THE ATOMIC MASS OF POTASSIUM. I1 THEPOTASSIUM CHLORIDESILVER RATIO CLYDE R. JOHNSON Frick Chemical Laboratory, Princeton University, Princeton, New Jersey Received August 23, lg34
The following report summarizes a determination of the potassium chloride-silver ratio by the new and now fully developed standard solution method (2). PURIFICATION O F REAGENTS
The general criterion of acceptable purity for any reagent used in this work was that it should introduce no impurities equivalent to more than 0.005 mg. of silver into any single analytical system or 1500 g. of standard solution. Potassium chloride. The potassium materials were obtained from five different manufacturers. The salts were purified by rapid crystallization from hot or cooling solutions in 500-cc. platinum dishes. In a few of the early crystallizations, and whenever free chlorine was present in the solutions, quartz vessels were used. As a general rule (applying also in the purifications of silver, lime, sugar, potassium nitrate, sodium nitrite, and ammonium chloride) small head and tail fractions were rejected in each crystallization or precipitation. The central fraction was always drained for about fifteen minutes a t 1500 R. P. M . in a centrifuge attachment in which the salt was inclosed entirely in platinum. Fusions which were part of the potassium chloride purification were made in a 180-cc. platinum cup in an electrically heated 200-cc. porcelain beaker. After these fusions the salt was dissolved in water and filtered through a platinum Munroe crucible. Sample 1 (40 g.) was separated from a 500-g. quantity of potassium oxalate from a Norwegian source. This material was crystallized six times, changed to the chloride by one treatment of the solution with chlorine gas, crystallized once, and fused. Sample 2 (42 g.) was obtained from a kilogram of potassium nitrate from a German source. This material was recrystallized ten times, then precipitated three times from solution with hydrogen chloride gas. After each of the three precipitations the potassium chloride was fused and crystallized once from water. Sample 3 (43g.) 78 1
782
CLYDE R. JOHNSON
was taken from 500 g. of potassium chlorate from a German source. The chlorate was recrystallized ten times and carried cautiously through the two stages of decomposition to a state of clear fusion. Rejected liquors from subsequent crystallizations (vide infra) contained less than 0.01 mg. of chlorate and perchlorate. Sample 4 (67 g.) was separated from 500 g. of potassium chloride from a German source. After one crystallization from water the material was precipitated twice from solution with hydrogen chloride gas. After each precipitation the salt was crystallized once from water, fused, and again crystallized from water. It was next crystallized from constant-boiling hydrochloric acid, then from water, and fused. Sample 5 (78 g.) was taken from an 1100-g. quantity of 98 per cent muriate of potash from Searles Lake, California. This salt was crystallized once from dilute aqua ammonia, twice from water, and once from 1 M hydrochloric acid made from recrystallized Searles Lake potassium chloride, It was then crystallized eight times from water and fused. To complete the above purifications, each of the five potassium chloride samples was crystallized three times from water, filtered through a Munroe crucible, and then crystallized, centrifuged, and dried with especial care to avoid the introduction of platinum scrapings. The centrifuged crystals were dried in a platinum dish for three hours a t 180°C., and then desiccated over broken pieces of fused potassium hydroxide a t pressures less than 0.002 atmosphere for a t least three days. By weighing all of the platinum before and after use in these final procedures it was found that a maximum of 0.14 mg. of platinum could have been present in the five samples (270 g,), Silver. Three silver samples were purified independently a t different times. Sample 1 (526 g.) was prepared from 1500 g. of c. P. silver nitrate. After one crystallization from water this material was precipitated from saturated solution by slow addition of 15 M nitric acid, using 0.38 cc. of acid per gram of silver nitrate. The crystallization and precipitation with acid were repeated, and the 925 g. of material remaining was made up to a liter with water and neutralized with ammonia. The dissolved salt was then reduced to silver by addition of 1.5 liters of solution made by neutralizing formic acid with ammonia, using 0.50 cc. of 78 per cent acid per gram of silver. The mixture was kept slightlyalkaline with ammonia and warmed toward the end of the reduction. Sample 2 (465 g.) was prepared from 750 g. of heterogeneous silver residues, which were reduced to silver with zinc and sulfuric acid. The silver was thoroughly washed and dissolved in 7.5 M nitric acid, using 1.85 cc. of acid per gram of silver. It was next precipitated as silver chloride. This compound was reduced to silver with a warm concentrated solution containing 1.80 g. of sucrose and 0.50 g. of sodium hydroxide per gram of silver. The silver was thoroughly washed, dried, and fused on lime in a current of methane. The solution in nitric
ATOMIC MASS OF POTASSIUM. I1
783
acid, precipitation as chloride, reduction t o silver, and fusion were then repeated. The etched silver bars were dissolved in nitric acid and reduced to silver with ammonium formate solution containing an excess of 0.5 cc. of 5 M aqua ammonia per gram of silver. Sample 3 (528 g.) was prepared from a kilogram of c. P. silver nitrate. The material was precipitated as silver chloride and recrystallized once from freshly prepared 5 M aqua ammonia. The crystals were digested in 6 N hydrochloric acid and aqua regia, washed, and reduced to silver with sucrose and sodium hydroxide. The 565 g. of silver remaining was washed, fused, etched, dissolved in nitric acid, and made up to 8 liters in a solution containing 100 g. of excess ammonia. The compound was reduced to silver by addition of a solution made by collecting 300 g. of sulfur dioxide in 1.5 liters of solution containing 160 g. of ammonia. The mixture was warmed to complete the reduction. Occasional filtrations were an essential part of the above purification schemes. All precipitations were made from dilute solutions. Precipitated silver and silver chloride were washed twenty to thirty times with l-liter portions of water. To complete the above purifications each of the three silver samples, supported on lime in a current of hydrogen, was fused in a quartz muffle into 50-g. to 100-g. bars. The bars were deeply etched, then transported at 0.02 to 1.0 amperes across a cell made entirely of quartz. The electrolytic crystals were thoroughly washed, dried in quartz, and fused into 0.1-g. to 10-g. buttons, supported on lime in a 5-ampere current of electrolytic hydrogen, in a clean quartz muffle (3). The formation of silver with the necessary smooth unbroken surface was favored by the following conditions. The current was first passed at 8.5 amperes, then held a t 9.2amperes until the silver had melted. After five to fifteen minutes, during which the furnace was rocked occasionally, the current was lowered to 7.0 amperes until the silver had solidified. The silver was cooled completely before removing it from the hydrogen atmosphere. The buttons were deeply etched, washed, dried in quartz, and finally heated for two hours in a quartz tube a t 400' to 500°C. in a 2 X lo-* atmosphere vacuum. They were kept in desiccators over fused potassium hydroxide until ready for use. Potassium nitrate. Two samples of potassium nitrate were each crystallized ten times from water. When portions of these samples were subjected to nephelometric tests capable of revealing unmistakably one part in a million of silver or its chloride equivalent, they showed no trace of these impurities. Approximately 9.4-g.portions of the material, fused in platinum and weighed to the nearest 0.1 mg., served as the starting point in the synthesis of the standard solutions. Lime. Three samples of lime were prepared from c. P. calcium nitrate by variations of a procedure including several crystallizations of the
784
CLYDE R . JOHNSON
nitrate, electrolysis to remove traces of iron or heavy metals, and two precipitations from solution with freshly prepared ammonium carbonate solution, followed by ignition in a quartz muffle. Wuter. The water used in the purification and analysis of samples 2 and 4 of the potassium chloride was obtained by distilling the ordinary distilled water of the laboratory without addition of reagents until the main ohm-' cm.-l fraction had a specific conductance of about 0.20 X when treated with pure nitrogen a t 25°C. For all other work water from the same source was redistilled in succession from dilute alkaline permanganate solution and very dilute sulfuric acid. Other reagents. Phosphorus pentoxide was resublimed in a current of purified oxygen in an all-glass apparatus. Aqua ammonia, formic acid, nitric acid, and hydrochloric acid were purified by one to three distillations of the c. P. concentrated or suitably diluted reagents in quartz apparatus. Sugar, sodium nitrite, and ammonium chloride were crystallized once from water. Solutions of sodium hydroxide were filtered through a Munroe crucible and electrolyzed in a platinum dish until free from iron and heavy metals. Commercial methane, carbon dioxide, chlorine, sulfur dioxide, oxygen, nitrogen, and hydrogen were washed in trains of towers containing glass pearls covered with suitable reagents. The hydrogen used in the final silver fusions was prepared by electrolysis of 20 per cent sodium hydroxide solution and purified by passage in succession through six 30 cm. X 2.5 cm. towers of 3-mm. pearls covered with concentrated sulfuric acid, a tube containing 50 cm. of No. 27 platinum wire carrying a current of 5.2 amperes, an 80 cm. X 2.5 cm. tube containing pieces of fused potassium hydroxide, and a 40 cm. X 2.5 cm. tube of glass pearls mixed with phosphorus pentoxide. The same train was later used in drying the purified nitrogen employed in the fusion of the potassium chloride. In analyses 3 to 7 inclusive, nitrogen was prepared by dropping a solution of sodium nitrite into a hot solution of ammonium chloride; in the other analyses the gas from a commercial tank was used. Both gases met the necessary requirements for purity and dryness when treated in succession with acid permanganate solution, hot copper gauze, potassium hydroxide solution, concentrated sulfuric acid, fused potassium hydroxide, and resublimed phosphorus pentoxide. THE ANALYSES
Weighings were made a t temperatures from 22°C. to 26°C. with relative humidities between 8 and 54. To eliminate a certain group of potential weighing errors the substitution method was extended to include a recheck of the first rest-point. The (true) mass values for the entire set of Class M weights were determined three times during the work, once at the U. S. Bureau of Standards. The greatest discrepancy in' the three
ATOMIC MASS O F POTASSIUM. I1
785
calibrations was one of 0.006 mg. in one of the platinum fractionals. For each weighing the air density was calculated from temperature, pressure, and psychrometric observations. Vacuum corrections were d(V, - VJ, in which m is applied through the expression: m = s the mass of the object weighed, s the mass of the corresponding weights, with rider and rest-point corrections applied, d the air density, V , the volume of the object, and V , the sum of the volumes of the weights. The volumes of the gold-plated brass weights were determined by hydrostatic weighings; the other volumes were calculated with the aid of the densities: Pt, 21.45;,A1,2.70;KC1,1.989; Ag, 10.49;KN03,2.11. Counterpoises were used in weighing the fused salts in their platinum containers. A statistical examination of the rest-point data in relation to the sensitivity of the balance showed that the recorded weights are significant on the average to about 0.005 mg. For each analysis a 7.1-g. sample of potassium chloride was fused in a 15-cc. platinum-rhodium boat in another quartz muffle of the same type used in the silver fusions, in a current of dry nitrogen. The muffle was lined with 0.001 in. sheet platinum. The weighed salt was dissolved out of the boat and made to a volume of 500 cc. In analyses 7 to 15 the salt samples were first tested for alkali in a volume of 50 cc., by a modification of the method of Honigschmid (1) ;the 0.05 cc. of 0.004 M chloride-free methyl red solution introduced in this procedure apparently had no effect in subsequent operations. A weighed quantity of silver was next dissolved in a calculated excess of 7.68 M nitric acid in the 3-liter Pyrex analysis flask and the solution was warmed to remove nitrous fumes, except in analysis No. 15.' The solution was made up t o 500 cc. and the salt solution was added drop by drop with constant rotation of the analysis flask, in the light of a Series OA Wratten Safelight. In analyses 3, 12, 13, 14, and 15, however, the silver chloride was precipitated by simultaneous drop by drop addition of the two 500-cc. solutions to 200 cc. of water, with constant rotation. In these five experiments the precipitation thus took place a t concentrations which could scarcely have exceeded 0.0005 M , on the average. In each case the analytical system was made up with water to contain 1520.7 g. of solution per 10.0000 g. of silver (9.3718 g. of potassium nitrate) and allowed to stand at room temperature for fifteen to fifty days, with occasional vigorous shaking. The acid concentration of the analytical solution was adjusted to 0.3025 molar after titration of a portion with standard alkali, and the system was brought to equilibrium by a t least three days cooling at O'C., with gradual reduction of the shaking to a minimum.
+
* Nitrite, which would seriously affect the potentiometric analyses, is best removed at this stage. I n analysis No. 15 i t was found that nitrite is removed only very slowly by bubbling nitrogen or oxygen through the analytical solution.
786
CLYDE R. JOHNSON
The supernatant analytical liquid was analyzed both nephelometrically and potentiometrically for silver and chloride by comparison of filtered samples withdrawn a t 0°C. with standard solutions having very nearly the same composition (2, 4). I n the nephelometric analyses the standard and test solutions were a t exactly the same temperature, i.e., room temperature; in the potentiometric analyses both solutions were a t 0°C. The standards contained measured equivalent amounts of silver and chloride, 9.3718 g. of potassium nitrate per 1520.7 g. of solution, and were 0.3025 molar in nitric acid. Seven of these solutions were made up to cover the range of silver and chloride concentrations from 0.590 to 0.610 mg. (as silver) per liter. To make standard solutions 8 and 9 two 14-g. samples of silver chloride from completed analyses were rinsed and brought to equilibrium with solutions made up without chloride or silver to the same concentration as the other standards, from two different samples of potassium nitrate and nitric acid. After preliminary analyses the supernatant analytical liquid was adjusted more closely to the end-point, the additions of chloride or silver being corrected to the original volume in the usual manner. The chloride and silver concentrations of the solution were finally determined by a t least three nephelometric and three potentiometric analyses made over a period of at least five days with three different standard solutions. The analytical system was shaken once only after each set of withdrawals. Measurements of the solubility of silver chloride in the analyt,ical and standard solutions permitted an improvement in the manner of determining the chloride concentration from the potentiometric analyses. These measurements are summarized in table 1. The data show that the chloride concentration may be calculated precisely from the expression: [Cl] = 0.3697j[Ag] or the corresponding approximation: [Cl] = 1.216 [Ag]. The silver concentration was Calculated from the E. M. F. of the cell Ag analytical solution standard solution Ag by the expression: log [Ag] = kE/0.0542 + log 8, where S is the silver concentration of the standard solution. All concentrations are expressed in milligrams of silver per liter, and E is in volts. The data obtained from the corresponding cell with silver chloride electrodes were used only in analysis 3, but furnished valuable confirmatory information in the other analyses. Equal-opalescence tests made in the usual manner, in connection with the standard solution analyses of various systems in equilibrium at O"C., gave further evidence that there is an essential difference in the stability of the sols precipitated with excess silver and excess chloride upon which various factors may operate to produce marked differences in color and opalescence, Comparison of the equal-opalescence ratios with the accurately determined values of the silver and chloride concentrations showed
I
1
787
ATOMIC MASS O F POTASSIUM. I1
that any estimate of the KC1:Ag ratio based on the assumption that the unit equal-opalescence ratio is characteristic of solutions at the correct TABLE 1 Solubility of silver chloride at 0°C. AVERAGE
A
~
~
90 6 6 3 3
~ TYPE ~ OF ~ ANALYSIS E s
Nephelometric Nephelometric Nephelometric Potentiometric Potentiometric
SOLUTIONS ANALYZED
QUANTITY M E A S U R E D
15 Analytical Standard No. 8 Standard No. 9 Standard No. 8 Standard No. 9
M G . OF Ag PER L I T E R
0.608 0.608 0.607 0.609 0.608
TABLE 2 S u m m a ? of end-point determinations
I NO.
Ag
INITIAL VOLUME I N LITERS
1
1.56
2 3
1.57 1.56
4
1.57
5
1.57
6
1.57
ADDED
I N SOLN. TOTAL MG.
(1 ’
{
{ {
7
1’57
8 9 10 11
1.58 1.57 1.59 1.57
12
1.57
{I
13
1.56
{
14 15
1.59 1.57
-0.795 -1.057 -0.704 -0.748 -0.475 -0.689 -0.683 -0.628 -0.729 -0.786 -0.594 -0.712 -0.733 -0.208 -0.307 -0.208 -0.139 -0.085
o’ooo -0.056 0.000 0.000
NEPHELOMETRIC ANALYSES MG. OF
Ag PER
LITER AS
POTENTIOMETRIC ANALYSES
MQ. OF Ag PER LITER AE
Chloride *
Silver
Chloride *
Silver
0.56 0.612 0.605 0.628 0.56 0.604 0.618 0.57 0.59 0.629
0.66 0.600 0.616 0.613 0.64 0.610 0.599 0.64 0.61 0.595
0.61 0.605 0.603 0.605 0.602 0.62 0.598 0.603 0.622 0.625 0.605
0.61 0.615 0.606 0.611 0.595 0.59 0.611 0.606 0.592 0.601 0.601
0.54 0.621 0.584 0.626 0.57 0.604 0.613 0.55 0.60 0.624 0.57 0.604 0.608 0.598 0.597 0.623 0.63 0.596 0.608 0.633 0.632
0.68 0.595 0.632 0.590 0.64 0.612 0.603 0.67 0.61 0.592 0.64 0.612 0.608 0.618 0.619 0.593 0.58 0.620 0.608 0.583 0.584
AVERAQE EXCESS Ag MG. P E R LITER
$0.12 -0,019 f O ,030 -0.026 $0.08 $0.007 -0.015 +O. 10 $0.02 -0.033 $0.07 f O ,004 $0 ,005 +o ,012 $0.014 -0.018 -0.04 +o ,019 +0.002 -0.040 -0.036 -0.004
* The values in this column are chloride concentrations in milligrams per liter, multiplied by the factor Ag/Cl. end-point would be subject to corrections which would vary with the temperature, the acid concentration and volume of the analytical solutions,
788
CLYDE R. JOHNSON
the size of the analytical samples, and the method of forming the test suspensions. Equal-opalescence tests made upon solutions containing equivalent amounts of silver and chloride and saturated a t 25°C. gave approximately unit ratios, but were insensitive to additions of excess silver nitrate or potassium chloride equivalent to 0.20 mg. of silver per liter. DISCUSSION O F RESULTS
The amounts of silver “subtracted” from the analytical solutions and the results of the standard solution analyses made a t the corresponding stages TABLE 3 Summary c analyses KCI IN
A g IN
VACUUM
VACUUM
TOTAL Ag
RATIO
NO.
KCI NO.
Ag NO.
4 15 6 7 3
1 2 3 4 5
1 1 1 1 1
7.174405 7.159125 7.139503 7.110874 7.119655
10.381709 10.358857 10.331248 10.289814 10.302444
10.38101 10.35886 10.33051 10.28910 10.30174
0.691109 0.691’111 0.691 108 0.691107 0.691112
13 9 10 12 2
1 2 3 4 5
2 2 2 2 2
7.113533 7.124221 7.241729 7,128585 7.163709
10.292932 10.308618 10.478735 IO. 314855 10.366249
10.29294 10.30839 10.47841 10.31474 10.36550
0.691108 0.691109 0.691110 0.691107 0.691111
5 11 14 8 1
1 2 3 4 5
3 3 3 3 3
7.132066 7.151197 7.230660 7,212326 7.102363
10.320468 10.347646 10.442442 10.436676 10.277835
10.31981 10.34747 10.46250 10.43594 10.27681
0.691104 0.691106 0.691103 0.691105 0.691106
grams
Average.. ............................... Probable error ...........................
grams
KCI
KC1:Ag
grams
...................... ......................
0.691108
f O .0000005
of adjustment to the end-point are given in table 2. From these data and the original weights of silver and potassium chloride added (table 3) the KC1:Ag ratio may be calculated. In table 3 the results of calculations based on the final sets of analyses are summarized. No corrections have been applied for hydrogen in the silver or nitrogen in the fused salt; they could hardly affect the final average by more than a few parts per million. While the analytical procedures were designed to reduce to a minimum all of the errors in the determination of the ratio, the plan of the research was calculated t o reveal any sources of error in the procedures. The only factor whose effect upon the accuracy of the procedures could not be
ATOMIC MASS OF POTASSIUM. I1
789
estimated in two different ways was the composition of the precipitated silver chloride. However, satisfactory evidence that silver chloride is a compound with an accurately reproducible composition was obtained in work on the atomic mass of sodium (2). The absence of effects due to greatly changed conditions of precipitation in five of the present determinations offered further evidence that silver chloride formed under the conditions of these experiments has a definite composition not measurably altered by adsorption effects. The agreement of the independent and basically different nephelometric and potentiometric analyses of the solutions obtained by the analytical reaction and by the reverse method of synthesis (table 1)offers proof of the soundness of the end-point. The absence of variations in the final results shows the adequacy of the purifications and the precision of the method as a whole. The fifteen determinations of the ratio have been made with sufficient independence to give general validity to the final average. It may thus be concluded that the value 0.691108 f0.0000005 represents an accurate estimate of the KC1:Ag ratio. This value agrees well with the average of Stas’s four series of determinations and is reassuringly close t o the mean of the various results which have been obtained in recent times with the equal-opalescence method. The corresponding value of 39.100 for the atomic mass of potassium (Ag = 107.880; C1 = 35.457) is also close to the mean obtained in determinations of this constant through other ratios. Acknowledgment is due for facilities supplied by the Chemistry Department of The Rice Institute, where this research was started in 1928. Special apparatus used in the work was purchased with a grant from the Cyrus M. Warren Fund of the American Academy of Arts and Sciences. REFERENCES
(1) (2) (3) (4)
HONIQSCHMID: Z. anorg. allgem. Chem. 213, 372 (1933). JOHNSON: J. Phys. Chem. 36,830 (1931); 38,1942 (1932) ;37,923 (1933). JOHNSON:Chemist-Analyst 22, 16 (1933). JOHNSON AND Low: J. Phys. Chem. 38, 2390 (1932); J. Am. Chem. SOC. 66, 2262 (1933).
