DENSITIES O F CERTAIX XQKEOUS POTASSIUM CHLORIDE SOLUTIOXS AS DETERMINED WITH h NEW PYCNOMETER BY HENRY C. PARKER AND ELIZABETH W. PARKER
Introduction Density measurements have been variously employed to determine the state of aggregation, the equilibrium existing in mixtures, the ionization in solution, hydrolysis, the distribution of a base between acids and for analytical purposes. Considering their wide application, the ease with which they are made and the high precision attainable, it is surprising that there are available in the literature so few reliable measurements with a high degree of accuracy. These measurements appear to offer the only purely physical method of determining the state of a substance in solution whose precision may prove sufficient to throw light upon even the most dilute solutione. Their importance would seem to warrant an extended series of accurate measurements. Work of Previous Investigators. There are four methods of measurement which so far have attained a high degree of accuracy. The displacement method was probably brought to its highest point of perfection by Rohlrauschl who claims a precision of two or three parts in ten million. X modified displacement method was used by Lamb and Lee2who claim a probable error of less than one in ten million. Piccard and Cherbuliez3 have described a method of using connecting tubes which was also used by Frivold4 to an accuracy claimed as one part in ten million. T’ery accurate measureniente with a pycnometer have been made by several investigators. Among these may be mentioned Tamniann6, Dij ken,6 Manley7, Bousfield,* Hartley and BarrettQ,Egerton and Leelo and Wade and Merrimanll. The mort accurate of these measurements attain a precision of about one part in a million. The two principal difficulties met with, in obtaining high accuracy in density measurements, are those of weighing and of temperature control. The displacement method reduces the first of these by removing a conaiderable part of the load from the balance, but introduces new complications in the form of capillary effects a t the point where the supporting wire touches the liquid. The corrections for these effects12require a knowledge of the surface Ann. Physik. 53, 14 (1894); 56, 184 (189j). J. Am. Chem. SOC.35, 1687 (1913). Arch. sei. phys. nat. 42, 324 (1916). 4Physik. Z.21, 529 (1920). Z.physik. Chem. 16, 91 (189j). Z. physik. Chem. 24, 108 (1897). Proc. Roy. SOC.Edinburgh 24, 356 (1902). J. Chem. Soc. 93, 679 (1908). 9 J. Chem. SOC. 99, 1072 (1911). l o Proc. Roy. SOC.103 A, 492 (1923). l1 J. Chem. SOC.101, 2430 (1912). l2 See Klaus: Chrm. Z t g 47, 85 (1922).
DENSITIES O F POTASSIUM CHLORIDE SOLUTIONS
131
tension of the liquid and are uncertain. Although the capillary effects are removed in the modification of Lamb and Lee, the principal difficulty in this method remains, namely that of obtaining temperature equilibrium in the interior of the large mass cf liquid required. Stirring is obviously out of the question and partial stratification of the liquid seems probable. The accuracy of the method of Piccard and Cherubiez can probably only be increased at the cost of producing a cumbersome and unwieldy apparatus. This leaves the pycnometer method which is the easiest and quickest of the three. There has been no systematic investigation as yet to determine to what limits of accuracy this method can be extended. Temperature control is evidently less difficult with a pycnometer, since it can be made of such shape as to expose a considerable surface to a thermostatic liquid. It is likewise evident that an electromagnetic stirring device may be utilized if necessary. The difficulties met in weighing will be discussed later. A new difficulty enters with a pycnometer, however, in that the volume must be accurately determined either by exact filling or careful calibration of its stem. In the most common type (Sprengel type) of pycnometer described in the literahre, an error is introduced by the necessity of bringing the liquid meniscus to an exact position in two places simultaneouyly. .A modification of this type hae two calibrated stems jn each of which a reading must be made, introducing a double error both as to calibration and reading. I n another type (Gay-Lussac) a single stem js employed but for convenience in filling this must be made large enough to introduce a considerable error1. It is believed that this difficulty je removed in a modified Sprengel type of pycnometer which was developed for measuring the densities of potassium chloride solutions, during the course of an jn~estigation~ on the absolute value of the specific conductance of these solutions.
Apparatus The pycnometer shown in Fig. I can be readily made by an amateur glass blower from materials found in most laboratories. The bulb, 7 , is made of thin soda glass and is large enough to hold about I O O cc. The capillary, 5, is made from a broken (0'-360'C.) thermometer which has a bore of about 0.3 mm. The capillary S, has a bore of about 0 .j mm. The brace, 4, serves t o support the pycncmeter on the balance. The enlargement, 1 , serves as an overflow cup when liquids are measured below room temperatures. The calibration mark) 2 , can be made conveniently by turning the capillary in a lathe and making a fine scratch with a sharp steel edge. The caps, 6, are ground to fit the corresponding tubes, and serve the purpose of preventing evaporation before weighing. 1 This type, moreover, cannot be employed without subjecting the liquid t o a vacuum which is not desirable in the case of solutions. J. Am. Chem. SOC. 46, 312 (1924).
132
H E N R Y C . PARKER AND ELIZABETH W. PARKER
If it is desired to make the pycnometer less liable to breakage and easier to wipe dry, the tube, 3, may be joined by an inner seal at the top of the bulb and be continued to the bottom similar to the corresponding tube in the pycnometer described by Davis and Prattl. Another modification which may be necessary in order to allow the pycnometer to fit in a balance case, is that of having two shorter bulbs connected at the bottom. In this case the tubes, 5, and 3, are joined to the tops of the two bulbs, somewhat similar to the pycnometer described by BOUPfield2. The new features of this pycnometer are the long calibrated stem, 5, which is of much smaller diameter than any used formerly, in combination with the stem, 3, which is larger and bears only a single calibration mark, 2. The method used for reading the volume is likewise an essential feature. The manipulation, which was developed for the use of this pycnometer, removes most cf the disadvantages which would seem t o accrue from the employment of capillary tubing of such small bore. Experimental Procedure and Results Manipulation.-The pycnometer was first cleaned with hot cleaning mixture, distilled water and alcohol. It was allowed t o age before using and thereafter no heat was applied for either cleaning or drying. The latter was accomplished b$ rinsing with redistilled alcohol and attaching to an aspirator at 1 . It was found that, if the liquid was always passed through the larger capillary and air through the smaller, filling or emptying was but slightly FIG.I delayed by the small bore 5. Filling was accomplished by attaching a siphon with a four-foot hydrostatic head at 1 , and required but five minutes. This method of transfer can evidently be used with very dilute solutions without danger of contamination. In weighing the pycnometer it wa,s wiped with a clean damp cloth, and 45 minutes were allowed for it to come to equjlbrium. In carrying out a meaeurenient the following prodecure was adopted :the pycnometer was filled until the overflow cup was half full. It was allowed J. Am. Chem. SOC.37, 1199 (1915). J. Chem. SOC. 93, 680 (1908).
DENSITIES O F POTASSIURI CHLORIDE SOLCTIOXS
I33
to stand for 30 minutes in ice water and was then packed into finely shaved ice, in the thermostat described in a recent article1. Thirty minutes were allowed before a reading was taken. To take a reading the overflow cup was emptied and carefully dried with cotton wrapped around the end of a small file. X glass stop-cock was fitted to the top of 5 by e short piece of clean rubber tubing. Pressure or suction was then applied at this point through a long rubber tube until the meniscus in 3 coincided exactly with the mark 2, when the stop-cock mas shiit off. The reading in 5 could then be conveniently obtained. After several check readings the ice mas repacked and I O minutes allowed before another reading was taken. This procedure was continued until the readings checked to I division on the stem. One division represented a temperature difference of only oO.006 C which speaks well for the temperature control. The water was drained from the thermostat. Otherwise higher readings were obtained which varied with the mater level. I n order to facilitate a reading in 5 a tiny drop of ink was placed at the top of the liquid in this tube. Calibrations and Correctzons.-The set of weights used for weighing the pycnometer had been carefully calibrated against a set with a 1920 Bureau of Standards certificate. The capillary, 5, was calibrated and all readings corrected back to an arbitrary mark on the stem. Calibration was made by weighing the r e d k tilled mercury which was sufficient t o fill 107 divisions on the stem, and calculating the volume from its density at that temperature. The weight of mercury was 0.05396 g. and its temperature 2ooC, making I division of the stem equivalent t u 0.0372 cu. mm. I n order to obtain the density of the potassium chloride solutions, their relative density in comparison with water a t oo was determined. The value ~ in grams per taken for the density of the latter was 0 . 9 9 9 8 4 0 6 ~expressed centimeter cube. It is evident that the density of a solution mill vary with the amount of air dissolved in it. Since the potassium chloride solutions could not be kept air free. the same water was used both for determining the volume of the pycnometer and for making up the solutions. The air was extracted from neither. It was assumed, therefore, that the amount of air dissolved did not change upon making up the solutions and also that this air caused the same decrease in density in the pure water, and in the solutions. For the dilute solutions measured, it is probable that these assumptions hold within the limits of accuracy obtained. Use was made of “conductivity” water with a specific conductance in the neighborhood of I X I O - ~ . It was kept in 4 liter bottles of resistance glass and after two liters had been used from a bottle the rest was discarded. To correct the weights of water to a vacuum, a factor of I.ooIoj5 was used. All the results obtained were corrected to an arbitrary reading (150) on the thermomet,er stem and to 750 mm. barometric pressure. J. Am. Chem. Soc. 46, 316 (1924). Thiesen, Scheel and Diesselhorst: Wiss. Abh. Phys.-Tech. Reichsanst. 3, 68 (1900). -Assuming I liter = 1.0000270 cu. dm.
HENRY C . PARKER h S D ELIZABETH W. PARKER
I34
To correct the weighings for change in barometric pressure, the volunie of air displaced by the pycnometer was found by adding to its inner volume a volume equal to that of the glass vessel. The latter was found from its weight by assuming the density of glass to be 2 . 5 . The total volume found thus was 141 cc. If the density of air under usual conditions is assumed to be 0.0012,the pycnometer would then displace 0.1695 g. of air. If the effect of temperature and humidity are neglected the density of the air varies directly as the pressure and a change of I em. in the barometric pressure would make a difference in weight of the pycnonieter of 0.000226 g. which is a sufficiently close approximation for this correction. Table I gives the results of five different determinations of the volume of the pycnometer with samples of water from the five different bottles used. It i s seen froni the data in the second column that the weights of the pycnometer filled with water were even more consistent than the tare weights. Since the densities of the different samples were thus within the experimental error an average volume for the pycnometer mas taken in calculating the densities of the solutions. This volume is given in the last column.
TABLEI Calibration of Pycnometer at o°C. Tare Weights
Weight with
66.504221
180.9178, 180.917-44, 180. 918307 180% 91 7-48, 180.917s6/
Hz0
66.5042~; 66.5036.~~
66.503 7 8 /
-
____-
Av. a
Average weight of water in air = I 14.4136~1 in vacuum = I I 4.53 43 ,/ ~01ume'=J14.5525~,
66. 503g5/ 180 * 91 7 5 5 / Discarded in the averaie. Density of water assumed = 0.9998406.
Experimental Results.-The density of three different solutions was determined, namely 0.099271 D , 0.1 D , and 0.01 D . ( D or clemal representing equivalents per cubic decimeter). The measurements on the first concentration were made to perfect the manipulation and to eetimate the accuracy of the method. The solutions were all made up by weight methods after the manner recently described1. To make up a solution to an exact concentration by this method requires a previous knowledge of its density. A seriee of approximations was used, therefore, and the density of the first solution was carefully estimated before calculating the required proportions of salt and water. The original density in each case was easily estimated to better than 0.01%. This error in the weight of salt used in making up the 1
J. Am. Chem. SOC. 46,
322
(1924).
DESSITIES O F POTASSIUM CHLORIDE BOLCTIOXS
135
D solution causes an error of only 5 parts in I O million in the density as finally determined. Consequently the first approximate solution gave nearly as great accuracy as the final. To reduce the weights of the o.oggz71 D and 0.1D solutions to a vacuum, the factor 1.001049 was used. For the 0.01D solution the corresponding factor was 1.0010;4. The results obtained for the three solutions are given in Table 11. 0.1
TABLEI1 Density measurements at o°C.
D" Solution
o.oggz71 D" Solution \'t. KCl t o 100ogH20b
7 4244a/ 7 42423, 7 42425,
0.1
\Tt. KC'l
Densit yc
to
'
I ' 0048397,
'
I . 00484031
7.47881, 7 47876/ 7.47874,
I . 0048400,
Accepted value
'
Accepted value
I
,004838,d
'
0.01
Density
1000 g. H206
I
,004883~;
I . 004880,, I . 004881,/
I
.004881,,
D" Solution
Wt. KC1 to 10008. H?Ob 0.746240:
Deiisi t yr I .000372,/
Expressed as equivalents per cubic decimeter; molecular weight 74.5 j 3 . Both weighed in air. Expressed in grams per centimeter cube. Discarded in the accepted value. I n the first column are given the weights of potassium chloride to 1000 grams of water (weights jn air) used in making up the solutions. The correct weight for the 0.1D solution as calculated from the accepted density is 7.47895 g. The difference between this andt he values actually wed would make a difference of but I in the last decimal place. The density estimated for the 0.01D solution was 1.00036 and hence the weight of potassium chloride used in making the solution was far more exact than necessary to obtain an accurate density measurement. The manipulation had been so well perfected a t this time that it was thought necessary to make but a single determination of this value. a
Discussion From the results given in Table 11, it is evident that a precision of about part in a mjllioii has been obtained. I n comparison with other investigators, this appears to be as great a precision as has ever been attained with a pycnometer. When consideration is taken of the fact that other inveetjgators have obtained this precision only when working close to room tempera&I
136
HEA-RY C . PARKER AND ELIZABETH W. PARKER
ture, it is evident that even greater precision could be obtained in measurements with the new pycnometer when working under these ideal condition?. I n the introduction it was stated that there had been no systematic investigation to see to what limits of accuracy density determinations with a pycnometer could be extended. The numerous investigations which have been made with a glass pycnometer have all pointed rather definitely to the limitations of this instrument. A precision of about I part in a million seems to be the highest attainable even in spite of the removal of the source of error caused by filling to an exact volume as has been accomplished in the present investigation. The cause for this limitation is not difficult to find. In Table I it is seen that in determining the tare weight of the pycnometer, an average deviation from the mean of 0.2 j mg. is made. I n weighing the pycnometer full of water the average deviation is 0.12 mg., providing one determination is discarded1. These deviations represent approximately the limits of accuracy obtainable in the weight of such a large glass apparatus, due presumably, to electrostatic effects and the absorption of mater vapour. No further proof is necessary that the use of glass must be discarded when an increase in accuracy is desired. It has been the writer's experience that a quartz vessel can be weighed reproducibly t o about 0 . 0 2 mg. The measurements of Edgerton and Lee2 indicate a still greater accuracy. In order to estimate the highest precision attainable in measurements with a quartz pycnometer we will assume that such an apparatus with a capacity of 500 cc. could be weighed (on a balance especially adapted to that weight) to 0 . 0 2 mg. This would be equivalent to 4 parts in IOO million. Such a precision would require a temperature control in the thermostat of = t o . o 0 2 ~ C . Several thermostats have been described which have claimed closer regulation. A series of more or less obvious corrections3 mould have to be applied to the results in order to obtain an accuracy of this order. A correction would have to be made for variations in the density of the air and this latter quantity would have to be determined with some accuracy a i each weighing. h difference of I cm. in the barometric pressure would cause an error of about 2 parts in a million4. Another correction which would become significant is the change in density caused by changes in external pressure. From the compressibility of water it is evident that a change of I cm. in the barometric Compare the accuracy obtained by Block: Z. angew. Chem. 33, I, 198 (1920). Proc. Roy. Soc. 103 A, 493 (1923). 3 A discussion of, and equations for most of these corrections are to be found in Refs. 5, 7, 8, and 9, p. 000. See also Saar: Chem. Ztg. 46,433 ( ~ 9 2 2 )48, ; 28j (1924);and Bordas and Touplain: Bull. Soc. encour. ind. nat. 133, 1052 (1921). 4 The use of a counterpoise would naturally be suggested to obviate the need for this correction. No great success has been attained with the use of a counterpoise in density measurements, however. (See Refs. 5 , 7, 8 and 9, p. 13"). There ix some difficulty in making the counterpoise of sufficiently identical volume to entirely remove the need for corrections. Correction must be made for the volume occupied by the additional weights in any case. On the whole, correcting the weighings without the use of a cousterpoise seems preferable. 1
2
DEKSITIES O F POTSSSIUM CHLORIDE SOLUTIONS
I3 7
pressure would cause an error of about seven parts in one hundred million. The changes in pressure on the solution caused by variations of surface tension in the capillary tubes, might also become significant. I n the pycnometer described in this article the surface tension of the liquid could be measured while reading the volume of the pycnometer, and hence, this correction could be easily made. A close regulation of the amount of air dissolved in the solution mould be necessary and might prove difficult to obtain. The error introduced by the necessity of filling a pycnometer to an exact volume would obviate the use of any other type of pycnometer but the one described in this article. The readings cn this type would evidently have t o be made to an accuracy of about I/Z of one division on the calibrated stem. Such an accuracy could undoubtedly be obtained with the use of a cathet oineter . Although the corrections and precautions which would be necessary t o attain a precision of 4 parts in I O O million, seem rather numerous, for the degree of precision involved, there are few physical measurements where the,y appear so few.
Summary I , X modified Sprengel type of pycnometer is described which eliminates the difficulty and error introduced by the necessity of filling to an exact volume. The volume can be read accurately to 4 parts in I O million. The manipulation required in its use is described. 2. Density determinations at oo have been made on three solutions: 0.099271 D, 0.1D and 0.01 D (equivalents per cubic decimeter), t o an accuracy of about I part in a million. The results are r.oo484oo, 1.0048813 and 1 . 0 0 0 3 7 2 ~respectively, expressed in grains per cubic centimeter and based on the vale 0.9998406 for water a t 0'. 3. A discussion of the errors, incurred in measuring densities with a pycnometer, is included. With the use of a quartz pycnometer of the new type it is predicted that a precision of 4 parts in roo million could be obtained. Research Department, The Leeds and Northrup Co., Phzladelphaa, Pa.
