brought about b y the addition of various types of added nonelectroactive species. Once these effects are quantitatively determined, thermal electroanalysis could become a rapid, routine method for the determination of any ion which may be quantitatively oxidized or reduced a t a n electrode without interference from other electrode reactions.
LITERATURE CITED
( 1 ) Eastman, E. D., J . Am. Chent. Soc. 50,283 (1928). (2) Gritsan, D. K., Bulgakova, A. M., Zolotareva, G. A., Zhur. Fiz. Khim. 28,337 (1954). ( 3 ) de Groot, S. R., “Thermodynamics of Irreversible Processes,” Interscience, New York, 1951. (4) Hasse, R., Trans. Faraday SOC.49, i 2 4 (1953).
( 5 ) Holmes, H. F., Joncich, AI. J., ASAL. CHEM.31,28 (1959). (6) Holtan, H., Jr., thesis, Cniversity of Ltrecht, 1953. ( 7 ) Lange, E., Hesse, T., J. Am. Chem. SOC.55,853 (1933). (8) Lange, E., Hesse, T., Z. Elektrochem. 39,374 (1933). (9) Onsager, L., Phys. Rev. 37,405 (1931).
RECEIVEDfor review June 2, 1959. Resubmitted Ji le 17, 19GO. Accepted June 17, 1960.
Direct Titration of Potassium with Tetrapheny borate Amperometric Equivalence-Point Detection DAVID L. SMITH, DONALD R. JAMIESON, and PHILIP J. ELVING University of Michigan, Ann Arbor, Mich. FTetraphenylborate ion gives two anodic voltammetric waves at the graphite electrode in aqueous solution, which waves can be used to follow the concentration of the tetraphenylborate ion. This electroactivity has been made the basis for the direct titrimetric determination of potassium via its precipitation as potassium tetraphenylborate. The resulting method i s simple and rapid; there i s no need to filter, wash, and redissolve the precipitate as in the indirect titration methods proposed for the determination of potassium by precipitation with tetraphenylborate. The method i s also more convenient, rapid, and flexible than other proposed direct titration methods for potassium. It i s relatively free from interferences, tolerating the presence of large amounts of chloride and other commonly encountered anions. The procedure has been satisfactorily applied to the direct determination of potassium in silicates and other refractory samples after sulfurichydrofluoric acid dissolution and fuming.
I
of ai1 investigation on the use of the graphite electrode for the voltammetric study of organic oxidation reactions. it n a s observed that the tetraphenylborate ion (TPB) was electrochemically oxidized in aqueous solution a t thiq electrode. Review of the literature revealed this to be apparently the first observation of the electrocheniical oxidation of T P B in aqueous solution. Geske ( I d ) subsequently described the electrochemical oxidation of T P B a t the rotating platinum microelectrode in nonaqueous solvents (best results n-ere obtained in acetonitrile) ; he reported that T P B could not be oxidized at the N THE COUIZSE
platinum electrode in aqueous solution because of the interfering evolution of oxygen. The observed electroactivity of TPB at the graphite electrode in aqueous solution mas recognized as a specific advantage of this electrode, and i t was decided to explore the possibility of determining potassium directly b y means of an amperometric titration based upon this electroactivity. Titrimetric Potassium Determination. Since K i t t i g et al. (28) discovered during a n investigation of organometallic compounds t h a t t h e tetraphenylborate ion quantitatively precipitates potassium ion in aqueous solution, t h e literature on t h e chemist r y and analytical application of T P B has been eutensive. Adequate bibliographies and reviews ( 2 , 3, I S ) are available, so only the most recent and most pertinent references n-ill be discussed. The first thorough investigation of the adaptability of T P B for the gravimetric determination of potassium was published b y Raff and Brotz (80); the gravimetric method continues to be b y far the most videly used. The principal diqadvantage of this method, in addition t o the normal time-consuming steps of the usual precipitationgravimetric procedure, is due to the physical characteristics of the precipitate, which is fine and often flocculent in nature. Consequently, i t tends to adhere to the walls of the beaker, and is difficult to transfer and wash, especially when the potassium is present in small amount (11). Several indirect titration methods for potassium based on precipitation with T P B have been reported, which eliminate the need for drying and weighing the potassium tetraphenylbo-
rate (KTPB). Xearly all of these methods, hon ever, require filtering and washing of the KTPB, follon ed either by determination of the excess T P B in the filtrate (f6-17, 22) or by dissolution of the K T P B in a n appropriate Eoll-ent and subqequent determination of the potassium ion polarographically (9) or of the T P B ion titrimetrically (10, 14, 16, 21). I n an indirect method based upon the chemical oxidation of K T P B (24, 26)>the precipitated K T P B is dissolved in acetone and oxidized n i t h excess Ce(1V); the exceis is back-titrated n ith Fe(I1). X direct conductometiic titration method, employing LiTPB as titrant, ha. been described (20), as ha- the use of a high-frquency owillator to follow the titration (18). Foreign electrolytes are generally undesirable in conductometric and high-frequency titrations, and thpir presence limit. the use of these methods. Direct potentionietric titration of some organic and inorganic nitrogen bases n i t h S a T P B u4ng a silver electrode assembly ( 6 ) has been reported ( 1 7 ) ; unfortunatelj-, poisoning of the electrode occurred. eipecially 17-hen halogens n-ere preaent, and many of the curves did not give sharp end points. I n a recently reported ( I ) direct amperometric titration of potassium mith N a T P B employing a dropping mercury electrode, the potential is set a t a value nhere the mercury electrode is oxidized; since Hg(I1) forms a precipitate 111th T P B , a n increase in current is observed after the end point becauqe of the increased rate of mercury oxidation rewlting from the shift in equilibrium tov ard insoluble Hg(TPB)?. Chloride concentration greater than 0 . 3 2 X and trace VOL. 32, NO. 10, SEPTEMBER 1960
1253
amounts of iodide and bromide interfered. Since between two and three hours are required for a titration because of drifting current values, this inet'hod is somewhat tedious (23). The method described here eliminates many of the disadvuntages of previous methods including the necessity of recovery and transference of the precipit'ate. Only 20 minutes are required for a single determination, whcre other proposed methods for potassium require considerably longer periods of time. The accuracy of the results obt:tined is good. EXPERIMENTAL
Graphite Electrode. Grade U-1 (medium density) spectroscopic graphite electrodes (United Carboil Products, Inc.) were iniprcgnated under vacuum with ceresin wax b y a procedure similar t o t h a t described by Morris and Schempf (19). The electrode was removed from the wax, wiped with absorbent tissue, allowed to cool, and then, using a pipe cleaner, coated with Seal-All cement (Allen Products Corp.) (6). A fresh graphite surface was prepared before each run by lathing (29),although other methods of electrode surfacing could be used +ice electrode area reproduribility is not a critical factor. Prior to each run, the elwtrode was clipped momentarily into a 0.00397, solution of Triton X-100 to ensure vomplete wetting of the elcctrode surfare; the excess solution was shaken off on removing the electrode from thc. yolution. Investigations in this laboratory ( 7 ) have showii electrode wetting to be an important factor in improving the voltammetric and chronopotentiometric results obtained with the waximpregnated graphite electrode. Electrical contact was made via a n :digator clip attached to the upper end of the rod, where the Seal-All coating had been removed. Four-inch lengths of rod were generally used. Apparatus. Current-potential cuives mere recorded n i t h a Leeds & S o r t h r u p T y p e E Electro-Chemograph. Titrations n ere carried out in beakers using either t h e Fisher Elecdropode or t h e Sargeiit Anipot :is potential source and current indicator in conjunction n-ith a graphitcl electrode and a n external saturated sodium chloride-calonicl reference electrode which was connected to the solutioii through an agar plug saturated with KaC1. Constant-speed stirring of the test solution during titration was effected by means of a small propeller stirrer driven by a Sargent conedrive stirring motor connected to the mains through a Thordarson voltage regulator. Burets of 10-nil. capacity, graduated to 0.05 ml., mere used. pH measurements were made with a Lerds & Northrup p H meter. Platinum dishes of 100-ml. capacity were used for the evaporation of rock and glass samples with sulfuric and hydrofluoric acids. 1254
e
ANALYTICAL CHEMISTRY
Table 1.
Stability of Sodium Tetraphenylborate Solution"
Time,
Titer, Mg. KZO/hIl. TPB 4 . 40b 4.38* 4.33 4 34 4.48 4.42 4 431 4.376 4.44 Av. titer 4 40 Std. dev. 0 05 (1 1%) aPrepared and stored as described in text. By aliquoting varying amounts of standard KCl solution, the amount of K20 taken varied from 24.0 t o 32.0 mg. for these titrations. Average of two determinations. The others are single determinations. Day> 0 1 2 3 6 8 9 10 16
Chemicals and Reagents. Sodium tctiaphenylborate reagent (J. T. Hakei and hIatheson, Coleman and Bell) was used nithout further purification. A standard 5OhltiOli was prepared by dissolving ahout 3 grams of N a T P B in 100 ml. of conductance water (or similar qalt t o volume ratio) in a borosilicate glass volumetric flask, in which the solution was stored a t room temperature in daylight. This solution was slightly turbid, but was not filtered, since i t gave satisfactory results without renioval of tlir turbidity. It was standardized by amperometric titration of standard KCI solution in acetate buffer of p€I 5.6. The titer of the solution, which was slightly basic, did not change significantly over a period of 2 weeks (Table I). Others (4, 17, $6) have also reported slightly basic aqueous solutions of N a T P B to be stable for long periods. A411other chc~micals were of reagent or C.P. quality grade, and were used 'i\ ithout further purification - . AMPEROMETRIC TITRATION PROCEDURE
Tlie size of the sample and the volume of distilled water, in which i t is dissolved, should be such that the potassium concentration is a t least 5mM (0.2 mg. of I< per ml.). T-olumes of 25 to 100 nil. in a 100- to 250-ml. beaker are convenient. Sufficient botliuni acetate to make the test solution about 0.255f should be added; the pH should thrn lie adjusted to about 5 by addition of acetic acid. This can be done conveniently by adding a more concrntrated p H 5 acetate buffer to dissolrr the sample or to the sample solution. (In the development and testing of the procedure here reported, weighed amounts of potassium salts were dissolved in distilled water with the requisite amounts of sodium acetate and glacial acetic acid or other buffer components being added so as to give the desired pH in the resulting solution.) The constant-specd glass stirrer, the reference elcctrode, and the graphite indicating elcctrode are then positioned in the titration vessel, the graphite
electrode having first been resurfaced and wetted with 0.003yo Triton X-100 solution. A potential of t 0 . 5 5 volt is then applied to the graphitc dectrode and the current srnsitivity E w t a t 45 pa. or u similar value, based on cxperiencc, for full-scale deflection. The stirring rate is preferably about 600 r.p.m. for a volume of about 25 ml. in a 100-m1. beaker; with larger Yolumes, the rate should be increased to provide adequate mixing. Small volumes of titrant arc then added from a 10-ml. buret (about 0 . 5 ml. increments before the end point and 0.05-ml. increments after the end point). After each addition, 0.5 to 2 minutes, less frequently 4, depending on the test solution volume, are needed for the current to become constant when a reading is taken. After the end point is reached, the time requircd for the current to become constant valur after each addition of titrant is about 30 seconds, regardless of the volume of the test solution. ' h e end point is determined from the plotted points by the U R U R ~e\trapolation method. DETERMINATION OF POTASSIUM I N SILICATES
The silirate and similar samples, analyzed in the present study, were decomposed by a sulfuric acid-hydrofluoric acid procedure ( 2 7 ) . About 5 nil. of I-to-1 sulfuric acid and an escess (ca. 10 ml. per gram of sample) of hydrofluoric acid (48%) were added to the neighed and moistened sample in a 100-ml. platinum dish. (In the case of limestone and other samples rn hich may react vigorously with acid, cautious preliminary treatment with hydrochloric acid is recommended.) ?'he mixture was then evaporated to near dryness on a hand bath. A second evaporation using smaller amounts of sulfuric and hydrofluoric acids (about 5 nil. of HF solution per gram of sample) was made in some vases to ensure complete decomposition. Hydrochloric acid, 1 to 20, as added to the residue; the resulting mixture was filterrd through Whatman Yo. 2 filter paper and the residue washed with l-to80 hydrochloric acid. The pH of the filtrate mas then adjusted to about 3 by adding concentrated NaOH solution until the proper color change of a universal indicator was observed. The resulting solution was then titrated with sodium tetraphenylborate a' previously described. I n the presence of large amounts of elemrnts, such as aluminum, which form hydrous oxides. i t is best to keep the pH as high as possible without causing precipitation during the titration. I n any event, i t is desirable to maintuin the pH greater than 2. DEVELOPMENT OF PROCEDURE
Oxidation of Tetraphenylborate. Investigation of the niechanism of tetraphenylborate ion ovidation a t t h e graphite electrode is currently under itudy in this laboratory (8). Figure 1 shovis a typical recorded curreiit-potential curve. Tlie halfvavc, potential for the fii>t n a ~ - cis
r---
r
t 4 5c
I
--/
~-
_-
~
\- L
Figure 1 . Current-potential curve of tetraphenylborate in aqueous solution Graphite indicating electrode vs. saturated sodium chloride-calomel reference electrode a t polarization rate of 2 0 0 mv. per minute in positive direction. NaB(C6H,)r, about 0.5mM; background electrolyte, NaOAc-HOAc buffer of p H 4.4, unstirred solution; temperature 25'C.
c 0
N o B(C,d,\d
(6).
Potassium Concentration. The initial volume of solution in respect t o t h e amount of potassium present does not seem t o be significant-e.g., 20.0
-
I e++--+ -
approsilllately 0.3 volt and for the second 0.9 volt vs. t.he saturated sodium chloridecalomel electrode at the polarization rate of 200 mv. per minute used. Addition of K + to the solution eliminates both waves. A potential of 0.55 was chosen for the amperometric titration 011 the basis of manual scanning of the potential range using the Fisher Elecdropode; this potential is approxi1u:itely in the center of the current plateau region between the two waves. Titration Procedure. T h e method \\-as tested by weighing out pure potassiuni chloride, dissolving i t in a hackground (alectrolyte solution, a,nd titrating b y t h e general procedure described. -1 typical titration curve is shown iii Figure 2 . As t h e tetraphenylborate solution is added t o t h e potassium solution, the current increases slightly; when the equivalence point is passed, the current increases rapidly. Two intersecting straight line.; can bedrawn easilythrough the experiniental points. The results. conveniently expressed in terms of the titer of the tet'raphenylborate solution, are reproducible t o n-ithin =t 1% (Tables I and TI). pH. T h c procedure described gives comparalh. results for potassium at pH valucs I w t w e i i 2.5 and 7.0; consequently, rigid control of pH is not necessary (Tablc 111). Erratic current readings were obtained when t h e titration was performed a t pH 1 or lower; this may be due to precipitate solubilit'y, instability of TPB in acidic media (22, 86), or electrode response
l
c Q
m
Figure 2. Titration curve showing effect of addition of tetraphenylborate solution to potassium solution Solution, 2 0 . 0 mg. of K in 25 ml. of 0.25M N a C l solution with HCI a d d e d to bring p H to 2.4
Table II.
Potassium Taken, llg.
Titration of Potassium with Tetraphenylborate"
TPB 1-nl 111.
,h
Titcr, K/TPB, l l g . 311.
18.1 18 8 24 0 26.7 29.7 43 3
Potassium Found,c Mg. 18.1 18.7 23.8 26.9 29.8 43.8
3.76
5.78 3.79 3.73 3.74 llean 3.75 Std. dev. IltO.031 ( 1 0 Sa:,
