Determination of Potassium by a Tetraphenyl borate Method R. M. ENGELBRECHT and F. A. MCCOY Research Department, Lion O i l Division, Monsanto Chemical Co.,
solution from a pipet, and swirl gently while adding the reagent. Cool to room temperature, swirling several times during the cooling period to ensure adequate mixing. The cooling period may be accelerated by placing the flask in a pan of chilled water. When the solution has reached room temperature, filter through a weighed, medium-porosity, Gooch crucible. Wash well with distilled water saturated with potassium tetraphenylborate. Dry a t 110' C. for 1 hour, cool in a desiccator, and reweigh. The gravimetric factor for potassium in the precipitate is 0.10912.
The use of sodium tetraphenylborate for the determination of potassium is becoming increasingly popular. A method described here for the determination of potassium in the presence of the ammonium ion is superior to the classical chloroplatinate method from the standpoint of accuracy, analysis time, and reagent cost.
N
El Dorado, Ark.
UMEROUS methods for potassium determination may be
DISCUSSION OF RESULTS
found in the literature; the chloroplatinate and flame photometric methods are the most widely used. The chloroplatinate method is extremely time-consuming and numerous ions coprecipitate with the potassium and affect the accuracy of the method. The flame photometric method is very rapid and finds excellent application where small amounts of potassium are to be determined. However, the dilutions required for samples high in potassium content may affect the accuracy of the method. The determination of potassium with sodium tetraphenylborate reagent has become increasingly popular. Gravimetric (6), volumetric (2, 5 ) , polarographic ( I ) , and conductometric (4) procedures utilizing this reagent have been described. The main interest in this investigation was finding a suitable procedure using sodium tetraphenylborate for the determination of potassium in the presence of the ammonium ion. Rudorff and Zannier (5) reported a volumetric procedure for the determination of potassium and ammonium with sodium tetraphenylborate in the presence of each other. The ammonium ion is known to react with tetraphenylborate in the same manner as the potassium ion. Therefore, the ammonium ion was first determined by the formaldehyde method of Marcali and Rieman ( 3 ) . The potassium was then determined by a volumetric procedure using sodium tetraphenylborate to precipitate the potassium. The precipitate was filtered, dissolved in acetone and titrated with a standard silver nitrate solution. The method described below is based on this scheme of analysis, except that a gravimetric rather than volumetric determination is made.
The results of the above method are s h o m in Table I. The test solutions were prepared by weighing reagent grade potassium chloride into a flask, adding a weighed amount of ammonium chloride, and dissolving in distilled water. In several instances aliquots were taken from a stock solution of potassium chloride and tested. It may be seen from the table that the ratio of potassium to ammonium ions varied from 2 : 1 to 1:4. The average recovery of potassium by this method was 99%. For the two cases shown, the recovery of potassium by the tetraphenylborate method was closer t o theoretical than by the chloroplatinate method. The analysis time by this method is a third that required by the chloroplatinate method.
Table I.
Determination of Potassium in Presence of Ammonium Ion K Found, Mg.0 K + Weighed, NH4 +, +
Mg.
3 22 40 60 40 45 80 60 140 85
TPB.
Mg. 5.9 10.3 13.1 13.1 15.6 24.4 31.2 31.4 32.1 43.2
Tetraphenylborate method.
TPB
5.9 10.3 13.0 12.7 15.5 24.6 30.8 30.6 31.6 42.3
CP... , . .
... ...
15 0 ,..
30.0
... ...
...
C P Chloroplatinate method
REAGENTS
Sodium Tetraphenylborate Solution. A 1% w./v. solution of sodium tetraphenylborate in 0.01N sodium hydroxide is used as the precipitant. Sodium tetraphenylborate owder, a Baker's analyzed reagent, was used. The solution sgould be prepared fresh each day. However, it was found to keep fairly well for several days in a refrigerator. Potassium Tetraphenylborate Solution. Potassium tetraphenylborate crystals are obtained by precipitation from a potassium chloride solution similar to that described in this paper. The filtered precipitate is dissolved in acetone and the potassium tetraphenylborate recrystallized from this. ii saturated aqueous potassium tetraphenylborate solution is used as the wash solution. Formaldehyde. h 3770 formaldehyde solution purchased from the hlallinckrodt Chemical Co. was used. Sodium Hydroxide. Reagent grade sodium hydroxide pellets are satisfactory.
Potassium is determined regularly in samples containing sodium, potassium, ammonium, chloride, nitrate, sulfate, and phosphate. Suspected high potassium results were attributed to the presence of sulfate and phosphate in the analysis mixture. Table 11shows a comparison of the tetraphenylborate and chloroplatinate methods on several samples of this type. In all cases the chloroplatinate results are high. The cation-anion balance for the analysis mixtures was better %-henthe tetraphenylborate
PROCEDURE
To an aliquot that contains no more than 45 mg. of potassium and up to 150 mg. of ammonium ion add 130 ml. of formaldehyde solution. Swirl gently to mix well and let stand, well stoppered, for about 5 minutes. Add about 6 grams of pelleted sodium hydroxide and dissolve; this makes the resulting solution about 1 N with res ect to the sodium hydroxide. As soon as the sodium hydroxide gas dissolved, heat just to boiling on a hot plate. Remove, slowly add 50 ml. of the 1% sodium tetraphenylborate
1772
Table 11. Comparison of Tetraphenylborate and ChloropIatinate Methods K + , 5% TPB 4.79 5.94 5.93 5.87 4.33 6.27 5.89 7.28
CP 4.90 6.17 6.26 5.96 5.93 6.27 6.03 7.59
V O L U M E 2 8 , N O . 11, N O V E M B E R 1 9 5 6 rcsult v-as used. This indicated the superiority of the method over the chloroplatinate method. The volumetric procedure of Rudorff and Zannier (6) has been desciibed. The determination of the ammonium ion and potasium in the same aliquot has not been tried; but the method should be most satisfactory, since the formaldehyde determination of ammonium is a very accurate method. Although the alkalinity of the test solution due to the addition of sodium hydroxide should conceivably eliminate any ammonium ion on heating, a large excess of formaldehyde ensures quantitative elimination of the ammonium ion. The ammonium ion is known (3) to react quantitatively with formaldehyde to form hexamethylenetetramine. Rudorff and Zannier reported higher than theoretical results if a large excess of formaldehyde were not present or if the solution were only weakly alkaline. The ptvimetric procedure was preferred to the volumetric procedure only from the standpoint of the analyst’s time. Both procedures require precipitation and filtration. Once the precipitate
1773 is filtered, an analyst need only reweigh after the drying period is completed. A conductometric method using lithium tetraphenylborate to determine potassium in the presence of sodium, magnesium, calcium, strontium, or barium has been developed by Raff and Brotz (4). The method is very rapid and possibly could be adapted to the determination of potassium in the presence of the ammonium ion. LITERATURE CITED
(1) Findeia, A. F., Jr., De Vries, T., ASAL. CHEX.28, 209 (1956). ( 2 ) Flaschka, H., Holasek, d.,Amin, A. JZ., 2. anal. Chem. 138,
161-71 (1953).
(3) LMarcali, K., Rieman, W., ISD. EXG.CHEM.,ANAL.ED. 18, 709-
10 (1946).
(4) Raff, P., Brotz, W., 2. anal. Chem. 133, 241-8 (1951). ( 5 ) Rudorff, W., Zannier, H., I b i d . , 140, 241-5 (1953). (6) Sporek, K., Williams, A. F., Analyst 80, 347-54 (1955).
RECEIVED for review March 26, 1956. Accepted July 9, 1956.
Polarographic Determination of Aluminum and Zinc in Magnesium Alloys D. G. GAGE N a v a l Research Establishment, Defence Research Board of Canada, Dartmouth, N o v a Scotia, Canada
A method for polarographic determination of aluminum in alloy steels has been adapted to permit simultaneous determination of zinc and has been applied to analysis of magnesium-base alloys. An illustration of the effectiveness of this method in studying inverse segregation in magnesium alloys is given.
I
S T H E course of an investigation of segregation of aluminum
and zinc in magnesium alloys, the need arose for a rapid and reasonably accurate method for determining these elements. A gravimetric method for aluminum ( 7 ) was unsuitable by reason of both the time required and the fact that zinc could be detected in the ignited residue. Gull ( I ) reported a polarographic method for aluminum, lead, zinc, and manganese in magnesium alloys. This method Tvas suitable for zinc determinations but in the case of aluminum there was some difficulty due to masking of the aluminum wave by a hydrogen wave at p H 3 or less. A similar method has been reported by Heller and Zan’ko ( 2 , S ) , who also found difficulty with the aluminum determination. Unfortunately, buffer solutions cannot be used to overcome this difficulty (5’.
Willard and Dean (9)have described a method for polarographic determination of aluminum in limestone, iron ores, copper-base alloys, and alloy steels containing 0.1 to 1% of the metal. The present paper deals Tvith the adaptation of the Willard and Dean method to the determination of both aluminum and zinc in magnesium-base alloys. 4PPAR4TUS AND REAGENTS
A Sargent Model XI1 polarograph was used in this work. S o other specialized apparatus was involved. Pontochrome Violet S W (sodium salt of 5-sulfo-2-hydroxybenzeneazonaphthol), obtained from E. I. du Pont de Nemours & Co., was used in 0 . 0 5 7 aqueous solution. Samples of the same
compound (designated Solochrome Violet) can be obtained from Imperial Chemical Industries, Ltd. STANDARD SOLLTIONS
The standard solutions of aluminum and zinc >\-ereprepared by dissolving the reagent grade metals in 5M perchloric acid and diluting with water to give each standard a concentration of 10 grams per liter. The magnesium standard was prepared from magnesium perchlorate and made up to contain 20 grams of magnesium per liter. PROCEDURE
A 0.5-gram sample of the alloy was dissolved by careful addition of 10 ml. of 5 M perchloric acid and diluted stepwise to give a final concentration of 5 mg. of magnesium in 50 ml. of the solution containing the dye. iYeutralization and buffering were carried out as outlined in the original work (9, and by Kolthoff and Lingane ( 5 ,6). A portion of the final solution was transferred to a polarographic cell, the dissolved air removed with nitrogen, and the polarogram recorded. The standard solutions were treated in the same way and the aluminum content of the unknown was determined by comparison of the heights of the second waves a t -0.5 volt us. S.C.E. Zinc content could be determined by comparison of height of the waves a t -1.2 volts us. S.C.E. but in cases where the aluminum-zinc ratio was greater than 1, this wave was too small for accurate measurement. In such cases, zinc was determined from the polarogram of an aliquot of the original solution equivalent to a 0.1-gram sample diluted to 50 ml. RESULTS AND DISCUSSION
The polarograms show the typical double wave reported by Willard and Dean (Q), the second wave (-0.5 volt os. S.C.E.) being that used for determination of aluminum. Diffusion currents of a series of standard solutions of aluminum plotted against their concentrations gave a calibration graph identical to that of Willard and Dean. A similar graph can he prepared for zinc, using the height of the third wave at -1.2 volts us. S.C.E. This also applies when zinc is determined on a separate aliquot of the original solution. The high sample dilution used was made necessary by the limited solubility of the dye. Possible substitute dyes are Super-
