ured by the regular procedure. The use the chlorOf mercuric amine cycle and the preliminary digestion for chloride oxidation. Since chloride are a separate chloride analysis is not necessary. The quantitative inhi bition of chloride oxidation in all the systems considered has demonstrated the usefulness of the mercuric sulfate modification in obtainOf the ing a more organic content of was1,e water samples.
ACKNOWLEDGMENT
(2) Gleu, K., 2. Anal. Chem. 95, 305
The authors are indebted to K. A. Bus& and Gerald Stern for the statistical evaluation of the data in ~i~~~~ 1, and to B. A. McDonald for her assistance in the early stages of the investigation,
(3) Medalia, A. E., ANAL. CHm. 23,
LITERATURE CITED
(1) Am. Public Health ASSOC.,"Standard
Methods for the Examination of Water and Waste Water; 11th ed. p. 399, New York, 1960.
(1933).
1318-20 (1951). (4) Moore, W. A., Kroner, R. C., Ruchhoft, C. C., [bid., 21,953-7 (1949). (5) M ~ W.~ A., ~ ~ ~~ d, F.~ J.,~ Ruchhoft, C. C., Ibzd., 23,1297 (1951). (6) Moore, W. A., Walker, W. W., Ibad., 28,164-7 (1956). RECEIVEDfor review March 12, 1963. Accepted April 22, 1963. Division of Water and Waste Chemistry, 143rd Meeting, ACS, Cincinnati, Ohio, January 1963.
Determination of Surface Areas of Phosphates from Adsorptioin Measurements in Nonaqueous Media R. E. MESMER and R. R. IRAN1 Research Deparfment, Inorganic Chemicals Division, Monsanfo Chemical Co., St. Louis, M o .
b A method for the determination of surface areas of powders in acetic acid-toluene solutions has been developed and applied to various phosphates with surface areas between 1.3 and 71 sq. meters per gram. O n powders with surface areas below 8 sq. meters per gram, cluplicates agree within 0.2 sq. meter per gram and are in agreement with va8ues determined b y the conventional BET method. The method i s based on the Langmuir equation for adsorption and it utilizes only common laboratory reagents and equipment. It i s especially useful for measuring small surface areas, and for use with materials that are reactive in aqueous solutions.
T
measurement of surface areas has long been of concern in applications of powders where reactivity or absorptivity is imporLant. Numerous methods for acquiring these data have been reported in the literature. The most important and widely used is that of Brunauer, Emmett, and Teller (BET) ( 2 ) . I n the B E T metho'i, the amount of gas adsorbed as a function of gas pressure is measured in a ';acuum system, and the surface area is calculated according to the B E T eqL.ation. More recently, continuous gas flow methods have been uscmd by Innes ( 4 ) and n'elsen and Eggerken (8). Other :tppruaches have been described, such as gas permeability (9), microscopic examination, x-ray scattering, and others which are listed by Adamson ( 1 ) and the Kational B u r e w of Standards
(3, 9, 12) have been used most commonly, since these materials form monolayers on the adsorbent, and therefore, approach a saturation limit on the adsorbent. Acetic acid forms multilayers a t concentrations higher than 0.25M, and this fact has probably concealed its usefulness in determining surface areas. We have found t h a t the adsorption of acetic acid from toluene on inorganic phosphates obeys the Langmuir isotherm in dilute solutions. With careful performance of titrations, greater precision than that achieved by the conventional R E T method was obtained on surfaces that are affected by the sample preparation procedures required with the B E T method.
HE
(T). l'hc detci,itiiri:Ltioii ( J f surface areits by adsorption from solution has also been invcstigatcd. DJ-cs ( 5 ) and fatty acids
EXPERIMENTAL
Materials. T h e calcium phosphates were prepared by reaction of lime with phosphoric acid under various conditions ( I S ) . The sodium polyphosphate sample was a commercial grade material. The toluene and acetic acid were reagent grade quality. Procedure. Handle a n d transfer all solutions as described below. K e i g h about 10 grams of t h e adsorbent for each of three 100-ml. glass-stoppered volumetric flasks. For surface areas below 5 sq. meters per gram, i t is advantageous to keep the solid-to-liquid ratio as large as possible vet permitting the re~novalof an aliquot after settling. Add by pipet 25 ml. of the toluene-. acetic acid solution. Three different concentrations in the range of 0.05iTf to 0 . 2 X should be used for precise work--Jf refers t o concentration in moles per kilogram of solution. Record the weights of these solutions to 0.5%.
Shake the flasks on a wrist-action Burrell shaker for 1 hour at room temperature. (Equilibrium is known to occur in this period, because there is no increase in the quantity of acetic acid adsorbed between 1 and 3 hours.) Allow 15 minutes for the solids to settle before withdrawing a 5-ml. aliquot. Add this aliquot to a 125-m1. Erlenmeyer flask containing 25 ml. of distilled water (COz-free) and 5 d r o p of phenophthalein solution. Record the weight of the aliquot and proceed with the titration, described later, using 30 t o 50 ml. of 0.01X to 0.02M standard base. Handling of Scluticns. Several precautions in t h e handling of these solutions are necessitated by their high volatility. Stock solutions must be transferred to t h e reaction flasks and from reaction flasks t o t h e titration vessel b y pipets which are filled b y t h e application of pressure instead of suction, as is the customary proccdure. This can be accomplished by using a wash-bottle-type apparatus with glass tubing and rubber stoppers or tubing for making the connections. Pipets are allowed to drain into a rcceiver in the usual manner without lobs due to volatility. Care must also be taken that the vessels are kept ne11 stoppered during the shaking and weighing steps. It is necessary t'o adjust the solid-toliquid ratio in the renction flask so that after settliiig there is srifficient clear supernatant solution for the withdrawal of an aliquot for analysis. A filter pipet (11) can be employed for this purpose, lmt thfJ ordinary pi1wt8 is i i w d l y :id(.q119te.
Titrations. l'he critical p r t of this nirtliotl is tlic c:iw and accuracy VOL. 35, NO. 8, JULY 1963
1067
~
k
,
O b I
I
0.4
0.8
I
1.2 moles CH 1000 gmssolu+ion)
I
c( Adsorption of acetic acid from toluene on
Figure 1 . CaHP04.2 H1O
with which tlie titration is made. Standard base solutions with concentrations of 0.01M and 0.02M are prepared from carbonate-free sodium hydroxide. T h e titration is carried out t o t h e phenolphthalein end point, using a 50-ml. weight buret. T h e use of weight burets eliminates the need to calibrate glassware when performing precise titrations. Because the toluene-acetic acid solutions are immiscible with water, about 25 ml. of water is added to the flask before the aliquot (ca. 5 grams) is added for titration. The acetic acid is rapidly extracted into the aqueous phase when this mixture is agitated with a magnetic stirrer. Near the end point it is necessary to swirl the flask vigorously to make certain a false end point is not reached. K i t h this procedure and the precautions described in handling the solutions, the titrations can be carried out with a precision of 1 or 2 parts per thousand, while the norinal volumetric procedures lead to errors of 1y0or greater. Since the change in concentration due to adsorption is normally of the order of a few per cent, errors in titration larger than 0.2% cannot be tolerated. Calculations of Surface Area. T h e surface areas are calculated using t h e Langmuir equation for adsorption. since the systems studied followed this rclationship in dilute solutions. T h e Langniuir equation
implies a linear plot of C us. C ' ( x / m ) for monolayer adborption \\here x l m denotes the amount of aretic acid adqorbed per gram of adsorbent, C i i the concentration of acetic acid in n i o l e ~per 1000 grains of ioliition, a i-; a mea-;iire of wrface aren, and b i, a measire of the intensity of the adsorption. Limited cvaluatiori of the dependence of b on temperature for some of tlie pliosphates 1068
0
ANALYTICAL CHEMISTRY
TY(AF) 1 - F ~
mliere It' is the total weight of the solution used per gram of solid, and F and AF are the weight fraction and change in weight fraction of acetic acid in toluene. The slope of the Langmuir isotherm (Figure 2 ) is the reciprocal of the constant a (in units of grams of acetic acid per gram of solid). The surface area beconies : Surface area = (2.056 X 103)n
C/5
1 025
020
2H20
gave iteats of adsorption of the order of 1 i 1 kcal. per mole, indicating straightforward physical adsorption of the acetic acid. I n calculating z ' m from the experimental data, a correction is included for the change in weight of the solution due to the removal of adsorbed acetic acid. Therefore, x/m is calculated from the data in the following 11-ay: x/m =
C h
c (TO& )Si : : ;?g Figure 2. Langmuir absorption isotherm for acetic acid on CaHPO,,.
2 .o
1.6
Ch5
(3)
in units of square mcters per gram. This relationship is based upon 20.5 sq. A. as the molecular area of a n acetic acid molecule (1).
conccntration. This ~ncthod is IC,., desirable than the acetic acid method for several reasons: acetic acid is readily available, its purity is much greater than that of long-chain acids, and its analysis is much simpier. The reported procedures for analysis in t,hc fatty acid method were either to el-aporate the solvent and determine the change in vieight of acid present or d o a lengthy conductometric titration of the acid. The quantity of acetic acid adsorbed by solid C a H P 0 4 . 2 H 2 0 is shown in Figure 1, over a vide range in acetic acid concentration. These data show that the Langmuir-type adsorption is not followed at higher concentrations, because no limiting value is approached. However, Figure 2 shows a good correspondence with the Langmuir equation at acetic acid concentrations ul, to 0.2.11. The other phosphates described in Table I also shon-ed similar behavior in this concentration range. The siirface areas have been calculated from data in this concentration region. For measuring areas less than about 20 sq. meters per gram, acetic acid concentrations up to about 0.231 in the starting solutions are suitable; however,
DISCUSSION
The measurement of surface areas by adsorption from solution requires less apparatus and less time than most conventional methods. However, dye adsorption is usually complicated by the question of how tlie large molecule lies on the adsorbent and, therefore, raises the question of the niolecular area t o be used in calculations. Good resiilts Iiave been obtained u4ng fatty a d s nit11 cliaiii lengths of 1)c.tn.rt.n 10 :tilt1 22 carbon atoms. Tlic-e mol(wdrs gil-i, monolaj-er coverage on sonic materials, as is s1io1r.n by tlic fact that n limiting value is a,pyro:dicd with iiicrca&ig acid
Table I.
Surface Areas
(Sq. meters per gram) Acetic acid BET (S2j
?lla~P& CaHPOd CaHP04 II 1
L i , 1.S 17 . 76 , 71 . 58
...
8(),'1'0.8
CaHP04.2H?01 3 . 1 , :?.0 2 . 0 , 4 . 6 CaHPO4.2H20II 1 . 8 , 1 7 .., CaHPOa .2Hy() 2 . 5 , '2.4 ... I11
because materials with large surface areas absorb larger quantities of absorbate, the concentraticins of the starting materials should be adjusted so t h a t equilibrium concentrations between 0.05M and 0.2M are used in constructing the isotherm. Likewise, the solid-to-liquid ratio may be diminished when measuring larger areas. A t least three points on the isotherm should be determined for precise work, but a value for the surface area can be calculated from a single point if the isotherm is drawn through the origin. If this is constructed from a point at about 0.2iM concentration, a n error of from a few t o 20y0reriults, depending on the material and th3 accuracy of the single determination. K h e n using this simplification, it is nc8:cssary to establish the accuracy, b y first constructing a more complete isotherm. The average working time requiresj for the performm c e of one of these single-point measurements is 45 minutes; the lapsed time is about 2 hours. It was not found necessary to outgas the material before use in the cases of dry, precipitated C a 3 P 0 4 and calcium hydroxylapatite. OLtgassing is known t o change the surface area of hydrates. Also, a n effect of o ~ t g a s s i n ghas been reported for the adsorption of acids from toluene onto silica ge: (10). Therefore, powders should normally be dried or outgassed prior to surface area measurements, unless evidence shows this to be unnecessary.
RESULTS
Table I lists the surface areas of various phc .phatcs which have been determined by the acetic acid and the conventional B E T methods. The precision of the acetic acid adsorption data is 0.2 sq. meter per gram for duplicates up to 8 sq. meters per gram. I n this range, the data are more precise than the B E T data and are at least as precise at the 170 sq. meters per gram region. The poor agreement between duplicates by the B E T method for CaHP04 and CaHPOe.2H20may be due t o the sensitivity of the surface area to the degree of surface hydration. Nonreproducible changes could h a r e taken place during the preparation of these t n o samples for the B E T measurements. The acetic acid adsorption inetliod iy not useful for surface area measurements on very basic materials such as lime, which reacts completely n i t h the acid. Results obtained on less polar materials such as silica shorn surface areas much less than those determined by nitrogen adsorption. This effect can be attributed t o t h e less polar nature of t h e adsorbent, which would enhance the competitive adsorption of the acid and solvent. hfaterials which Chemically sorb acetic acid might also give complex results when studied by this method. KO solvents other than toluene were studied. However, the choice of solvents is limited by several requirements. The solvent must be miscible n i t h
acetic acid, inert chemically toward it and toward the adsorbent, and nonpolar and nonpolarizable, in order that the adsorption of the solvent on a polar adsorbent does not compete with the acetic acid. Systems where binary adsorption occurs are too complex for routine surface area measurements. LITERATURE CITED
(1) Adamson, A. W., “Physical Chemistry
of Surfaces,” Interscience, Sew York,
1960. ( 2 ) Brunauer, S., Emmett, P. H., Teller, E., J. Am. Chem. SOC.60, 309 (1938). ( 3 ) Harkins, W. D., Gans, D. M., Zbid., 53,2804 (1931). ( 4 ) Innes, W. B., ANAL.C m x 23, 769 (1951). (.5) Kolthoff, I. hZ., hlacNevin, W. AI., J . Am. Chem. SOC.59, 1639 (1937). ( G ) Kozeny, J., Sztzhw. Akad. Il’iss. Wien Math. naturw. Kl., Abt. ZIa, 136, 271 (1927).
( 7 ) Natl. Bur. Standards, “Bibliography of Solid Adsorbents,” Circ. 566 (1956). ( 8 ) Yelsen, F. J‘I., Eggertsen, F. T., ANAL. CHEM.30, 1387 (1958). (9) Orr, C . , Jr., Bankston, P. T., J . A m . Ceram. SOC.35,58 (1952). (10) Puri, B. R., Sud, R. K., Lakhanpal, M. L., J . Indian Chern. SOC.31, 612 (1954). (11) Rapaport, If., Hancock, C. IC., J . Chem. Educ. 39, 98 (1962). (12) Smith, H. A,, Fueek, J. F., J . .4m. Chem. Soc. 68, 229 (1946). (13) Van Wazer, J . R., “Phospl-iorus and
Its Compounds,” Vol. I, Interscience, New York, 1958.
RECEIVED for review September 27, 1062. Accepted April 22, 1963.
Spectro pola rimetry Acid-Base l’itrimetry with Asymmetric Indicators and Determination of Metal Ions and Asymmetric Substances STANLEY KIRSCHNER and DINESH C. BHATNAGAR Wayne Sfafe University, Defroif, Mich.
b Because of the recent development of high-precision photoelectric spectropolarimeters, it is possible to purchase or construct insiruments which will permit the rapid determination of optical rotation a t many different wavelengths. A meihod is presented, utilizing such an nstrument, which allows for the titration of acids or bases using optically active substances as indicators. The titrution curves resemble those of conductometric titrations. The method also provides for the determination o i metal ions and asymmetric coordinating agents, and permits the determination of the ligandmetal ion ratio in coordination compounds, as well.
B
of the limitations inherent in making determinations with a visual polarimeter, the instrument is not readily adaptable for titrimetry and, in analysis, has found its prime use in the comparison of optical rotation of samples n i t h that of standards. Matand others have tock (6), Freegard also used the visual polarimeter in continuous variation studies of metal complexes with asymmetric ligands in the determination of ligand-metal ion ratios. I n recent years versatile spectrophotometers of high quality, light sources of high intensity in both the 1 isible and ultraviolet regions, and polarizing prisins 11ith appropriate cements alloaing trnnsmissiori into the ultra\ iolet ECAUSE
(e),
have become commercially available. Some individual investigators have also constructed similar instruments in their own laboratories ( 2 ) . This has allowed construction of spectrophotometric photoelectric polarimeters which utilize phototubes in place of the human eye as a detector, monochromators in place of special lamp and filter combinations for producing monochromatic light of different wavelengths, and intense light sources for determining the optical rotation of solutions which are also strongly absorbing ( 3 , 5 ) . Several such polariineters are coininereidly available ( 5 ) , and some can fairly readily b e adapted for use in titrimetry--e.g., photoelectric polarimeter, Carl Zci-a, Lot. 35, NO. 8, JULY 1963
1069
