water are not necessary in order to get reproducible results, TABLE I. DETERMIS.~TIOS O F POTASSICM as the amount of potassium salt in the liquid adhering to the TemperaK K ture I/Io X 100 Found in Sample Error precipitate and apparatus is negligible and at the most requires c. Mg. MQ. % a slight constant blank correction. 3.5 0.621 0.600 58.5 18 It is also advantageous to run the whole determination in a 4 .0 0,600 59.0 0.624 18 1.0 0.600 0,606 large Pyrex test tube calibrated at 50 ml., and to perform the 55.0 22 0 . (1 0 600 0.600 54.0 25 final dilution in it, thus eliminating the chance of loss during - 0 ,? 0,597 0 600 53.5 27.5 0 . 0 0 . 6 0 0 0 . 6 0 0 54.0 27.5 the transfer from a crucible to a volumetric flask. -5.0 0.600 48.5 0,570 34 5 . 0 0 . 5 7 0 0 . 6 0 0 Three 100-microgram samples were determined using these 4 8 . 5 34 modifications with errors of 0, 0, and fl per cent. Two 50~
DECEMBER 15, 1940
ANALYTICAL EDITION
735
Likely Cause of Uncertain End Point
TABLE 11. ACCURACY OF METHOD (Potassium iodide Pquivalent t o . 2 . 0 mg. of iodine w e d instead of powdrred thyroid a t different hydrogen-ion concentrations of solution titrated. U. 5. P. method except for refinemehts mentioned.) N/?OO X 1.046 Color of Solution \'mol Illue Used Sodium Procedure ( T b h Indicator) Thiosulfate Blank Iodine Recovery
kI
U. S. P. XI all the nay Recommended pH and teriinerature
Strong pink J u s t barely pink
cc.
cc.
18.4 18.7 18.0 18.0 19.3 18.8
0.2 0.2 None None Indefinite
adjusted by adding 5 drops of the thymol blue indicator solution and then 50 per cent sodium hydroxide until the solution is barely on the pink side. .:This corresponds to the T. B. color of pH 2.2 to 2.4 on the,Clnrk color chart and can best be described as a salmon pink. {The actual pH measured with a glass electrode is about 2.G. The discrepancy between electrometric and colorimetric readings is, of course, due to a colorimetric salt error. Any uncertainty as to this color the first few times the test is made can be avoided by making the solution yellow and bringing back just to salmon pink with phosphoric acid (1 to 1). The temperature of the solution is adjusted to 32' to 34" C., the potassium iodide is added, and the titration is made with thiosulfate solution in the usual manner. The pink color of the indicator does not interfere vith the iodine-starch end point, If any find its resence objectionable, they can use the thymol blue as an outsig indicator. I t is also likely that thymol blue test papers could be successfully employed. The U. S. T. method calls for the titration to be made at about 25' C. When alkali is added to raise the pH, the 25' C. of the U. S. P. method rises to about 33" C. and best results are obtained at this higher temperature. If a higher temperature is not used at the higher pH, the liberation of iodine proceeds slowly when the potassium iodide is added. The hypochlorite made according t o the older U. S. P. X and hypochlorite made in the packing-house sanitary department from commercial chemicals all give satisfactory results when the method is modified by pH adjustment. Others may have convenient sources of hypocliloritc wliich, when adjusted to a chlorine content of about 2.5 per cent will make its special preparation unnecessary.
Experimental The experimental work presented is representative of considerable data accumulated on this subject over a period of years. I n all cases the results have consistently pointed in the same direction. An analyst will invariably check himself within 0.1 cc. and two analysts will not ordinarily differ more than 0.2 cc. (0.1 per cent) b y the modified procedure. I n Table I is shown the effect of varying the p H of the solution to be titrated from the low p H encountered in the old U. S. P. X method u p t o a p H above the optimum. Electrometric p H readings were made for comparison with the colors of the various solutions. The present U. S. P. procedure gives solutions which vary in p H from 2.0 to 2.4 even with careful measurements of the same set of reagents. The electrometric p H readings of the adjusted solutions will vary between 2.5 and 2.7. The figure 2.6 given is therefore an average pH. The table is self-explanatory. Table I1 shows that the accuracy of the refined method is good. Table I11 shows that the absence of a blank a t the recommended p H is not due to lack of sensitivity to small quantities of iodine. Recovery was good u p to and including a p H of 3.0 and with only 0.2 cc. of 0.005 N iodate. Using the recommended procedure, significant amounts of iodine in the reagents used would appear in the blank and would have meaning; whereas, by the 8.S. P. XI method, blanks on the same reagents are a p t to be variable and therefore of doubtful value.
The fact that the authors obtain no blank by their method would seem to elimiMa. % nate the possibility that re2.014 100.7 sidual chlorine is a t fault. 2.047 102.35 1.992 99.6 Hojer (4) claims that the 1,992 errors, when chlorine is used 2.136 106.8 2.092 104.6 t o oxidize iodides to iodates, are due to residual chlorates and perchlorates, and t h a t slight effects can be caused b y t h e oxygen of the air in acid solution. While Hojer realized that the effect of the oxygen in the air could be minimized by the control of acidity, he did not realize that the interference of chlorates could be avoided if the p H was high enough. He finally resorted to the use of bromine for his oxidations. Bray (2) found that within certain limits the rate at which iodine is liberated in solutions containing potassium chlorate, potassium iodide, and hydrochloric acid is proportional to the concentration of the chlorate and to the square of the concentration of the hydrogen ion, and is a linear function of the concentration of the chloride ion and of potassium iodide. It appears, therefore, t h a t the liberation of iodine from iodides b y chlorates at the pH of the U. S. P. titration is slow and incomplete but is sufficient to make the end point uncertain. This effect is increased by lowering the p H or increasing the chlorate content of the solution. When the pH of the solution is raised to about 2.6, the reaction proceeds so slowly that the amount of chlorate ordinarily present does not interfere and no blank is obtained on the reagents. This p H is, however, low enough for the desired quantitative reaction between the iodide and the iodate.
TABLE 111. RECOVERY OF SMALL AMOUNTSOF 0.005 AT IODATE
SOLUTION
[In 150 cc. of water adjusted t o various p H values with phosphorio acid (1 to 1) and titrated with 0.005 N sodium thiosulfate a t 32' t o 34' C.] H of Solution 0,.005LV, 0.005.N (&ass c< lectrode) Potassium Biiodate Sodium Thiosulfate
2.98 2.70 2.04 2.38 3.00 2.82
cc .
cc .
0.50 0.50 0.50
0.50 0.50 0.50 0.50 0.30 0.20
0,50
0.30 0.20
Summary The uncertain end point and the blank are eliminated and accurate results obtained by the U.S. P. XI assay for iodine in thyroid when the p H is adjusted t o about 2.5 to 2.7 and the temperature to about 33" C. before titration. Preliminary investigation of the use of this modified method for determining iodine in mineral feeds has indicated possibilities for use in other fields.
Literature Cited G. D., and Szalkowski. C . R.,J . A m . Phnrm. Assoc.. 24,
11, 637 (1939). (4) H o j e r , J. A,, Biochem. Z., 205, 273 (1929).
Electrolytic Determination of Copper in Steel H. A. FREDIANI' AND C. H. HALEZ, Louisiana S t a t e University, University, La.
A
the current permits the use of high current densities without overheating the solution. With this ccll, after complete deposition of the met,al, the electrodes may be convenicntly washed by opening the stopcock and gradually displacing the electrolysis solution with distilled water until zero current is registered on the ammeter.
S EARLY as 1910 Blasdale and Cruess (2) remarked that
the electrolytic determination of copper had been studied since 1863 and 1.hat the literature on the method was voluminous. The literature has been greatly increased since that time and a comprehensive review would be a major report in itself. Numerous publications have dealt with the electrolytic separation of copper from iron-bcaring solutions, but few have had to do with the actual concentration proportions met in the analysis of copper-bearing steels and cast iron. Prescnt practicc appears to bc b a d upon the sepamtion of copper as the sulfide, followed either by its oxidation and weighing as cupric oxide or resolution of the cupric sulfide and electrolysis of the resultant iron-free solution. The presence of ferric ion during the electrolytic deposition of copper is undesirable (6), inasmuch as there is a strong tendency for the iron to be reduced to the ferrous state and thus either prevent the deposition of copper or retard it to such an extent as to make the separation impracticable. Removal of the iron by neutralization with ammonia and filtration, as recommended by Schong (9),followed by electrolysis of the acidliied filtrate, is a tedious procedure a t best. This is especially true in the analysis of low-copper samples, since Toporescu ( I d ) has shown that copper is occluded to an appreciable extent under these conditions, thus making double precipitation necessary. The same remarks apply to the method suggested by Fife and Torrance ( 5 ) . The preferential solubility of iron by dilute sulfuric acid has been suggested by Anderson and Swett ( I ) as a means of effecting a separation of iron from co per in steel. The copper remains in the undissolved residue, wbch is then dissolved in nitric acid, and the resultant solution is electrolyzed. Unfortunately, many samples are difficult to dissolve by the mere use of sulfuric acid. Furthermore, Zinberg (15), who took great pains to exclude air during the solution process, found that some of the copper was invariably lost in the filtrate. He nevertheless claimed satisfactory results with samples containing from 0.5 to 5 per cent of copper by dissolving out the iron with sulfuric acid in a carbon dioxide atmosphere and igniting the residue to cupric oxide in a porcelain crucible. Effective removal of the iron by the formation of ferric comlcxes has often bcen resorted to for analytical separations. imith (10, I f ) and Fernberger and Smith ( 4 ) discussed the use of phosphoric acid for this purpose a t an early date, although the proportions of iron to copper investigated were small as comr d to actual conditions obtaining for present-day alloys. aInbcrg and Troitzkaya (3) used potassium hydrogen fluoride for the formation of the FeFe--- complex, although they did not investigste the feasibility of subsequent electrolytic removal of copper from such a solution. Hoar ( 7 ) prevented the interference of ferric ion with the colorimetric determination of copper by using citric acid, ammonia, and gum arabic, or sodium pyrophosphate.
Method The procedures used involve relatively rapid dissolution of
the sample steel, combined with the addition of phosphate or fluoride ion to tie u p any ferric ion that may be formed. The resultant solution is then electrolyzed a t R rather high current density while being maintained a t a relatively low temperature. APPARATUS.The electroanalyzer used for plat,ing the copper was a rather old Fisher, single-spindle modcl which has bcen in use for many years in one of the advanced undergraduate laboratories. For electrolysis of the copper-bearing solution the vessel shown in Figure 1 (obtainable from the Fisher Scientific Company, Pittsburgh, Penna.) has been found most satisfactory. Circulation of ice water through the outer jacket during passage of 1
Present address, Fisher Scientifio Company, Pittsburgh, Penna. address, Standard Oil Company of Louisiana, Baton Rouge,
1 Preaent
La.
736
$
wl
FIGUKE 1.
DlAGR.4M OF
APPARATUS
PROCEDURE. For relatively easily soluble samples the following procedure was used: A sample between 1 and 5 grams (depending upon the copper concentration) was weighed into a 400-nil. beaker. Five milliliters each of concentrated sulfuric and sirupy phosphoric acid were added, followed by 260 ml. of distilled water. Gentle warming for 15 to 30 minutes usually effected solution. The resultant solution was then filtered into the electrolysis vessel, 5 ml. of a saturated disodium phosphate and 5 ml. of a 20 per cent ammonium sulfate solution were added, and the solution was electrolyzed at 3 volts and 2.5 to 3.5 amperes for one hour. The anode stirrer rotated a t 500 r. p. m. For Sam les more difficult to dissolve, 5 ml. of nitric acid were initially axded with the sulfuric acid. When reaction ceased the solution was evaporated until crystallization started, then cooled, diluted to 250 ml., and filtered. An :Iltcrnative procedure which also yielded satisfactory results, especinlly \vith sarnplcs ordinarily difficult to dissolve, is the following: The sample (from 1 to 5 grams) is weighed into a 100ml. platinum dish. An acid mixture consisting of 5 ml. each of concentrated sulfuric, nitric, and hydrofluoric acids and 20 ml. of distilled water is sloivly added. All samples that have been tried up to the present appear to dissolve very rapidly. After reaction has appeared to cease, the solution is gently warmed for 5 minutes and then diluted, filtered, and placed in the electrolysis vessel.
