Vapor Pressure Determination See RADLEY, E. T., J. Chem. Edue.,37, 35 (1960) Most measurements of the vapor pressures of liquids require several millilitem of liquid, but i t is possible, as Radley shou-s, t o determine the vapor pressure using only a. very small amount of liquid. The length of an air column, confined between two globules of mercury in a glass tube ( 1 or 2 mm i d . ) is measured. A small quantity of volatile liquid is introduced into this air column and, after equilibration, the increased length of the air-vapor column is measured. Provided the temperature of the glass tube has remained unchanged, the tubing is of uniform bare, the gases are ideal, and some residual liouid (whose volume is neelectedl remains in the space between the mercury globules, the vapor pressure of the liquid o m be calculated simply from the changes in length of the confined gas column
.
.
-
Questions:
Select any moderately volatile liquid or solid and determine its
1s
it necessary,
&
Ra&ey'suggests, that the initial column
length be approximately 40 cm; can a shorter initial length be used without adversely affecting the precision of your determination of vapor pressure? Within what limits can t h e diameter of the tube not he uniform throughout its length without affecting the precision of your measurement of vapor pressure? Radley's procedure requires the use of a very long tube, if vapor pressures over a wide range of temperatures are to be determined. I t is possible t o measure vapor pressure over a vide range of temperatures by using the apparatus described b y Darnerell [J.Chem. Educ.,32, 5'34 (1955)l in which one end of a short glass capillary tube is fused shut and a small quantity of liquid is confined under s, mercury globule. For this purpose t h e short length of tubing, containing the liquid and mercury globule, is placed in a. vessel in which the pressure can be adjusted so as t o retain the globule in the short. capillary tube as the temperature is increased. Measurement of this pressure, of the length of the mercury globule, and of the temperature then yield thc necessary data. Design and set up the necessary apparatus and determine the variation of the vapor pressure as a function of temperature for any liquid you select, from its freezing point t o its boiling point.
Two Experiments in Spectrophotometry S ~ ~ P H I L L I J. P SP., , J. Chem. Ed'dzic.3,1 , 81 (1954). The concentration of a, colored solution can be roughly estimeted by comparing the intensity of its color with that of anothw solution of the same solute a t a known concentration. For precise evaluation a calorimeter is used; if the experimenter wishes to obtain detailed information, the light absorbing characteristics of the solute are measured a t several selected wavelengths, using a spectrometer. The simplest application of these kinds of data vields information concerninr the aumtitv of unknown present, in stctightforwrtrd quantitative analysis. If the concentration of a.salute can be measured, however, this knowledge can also be used to determine other characteristics. Knowledge of the concentrtltions of the ionized and unionized species present in several benzoic acid solutions, each a t knom-n pH, yields suffioient information to calculate the ionization constant of this acid. Many indicators are weak acids or weak bases; their ionization constants can be determined by this method. Many compound8 form eolorcd eomplcses in solution; the formula of the complex may be enleulated by taking a series of measurements of the absorbencies of ~everalsolutions containing varied, known quantities of the two species farming the colored complex.
These measurements may be made with a. spectrophotometer that is sensitive only to visible light or (alternatively and with increased versatility) with a. ~peetrophotometerthat is sensitive to light beyond the visible region. Phillips suggests the use of the latter, and it is necessary for the determination of the ionization constant of benzoic acid. But the simpler type of spectrophotometer can be used to-determine the ionization eanstant of a colored indicator or the formnla of a. colored complcx. Questions
Determine the ionization constant of a colored indicator. Determine the formula, of a soluble colored complex. Show that the ionization constant of a weak acid varies as the temperature varies. Does the ionic strength of the solution affect the ionization conEtallt of a weak acid? What reasons can you give for this effect or its absence? Test your explanation by further experimental invcstigations. The mebhod of continuous variations can be used in other types of investigations. Demonstrate one or more of these uses.
Stability of a Complex Ion See KRUGER, P . AND SCAUBERT, J., J. Chem. Educ.,30,196-8 (1953) The stability of a complex ion can be determined in many different ways. I n this experiment, the distribution of radioactive strontium between buffered solutions containing varying concentrations of citric acid and a cation exchange resin immersed in these solutions is measured. From these data, the stability quotient of the oomplex ion formed between strontium ion and citrate ion is caloulated. Though the technique is not demanding, applicstion of the principles discussed by Kruger and Sohubert to other related questions will require a certain facility with mathematics. Hence the questions listed below will be more interesting to a student who wishes to learn about radioactivity and who also understands the applicability of abstract mathematical concepts to s. chemical problem. Questionr:
Carry out the procedure described by Kruger and Sehnbert. From further experimental work, identify the major sources of error and determine their relative magnitude.
A502
/
Journal o f Chemical Education
.,
\\'la1 erftcr docs vari:firmn i n lcnqwn~turror a vnri:ttiw i n t thr C I T ~ W I . ~ i c h srrrngrh hdve u p m 111,. i i ~ . ~ t dq~m?~ k n of citrztc complex ion? What is the stability quotient of another complex ion, such as the ion formed between calcium or barium ions and anions of an organic acid other than citric acid7 I s the value of the formation quotient of the strontium-citrattte complex ion (or of another similar ion) altered when the organic acid is dissolved in 8. solution of low pH, such that not all of the acid is dissociated into ions? Carry out the procedure described by Kruger and Schuhert. Include, in your c d c u l ~ & mof the find results, corrections which depend upon complex famatian by buffer anions. Calcdate fdrniation quotients for the series of complexes with the general formulas MA, MAs MAs, etc. Can the same procedure, suitahly modified, be used to determine the stability quotient of other complex ions, such as those formed between ions of a transition metal and halide ions?
~
