Oct., 1960
DIFFUSION COEFFICIENT OF FORMAMIDE IN DILUTESOLUTIONS
1537
THE DIFFUSIOX COEFFICIENT OF FORMAMIDE IK DILUTE AQUEOUS SOLUTIONS AT 25' AS MEASURED WITH THE GOUY DIFFUSIOMETER BYJOHN G. ALBRIGHT AND LOUISJ. GOSTING Department of Chemistry, University of Wisconsin,Madison 6, Wisconsin Received A p t il 90, IQ60
The isothermal diffusion coefficient for the system formamide-water was measured with a Gouy diffusiometer over the concentration range 0 to 51 g. of formamde per liter of solution. Each diffusion experiment also provided data for the difference in index of refraction between the two initial solutions used for that experiment. The densities of most of the solutions prepared for these experiments were measured a t 25'. Relations are given which express the diffusion coefficient, the index of refraction and the density of the system as functions of concentration a t 25O.
Formamide was chosen as the solute for this year the decomposition to ionizable products contributes less than 0.05 equivalent per liter (or less than 0.3% by study because it was expected to have a high dif- weight if computed as ammonium formate). These tesbs fusion Coefficient as a consequence of its low molecu- indicate that no significant difficulty was encountered from lar weight. Such a non-electrolyte may be an decomposition. The purity of the freshly prepared third sample was tested interesting solute to use in future diffusion experiby conductance and freezing point measurements; the ments with ternary systems. Furthermore, it is specific conductance (at about 25") and the freezing point useful to have data for diffusion in more binary were found to be 6.5 X 10-6 mhos cm.-l and 2.4O0,?especsystems. Formamide has the interesting property tively. A new platinum resistanc,e thermometer whlch had that its dielectric constant is higher than that of heen calibrated by the Bureau of Standards was used for the water, and it has found application in some studies latter measurements. It is of interest t,hat if 2.55" is the correct freezing point of pure formamide (which has a of proteins and other biological materials. The cryoscopic constant of 3.50 degrees per mole, per kilodata reported in this paper confirm that formamide gram of formamide),e the freezing point 2.40" corresponds has a high diffusion coefficient; its value in dilute to about 0.04 molal (non-electrolyte) impurity, or less than aqueous solution lies between those for urea and 0.1% by weight if the impurity is water. Apparatus.-The diffusion apparat,us used has been deheavy water. scribed in previous articles.7 A new Tiselius elevtrophoresis
cell was used for these experiments. The optical qualit8y and precision of construction of the cell were first checked Purification.-Doubly distilled water, which had been by a telescope with a Gauss eyepiece and were judged t o saturated with air, was used as solvent for all of the solu- be satisfactory. Then the cell was mounted in a new cell tions. During the course of this experimental work, three holder and aligned so that the cell windows were parallel t o different samples of formamide were purified by methods the bath windows. A Gaertner M2001RS toolmakers' similar t o those of earlier researchers.',' In each case Math- microscope was used to measure the cell dimension a, eson, Coleman and Bell 99% pure formamide was distilled the interior width of the cell along the optic axis. The a t least twice at 1-2 mm. pressure a t temperatures between optical distancelo b, between the center of the cell and the 70 and 80"; only the middle 2/, fraction was retained from emulsion of the photographic plate, was measured by using each distillation. The product of the last distillation was a combination of stainless steel rods and micrometers which then fractionally crystallized a t least six times; each time had been calibrated against gage blocks. Distances a and about 85% of the formamide was allowed to freeze and the b were l o u d tto be 2.5092 and 306.670 em., respectively. remainder was discarded. Because of the hygroscopic Method.-The general exprimental methods used for the nature of formamide, care was always taken to avoid expos- Gouy diffusion experiments itre essentially those described in ing it to moist air. :L previous paper." A11 solutions were prepared by graviThe freezing point of the first purified sample w:ts mess- metric inethods with an accuracy of about one part per t,en ured with a Beckmann thermometer which had been cali- thousand. From the weight i ? L vacuo of each component, brated a t the freezing point of water; a freezing point of and the der1sit.y which was measured a t 25' with a set of 2.55 f 0.05" was found for this sample, which was in three single-stemmed 25-nil. Pyrex pycnometers, the conagreement, with valucs reported in the l i t e r a t ~ i r e . ' ~ ~rentration -~ of each solution n'as cnlculated and tabulated This sample of formamide wag used for euperiments 1-5 as grams of solute per liter of sollition. (see Table I); these experiments were performed over a In each experiment thc cell ivns allowed to stand in t,he period of approximately a year. Because it has been ob- constant tcmperature bath for a t lcast 45 miri. to re:tch teniserved that formamide is slightly unstable,* the possibility peraturc cqui1ibri;im bcforc t:i!
