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. T o 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
Polyethylene
G
Glass
Figure 1. thermostat
Schematic
rThermornetr
cross-section
Well
diagram
of
been placed on simplicity of construction, and there are no critical dimensions or indeed, any fixed dimensions. It has been used in the range of $10" to -70" C. with regulation t o = t O . l " C.; there is no reason a hy this could not be extended on either end. It has proved reliable in operation, and may be left unattended for several hours. The thermostat is shown in schematic cross section in Figure 1. I t is essentially a massive copper block in a n insulated box, with a spiral channel for cooling gas. Gas flow is regulated by the solenoid valve (Automatic Switch Company, Florham Park, S.J.) 1%hich is in turn actuated b y the controller (Fenwal, Inc., Ashland, Mass., Model 56052 temperature-indicating controller). The valve may be remote from the thermostat, and its vibrations isolated b y a rubber tubing SECtion in the gas line. Gas is cooled by passing through the copper tubing coil immersed in the refrigerant in the large Dewar flask. Dry ice-alcohol is suitable for temperatures down to -20"; liquid nitrogen is always satisfactory, and one filling lasts several hours once the thermostat is a t the desired temperature. With a little experience, it is easy to set the pressure regulator so that for any tem-
perature gas is flowing about half the time. (The controller includes visual indication of the on-off cycles.) I n case rapid raising of the temperature is desired, warm gas can be backflushed through the line at high velocity, emerging a t the opening shown closed with a rubber stopper. The cylindrical copper block is cut from bar stock and turned down in the lathe until it Kill just slip snugly into a piece of copper pipe. Leaving a little distance a t each end, a helical groove is cut around the block. Convenient dimensions are 3/16 inch deep, 3 turns per inch. A silver-solder seal is made between the block and pipe on the ends to ensure t h a t the "coil" is gas tight. Holes are drilled through the ends of the block t o reach the end turns of the groove, and are tapped to receive the threaded polyethylene tubing. The threads are made tight with neoprene cement. The holes in the block for the alcoholfilled thermometer and the thermistor probe are as close as possible to the cavity for the sample cell, as shown. A little alcohol in the thermometer well prevents condensation of atmospheric moisture from freezing the thermometer in place. Practically no construction of auxil-
Fold-Over Zero-Suppression Circuit for
iary equipment is required. The thermistor-probe control unit is comniercially available. There is no bath liquid, no stirring, and no pumps; in fact, there are no moving parts a t all, a t least in the immediate vicinity of the thermostat. This makes the device especially suitable for use with silicaspring balances, strain gages, and other vibration-sensitive equipment. Heat leak t o the room, n-hile low, is of such magnitude that no heating circuit is necessary. Reading the temperature on the thermometer is B concession t o simplicity; a thermocouple may be used if desired. LITERATURE CITED
(1) Allsop, H. L., Gibbs, D. F., J . Sei. Instr. 35, 395 (1958). (2) Bose, A., Indian J . Phys. 21, 275 (1947). (3) Fuschillo, N., Krautkopf, D. W,, Rev. S a . Instr. 28, 1060 (1957). (4) Mayence, J., J . phys. radium 12, 74450 (1951). (5) Morrison, J. *4., Young, D. ll,,Rev. Sci. Instr. 25, 518 (1954). (6) Ross, S., Clark, H., J . Am. C h e m SOC. 76. 4291 (1954). (7) Taylor,~W.J., Smith, A. L., Johnston, H. L., J . Opt. Soc. Am. 41, 91 (1951). (8) Ubbelohde, A. R., Woodward, I., Proc. Roy. Soc. (London) A185, 448 (1946). (9) Waldman, M. H., McIntosh, R., Can. J . Chem. 33, 268 (1955).
Use with Gas Chromatography
Lyle H. Hamilton, Research Service, Wood Veterans Administration Center and Department of Physiology, Marquette University School of Medicine, Milwaukee, Wis.
T
HE ACCURSCY O f peak height measurements in gay chromatography can be increased by shifting recorder zero with a bias (bucking) voltage introduced a t the input to the rccorder ( I , 2 ) . This report describes a zerosuppression circuit n-hich applies a bias voltage in stcpnise increments and re\ m c s the input polarity a i t h each stcp, so the rrcordc.d curve is folded back and forth on the. chart paper. The unit is available commrrcially from QuinTron Instrument Co., Inc., AMauker 18, Vis. 'The principle used in the circuit is h o n n in Figure 1. Snitcah position 1 is actually the "OFF" position, a t which the instrument has no effect on the signal from the chromatograph. K h e n the input signal from the chromatograph increaqes until it equals 1 mv. (the span of the recorder), the operator rotatrs the snitch to position 2 . This action produces two effects: The polarity is reversed, so that the 1-niv. signal is shifted to a point below the base line A ; and a zero-suppression voltage of 2 inv. is applied (as indicated by the heavy arrow in Figure I), to bring the
pen back t o the top of the chart -4'. As the signal from the chromatograph increases to 2 mv. B, the pen n-ill be driven back to the bottom of the chart B' because polarity is now negative.
K h e n the switch is rotated to position 3, the polarity is positive, and the 2niv. signal from the chromatograph C is cancelled by the 2-mv. zero-suppression voltage, so the pen will remain on
I
I
I
I
c7-2mv POSITION 1 Polarlty positlve; No ruppr. voltage
POSITION 2 Polarity negative ; 2 m v ruppr. voltage
r f l c 0
1 I
,
-
:I
/ I mv. Figure 1. Principle used to provide a folded record of a 3-mv. signal on a recorder with 1 -mv. capacity
',\
B'
I
\\
*E \ N
A
+-
'C POSITION 3 Polarity positive ; 2 mv. suppr. voltage
I mv.
\
\
\
E '2mv
VOL. 34. NO. 3, MARCH 1962
445
