Automatic Apparatus for pKa and Molecular Weight Determinations of Micro Samples of Organic Acids Arthur M. Wilson and Morton E. Munk,' Department of Chemistry, Wayne State University, Detroit, Mich.
T
HERE are several commercially available recording potentiometric titratorsforabout $3000. Theapparatus described in this paper was fabricated from commercially available components for about $1200. Parke and Davis described a method of obtaining manual pH titration curves and their interpretation as fraction of bound hydrogen us. pH [Parke, T. V., Davis, W. My., ANAL.CHEW26, 642 (1954)l. Their procedure requires a bound hydrogen us. pH overlay and considerable operator time obtaining data and converting the raw data for interpretation. Directly recorded p H titration curves allow one to read the pK, of the acid directly from the graph. It can be shown from DeFord's treatment of Keak acid-base equilibria [DeFord, D. D., J. Chem. Educ. 27, 554 (1950)l that:
drive switch is coupled via an Amphenol 80-MC2M plug and a suitable length of coaxial cable to the buret titrate switch, thus assuring simultaneous delivery of titrant and chart paper. The chart delivery is 8 inches per minute and the buret delivery is 1.000 ml. per minute. Thus an engineer's scale divided into 1/60 inch allows a volume readability of 2.081 d., which is comparable to the buret reading of 2.000 ~ 1 . The recorder is calibrated a t the factory to read 14.3 mv. which permits the presentation of 10 p H units full scale. The recorder is coupled to the p H meter output with an Amphenol 80-MC2M plug and a suitable length of coaxial cable. Calibration of the p H meter and the recorder are performed in the usual
Table 1.
APPARATUS
The apparatus is shown in block diagram in Figure 1. The recorder chart
Benzoic
o-Sitrophenol
REAGENTS AND COMPOUNDS
All reagents were C.P. grade or better, and buffers and standard solutions were obtained according to standard procedures. A sample of benzoic acid was purified by repeated recrystallization from water, m.p. 122O C. oSitrophenol, commercial grade, was recrystallized from an alcohol-water mixture, m.p. 45' C. Aniline hydrochloride, technical grade, was purified by repeated recrystallization from an alcohol-ether mixture, m.p. 198' C.
Formal pK,'s and Molecular Weights of Some Organic Acids
-4cid Aniline hydrochloride where C. and Cg are the stoichiometric concentrations of the acid and conjugate base, respectively. Thus, when the acid is 50% titrated: C, = Cb, and if C, and Cb 2 5 X 10-3X, for 4 < pK. < 10, Relation 1 reduces to: pK, = ~Hsos,.
way. The reproducibility of pH measurements are within the readability of the p H meter and recorder, k0.03 p H unit.
Wt. of Sample (mg.) 7.05 6.77 7.50 9.90
9.00 7.92 5.58 7.13
7.65 5.35 6.00 6.80 6.90
Present address: Department of Chemistry, Arizona State University, Tempe, Aria.
Gram Molecular Weight (g.) Found Theoretical
PK.
124 4.47 132 4.35 138 4.40 130 4.40 r2v. 129.5 Av. 4.41 s = 0.05 s = 6 5.35 5.35 5.27 5.25 Av. 5.31 s = 0.05
126 122 121 121 Av 122.5 s = 2
7.90 151 138 7.80 7.80 139 7.85 135 7.92 129 Av. 7.85 Av 138.6 s = 0.05 s = 8
129.6
122.1
131),1
4 Figure 1.
Titration apparatus
6. Sorgent constant rate buret, 10-ml. capacity, 1,000 ml./min. delivery rate Et, E, T. Metrohm T buret tip, combination micra glassreference electrode and micra titration cell (Brinkmann Instruments, Inc., 115 Cutter Mill Rd., Great Neck, L. I., N. Y.) H. Leeds & Northrup Model 7 6 6 4 pH meter M. American Instruments Co. 1525-r.p.m. motor P. Varian Model G-1 1 strip chart potential recorder R. KOH reservoir S. Ace Glass Ca. 14/20 -?$true bore stirrer
-s
VOL. 34, NO. 3, MARCH 1 9 6 2
443
RESULTS AND DISCUSSION
The micro titration cell a ill accommodate volumes as small as 5 and as large as 15 nil. Volumes larger than 10 ml. cause serious deviation from equilibrium p H readings because the mechanical stirrer cannot circulate the uppermost solution fast enough into the 3-nd. well of the cell. At least 5 ml. must be present to cover the reference electrode's fiber. To test the response time of the p H recording circuit, a 5-ml. sample of 0.0500.V potassium acid phthalate F a s titrated with 0.164X potassium hydroxide-50% methanol reagent. KO error !vas produced in the buffer region. However, a t the equivalence point the instantaneous p H reading was 0.5 p H unit above the
equilibrium value. This should produce no systematic error in molecular weight determinations because the base is standardized a t the same rate that unknowns are titrated. To increase the solubility, acid samples were dissolved in a 50% methanol-water mixture and titrated with 0.164X potassium hydroxide in the same solvent. Fifty micromoles of the acids were weighed directly into the lower half of the titration cell (tare weight about 47.6 grams) to a precision of b0.02 mg. The samples were dissolved in exactly 5 ml. of solvent and titrated with the apparatus a t 25.0 =t0.5" C. The results are summarized in Table I. It is apparent from the results that
the standard deviations of the pK, determinations are comparable to the reading error of the recorder chart. The standard deviations of the molecular weight determinations are a little larger than might be expected in the case of o-nitrophenol if one considers that the uncertainty of the sample weight and the reagent volume are both about 2%. The described apparatus, operated in the manner specified, will determine formal pK,'s of organic acids in the pK, range 4 to 10 to a precision of ztO.05 unit and molecular weights to a precision of i 6 7 0 . Its chief advantages are: small sample size, permanent record, speed, and relatively low cost compared to comparable commercially available units.
Simple Thermistor-Controlled l o w Temperature Thermostat Myron J. Rand, Bell Telephone Laboratories, Inc., Allentown, Pa.
SMALL THERhlOSTAT 1%hich I\ ill
oper-
A ate down to 100" below room tem-
perature is often required in the laboratory. It is particularly useful for gassolid reactions and adsorption studies, either to maintain the sample temperature or to contain a low teniperature reservoir to provide a constant low reactant vapor pressure. While the literature contains many examples of cryostats, particularly for the temperature of liquid nitrogen and below, most of these are complex in design and intended only for a particular purpose. Several thermostats capable of operating down to -80°C. or so (and, in some cases, above room temperature also) have been described. The most common type of thermostat surrounds the sample chamber with liquid refrigerant, with a n intermediate space which can be evacuated. Heat leak through this space is regulated by the pressure, and steady state is achieved by adjustable electrical heating. Vacuum systems are required (4-8). Other devices operate by the circulation of a liquid (9) or gaseous (2-4) coolant by a pump. Still another type is the gasflon cryostat (1, 4). Regardless of the method of operation, those devices which have automatic control generally employ a gas or vapor-pressure thermometer with some auxiliary system sensing small pressure changes. The thermostat described here is of the gas-flow type, and for general laboratory work it possesses a combination of virtues unattainable with any published design. Emphasis has 444
ANALYTICAL CMMISTRY
l
l Styrofoam
P
G
Polyethylene Glass
Figure 1. thermostat
Schematic
rThermornetr
cross-section
Well
diagram
of
