Simplified Apparatus for Photometric Titration E. J. Agazzi and G. W. Bond, Shell Development Co., Emeryville, Calif.
w
ITHIN the
last few years the use of photometric titrations has increased greatly. Discussion of the principles involved and reviews of published papers have been presented by Goddu and Hume ( I ) , Hcadridge (@, and Osburn, Elliott, and Martin ( 3 ) . Apparatus described vary from modified filter photometers or spectrophotometers, to units designed specifically for titration. This paper describes an easily constructed arrangement of light source and phototube for detecting end points photometrically. The potential of the phototube output can be measured by any high-impedance measuring device. A dual-unit Titrometer (Precision Scientific Co.) has proved convenient for such potential measurements because it permits one unit of the Titrometer to be used with the photocell and the other unit for adjusting the p H of solutions prior to titration. The apparatus enables the analyst to make photometric titrations without purchasing or tying up a more expensive spectrophotometer. APPARATUS
The unit, shown schematically in Figure 1, consists of a phototube and light source enclosed in glass tubes so they can be immersed in the solution
being titrated. The glass tubes housing the phototube and light source are inserted through holes in a Bakelite plate and held in place with sealing wax. A steel rod is fixed to the Bakelite plate so the assembly can be supported on a ring stand. A hole is drilled in the plate for insertion of a buret. The light source is a miniature lamp (Nazda KO.46) screwed into a socket which is attached to a brass tube. The other end of the brass tube is fixed to a n inner glass joint with cement. A brass tube with a 3-mm. hole near the closed end is placed over the bulb and positioned to direct the light at the phototube. The entire assembly is inserted into a glass tube sealed a t one end and fitted with a n outer glass joint as shown. Electrical energy to the bulb is supplied by a battery, or better, a 6.3-volt constant voltage transformer. The intensity of the light is regulated by a rheostat. To prevent condensation of moisture on the inner wall of the glass tube which would obstruct the light, a small hole is blown in the glass tube as shown and a length of hypodermic tubing inserted into the hole and attached to an air line. By keeping a slow stream of air flowing in the glass tube, condensation is prevented. The phototube (Type 934) has its glass envelope covered with plastic tape. A 1-cm. circular hole is cut in the tape to admit light from the light source.
The phototube is inserted into its socket, wired with a shielded cable, and the entire assembly inserted into a glass tube. A small drop of adhesive is placed a t the base of the tube to hold the phototube in place. The cable from the phototube is brought through a grommet and connected to the components shown. The measuring circuit is attached across the &megohm resistor n-ith necessary shielding of leads if a Titrometer is used. The 1-mfd. capacitor eliminates oscillations of the meter needle which occur when the sample is stirred by a magnetic stirrer. Figure 2 shows details of the filter holder. A 1-cm. disk of colored glass transmitting the desircd n ave-length region is cemented to a split ring of Bakelite. The filter is slid in position over the glass tube housing the phototube. A number of such assemblies are prepared with different filters. The glass should not be PO highly colored or thick that it transmits little light. The transmitted light should be intense enough t o give 600 to 800 mv. across the 5-megohm resistor. PROCEDURE
To A i r Line
Place the solution to be titrated in a 250-ml. beaker. Set the beaker in a metal cylinder and on a, magnetic stirrer. Add indicator, if necessary. Immerse the light source and phototube assembly in the solution and turn on the stirrer. Adjust the speed of stirring so that the turbulence cone does not obstruct the light path. Turn on the light and adjust its intensity. Immerse the buret tip in the solution and add titrant slowly until the meter reading begins t o increase or decrease. Then add titrant in increments until sufficient points are obtained to define the curve. Make a plot of milliliters us. meter reading and draw the best, steepest straight line through the points before the end point and the best line through the points after the end point. The intersection of the two lines is the end point.
ypodermic Tubing
DISCUSSION
Bakelite Plate 3" D i a m .
Unshielded glass beakers were first used as titration cells but did not prove entirely satisfactory. At times meter readings of the Titrometer were un-
B r a s s Tube 1 7 0 D . G l a s s Tube
B r a s s Jacket
Phototube (cement t o bottom of C l a s s Tube)
Figure 1 ,
972
A l l Dimensions i n m m .
Phototube and light source assembly
ANALYTICAL CHEMISTRY
Figure 2.
Details of filter
holder
steady. This was attributed to electrostatic effects and was eliminated by setting the titration beaker in a cylinder of sheet metal. Measurements were taken across the 5-megohm resistor, which we have found satisfactory for most titrations. Sensitivity may be increased by increasing the value of this resistor. The apparatus, a filter photometer with a tungsten lamp as the light source, is limited to use of wave lengths from 400 to about 750 mp. Because filters do not provide monochromatic light, titrations requiring mono-
chromatic light to avoid interferences cannot be performed with this apparatus. The apparatus has been extensively used in this laboratory for the titration of calcium and magnesium with EDTA to the murexide and Eriochrome Black T end points. Results for 2 to 50 mg. of magnesium and 1 to 8 mg. of calcium showed a n error of less than 0.5% of the amount present. The apparatus has also been applied to the titration of permanganate with arsenitenitrite solution and many other titrations involving EDTA.
ACKNOWLEDGMENT
The authors thank J. F. Fidiam, Jr., and W. B. Milligan of this !aboratory who constructed the apparatus and suggested improvements in LITERATURE CITED
(1) Goddu, R. F,,H ~ D, N,, ~ A~ ~, C H E M . 1740 ~ ~ , (1954). (2) Headridge, J. Be, Tatanta 1, 293 (1958). (3) Osburn, R. H., Elliot, J. H., Martin, A. F., IND.ENG. CHEM., ANAL. ED. 15, 642 (1943).
Recovery and Identification of Mercaptans from Aqueous Alkaline Solutions by Gas Chromatography
H. D.
LeRosen, Texaco Inc., Port Arthur, Tex.
chromatographic analyses have G been reported for mercaptans in organic and hydrocarbon mixtures (I+),
definitely acidic. (The methyl orange may fade.) Block off the gas pipet, disconnect it, and hold for gas chromatographic analysis. Gas Chromatographic Analysis. Using helium a s carrier gas, select a rate permitting complete separation of COz and HzS and giving a 13-minute (approximate) retention time for CzH&H (83 ml. of helium per minute of flow and 30" C. column temperature used in this work). Introduce 75 mm. of the collected acidic gases. Scan 5 to
AS
but none for aqueous alkaline solutions. Release of mercaptans by caustic solution neutralization, into a light liquid hydrocarbon and analysis by gas chromatography proved unsatisfactory, for rapid evolution of hydrogen sulfide and mercaptans caused incomplete absorption of these gases into the nheptane supernatant layer. I n the scheme reported the caustic solution is neutralized under reduced pressure, and the acidic gases are evolved, collected in an evacuated flask, and analyzed gas-chromatographically.
500 mi.
Gar Pipet
Vocuum PUmp
Table 1. Relative Retention Times (Air-Free) for Mercaptans and Acidic Gases
CoC12 Dryer Acid
Reservoir IO ml.
EXPERIMENTAL
Apparatus. T h e equipment for recovery of mercaptans is shown in the figure. The evolved gases were analyzed using a Perkin-Elmer Vapor Fractometer Model 154A (other commercial types may be used) equipped with a 1-ml. volume gas-sampling valve and with two 2-meter columns in series, the first containing 60/100mesh Celite coated with Silicone 200 and the second containing 60/100mesh Celite coated with tetraisobutylene. T h e concentration of both substrates was 30 grams per 100 grams of dry support. Mercaptan Recovery. Disconnect the neutralization flask and pipet in 15 ml. (more if necessary) of the caustic sample. Close the stopcock and nearly fill the acid reservoir with 1 t o 1 sulfuric acid containing methyl orange as a n internal indicator. Attach t h e freshly filled CaClz dryer and gas pipet assembly. Evacuate the 500-ml. gas pipet, then the dryer and neutralization flask until the sample begins bubbling. Block off the vacuum pump so as to leave the gas pipet open to the neutralization flask. Carefully admit acid, dropwise, to the neutralization flask, shaking well after each addition. Continue until the sample is
Retention Time
Compound
coz
0.02 0.06 0.40 1.00 1.77 2.62 3.01
H2S
CHaSH CzHsSH I~o-C~HBH tertCdHoSH n-CaH?SH
250 ml. Neutrolizotion Flask
Table II.
Analysis of Gases Liberated from Synthetic Mercaptan in Caustic Blend
Av. Dev. 1
2
Av.
3
WeiPht, % .. _._-_ HnS 51.5 51.8 53.1 52.4 52.2 CHaSH 15.7 16.1 16.4 16.8 16.2 CnHsSH 9.3 9.2 9.0 9.3 9.2 Iso-CIH~SH 8.3 8.6 8.3 8.3 7.9 n-CsH7SH 7.8 7.4 6.9 7.1 7.3 td-C,H&H 7.4 6.9 6.3 6.5 6.8 Liberated gases Total Sulfur Distribution, % As H a 66.2 66.4 67.4 66.7 66.7 &mercaptans 33.8 33.6 32.6 33.3 33.3 Original caustics A8 HzS 68.7 67.7 68.2 As mercaptans 31.3 32.3 31.8 o Determined by potentiometric titration with AgNOs.
from
Blended
Blended
50.2 13.9 8.7 9.4 10.0 7.8
+2.0 +2.3 +0.5 -1.1 -2.7 -1.0
65.7 34.3
+1.0 -1.0
I"
VOL. 33, NO. 7, JUNE 1961
$2.5 -2.5
973
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