V O L U M E 25, NO. 11, N O V E M B E R 1 9 5 3 tested, it is well always to obtain readings a t a given time interval, sag 30 seconds, after load application. This was done in the above evaluation of tubing. The viscoelastic nature of rubbers and plastics is responsible for the nonlinearity of the stress-strain curve. Consequently in a test such as was conducted on the steel spring, where weights were applied in increments, a t a certain rate, the flexural rigidity of rubberlihe products will not remain constant with increasing degree of bend, but will vary. The amount of variation depends on the material. This nould be true in any kind of bend test, however. iz completely satisfactory solution could be obtained by taking the viscoelastic spectrum of the material into consideration. The
1733 pertinent theoretical development, taking time or rate into consideration, for cantilever bending has been worked out by hlfrey ( 1 ) . The theoretical development for the proposed test method is beyond the scope of this paper. LITERATURE CITED (1) ;ilfrey, T., “11echanical Behavior of High Polymers,” pp. 215-18, Xew York, Interscience Publishers, 1948. (2) Am. SOC. Testing Materials, “A.S.T.M. Manual on Quality Control of Materials,” Part 2, 1951. RECEIVED for review March 19, 1953. Accepted August 25, 1953. Presented before the Division of Rubber Chemistry a t the 123rd Meeting of the AXERICAV CHEJIICAL SOCIETY, Los Bngeles, Calif.
Universal Anticipation of End-Point System for Automatic Titrations W. N. CARSON, J R . General Electric Co., Hanford Atomic Products Operation, Richland, Wash. This work was done to find a means for preventing overtitration in automatic titrations. Overtitration arises principally because of a time lag in the response of the indicator system. The proposed system circumvents this time lag and can be used with almost any indicator system. It consists of withdrawing part of the sample, titrating the remainder at a rapid rate until an end point is reached, then returning the withdrawn portion and titrating at a slow rate to the final end point. Variation in the amount of sample withdrawn, and in the rate of the slow titration, permits adaptation to a specific titration. Such an anticipation system has been designed for an automatic titrator used to titrate microsamples of oxidizing agents. The results of titration of dichromate samples show a standard deviation of less than 1%. The system extends the applicability of automatic titrators by making i t possible to use sluggishly responding indicator systems. It also permits faster titrations without danger of overtitration.
0
S E of the major problems of automatic titrations is stopping
the addition of titrant in time to prevent overshooting the end point. This generally calls for some means of anticipating the end point, either by the behavior of the indicator system itself or by some external system. This problem is discussed in a previous paper (1). The need for this anticipation of end point arises because the indicator response is never instantaneous. The response is affected by the rate of stirring of the solution, the rate of addition of the reagent, the type of indicator, and the physical form of the titration vessel with respect to the location of indicator electrodes, reagent addition point, nonuniform mixing of the solution and sluggish, but eventually stoichiometric, reactions. The more sluggish the response of the indicator system, the more necessary is the need of anticipation to obtain accurate results. In view of the numerous indicator systems that are useful in automatic titrations, a desirable anticipation system should be independent of the indicator system, and should also be independent of the speed of indicator response in the sense that either fast or slow indicator systems can be used. Muller (3) has discussed such an anticipation system in his column on instrumenta-
tion, but up to this time the writer is unaware of any reported titrator that has actually used the system. The device described in this paper operates essentially in the manner proposed by 3Iuller. The svstem consists of withdrawing part of the sample before titration, titrating the remainder at a rapid rate until an end point is obtained, returning the withdrawn portion, and titrating the remaining sample by the addition of small increments of titrant. The first end point is overshot by a small excess of ti-‘ trant; the amount of solution withdrawn must contain sufficient sample to react with this excess. The remaining sample is titrated m-ith small increments of titrant; sufficient time is allowed between increments to allow the indicator system to come t o equilibrium. The titration can then proceed to an end point that is not overshot by more than a nominal excess of titrant. Variation in the amount of withdrawn sample, increment size, and time b e h e e n addition of increments permits adaptation t o any indicator system, no matter how sluggish its behavior. An automatic titrator using this anticipation system has been tested for the titration of oxidizing agents with electrolytically generated ferrous ion according to the method of Cooke and Furman ( 2 ) . The basic design of the titrator has been given ( 1 ) . The controller circuit is given in Figure 1. At the start of a titration, all of the relays are in the open position shown in the figure, the trigger relay (not shown) operates when the indicator potential is above the end-point potential (excess oxidant present). Closure of the “operate” switch t o the titration position starts the titration The circuit (via K-5) to the anticipation system solenoid is energized, as is the circuit (via K-5 and the trigger relay) to the output rela! s, K-2, K-3, which control the addition of titration current and the stopwatch, Energizing the solenoid withdran s a portion of the sample solution from the titration zone (see Figure 2). The titration continues until the remaining portion of the sample has been titrated t o the stage where the indicator potential decreases below the end-point potential, which opens the trigger relay contacts. The output relays, K-2, K-3, now open and the addition of titration current is stopped. Opening K-3 closes a duration of end-point timing circuit (the 2050 thyratron and the R-C network C-1, R-l), which is set for 1 to 2 seconds. When an end point persists for this time, the thvratron fires, and relay K-6 closes, 15 hich in turn closes K-5, which locks in electrically. Closure of K-5 activates the increment addition circuit (K-7 and the 12AU7 network), and opens the solenoid circuit, permitting the M ithheld portion of the sample to return to the titration zone Part of this portion of the sample
ANALYTICAL CHEMISTRY
1734
Figure 1. Wiring Diagram of Oxidant Controller R-1. R-2. R-3. R-4. R-5.
Resistor, 1 megohm, 0.5 watt Resistor. 5-menohm Dotentiometer Resistor; 300,030 oh&, 1 watt---Resistor, 220,000 ohms, 1 watt Resistor, 500,000-ohm potentiometer R-6. Resistor, 150,000 ohms, 1 y a t t R-7. Resistor 1-megohm potentiometer R-8. Resistor: 180,000 ohms, 1 watt R-9 Resistor 22 000 ohms 1 watt R-IO. Resisto; 10'000 ohms' 0.5 watt C-I. Condens'er i mfd 15b volts dry electrolytic C-2. Condenser: 4 mfd:: 450 volts: dry electrolytic (7-3, C-4. Condensers, 4 mfd., 450 volts, dry electrolytic C-5, C-6. Condensers, 0.01 mfd., mica
ieacts with the excess reagent generated in the overtitration, arid the remainder raises the indicator potential above the end-point potential, causing the trigger relay to close. HoTvever, no titration occurs until the increment addition circuit (K-7 and the 12AU7 circuit) operates in the "on" cycle. Both K-7 and the trigger must be closed a t this stage for titrant to be added. The titration now proceeds bv the addition of small increments. The solenoid operates each time an increment is added; this action permits thorough rinsing of the h d d chamber to free it from untitrated sample. Titration continues until the indicator potential reaches the second end point. The first increment addition breaks the end-point timing cirw i t a t K-3. This permits K-6 to open, which closes K-4. Cloiing K-4 inserts resistance R-2 into the end-point timing cirw i t . which permits a delay of up to about 40 seconds; this delay is required to allow for sufficient time between increments. The titration is stopped after the indicator potential decreases to the end-point potential a second time. This opens the trigger circuit. and hence K-3 and K-2 as before. Opening K-3 again closes the end-point timing circuit, now set a t 30 to 40 seconds. After this time, K-6 again closes, and operates K-1, which locks in electrically, and opens the circuit to K-7, preventing any spurious titrations after an established end point. Opening the operate witch to "reset" restores the circuit. The requirements and functions of the trigger output relay circuit, K-2 and K-3, the duration of end-point circuit (K-6, K-3, and the 2050 thyratron circuits), and the lockout circuit, K-1, have been described (1). The function of K-4 and K-6 is to change the titration from continuous to incremental. Two relays are used to effect the change-over in order to permit the use of one thyratron relay circuit and to prevent false lockouts. The increment addition circuit is a conventional multivibrator circuit, although the frequency of oscillation is lower than is usual in this type of circuit.
C-7 C-8 C-9 (7-10. Condensers 20 nlfd 450 volts dry electrolytic C-li. Cbndehser, 40 mfd.. 450 v6lts, dry '&trolytic' 7'. Transformer, filament B. Battery, 22.5-volt C P.L. Pilot light X. Rectifier selenium 100 ma. K-1 Relay S k D T ll0:volt a.c coil Dunco (Struthers-Dunn Co.) lXAX K-2' Relay' DPD?: 110-volt a f: coii Dunco I X B X 'coil 'Dunco I X C X K-3' Reiay' 3 P D T '110-volt a K-4' Relay' BPDT' 110-volt a:c" coil' Dunco I X C X K-5' Relay' D P D T 110-volt a,;' coii Dunco l X B X K-6' Relay' DPDT' 5000-ohm d'b cdil C P. Clare T pe C K-7: Relay: SPBT, i0,OOO-ohm d.;.', coii. L a c h 1024-1
A
c S O L E N O l D COIL FROM AUTOMATIC SWITCH CO CAT No 83061 VALVE TO CONTROLLER TEPMINALS (SOLENOO)
CORK CUSHION
-30
CC. SYRINCE
TO E R I V A T I M POLAROGRAPHIC INDICATOR CIRCUm
I2 MM. SlOEARM
Pt ELECTRCOES
MAGNETC STIRRER
A titration assembly is shown in Figure 2. Only the dimenuions of the side arm of the titration cell are critical.
The design
Figure 2.
Titration Assembly
1735
V O L U M E 2 5 , N O . 11, N O V E M B E R 1 9 5 3 Table 1.
iutoniatic Titration of Dichromate Standards S Taken N Found Precision, 70 0 09810 0 0500 0 0100 0 00500 0 00100 0 1021
0 0 0 0 0
09818 04997
01005 004943 001004 0 1022
+O
67 0 48 0 76 0 G9 0 74 0 36
mubt be essentially as shown in order to prevent pumping of air fiom the side arm into the vessel in place of the solution. Pumping occurs if a bulb is used in place of the long tube or if capillary tuhing is used t o connect the side arm. The amount of solution a-ithdrann is determined by the position of the barrel of the hypodermic syringe in relation to the plunger assembly. From 10 to 20% of the solution should be withdrawn; the rate of stirring, indicator response, titration current, and amount of sample all affect the amount of “overshoot” that must be overcome. -4 cork or rubber cushion bet-ireen the solenoid plunger and the syringe plunger is required to prevent breakage; a similar cushion at the end of the syringe plunger is recommended. Table I gives the results of a set of titrations of dichromate vtandards using the anticipation qystem. It was not possible toper-
form this titration automatic all^ \+ithout end-point anticipation, as the indicator lag is large. The derivative polarographic indicator system ( 4 )was used. The electrolyte was 4 V sulfuric acid0.6.V ferric ammonium sulfate as prescribed by Cooke (2). In the procedure, an unmeasured sample was added to 5 ml. of the electrolyte and titrated to the end point; this pretreated the electrolyte to remove traces of oxidizing or reducing impurity. d series of two to five measured samples (100 J)was then run in succession with this treated electrolyte. The dichromate standards were made from triply distilled water and the dried salt. The precision is the standard deviation of a single value, as calculated from 10 results. The results show that the anticipation system xorks satisfactorily. The titrator is now heing invr3tigated for use in the titration of other oxidizing agents and reducing agents. LITER4TURE CITED (1) Carson, W. X . , Jr., ASAI.. CHEM.,25, 226 (1953). (2) Cooke, 1%D., ‘. and Furman, S . H , Ibid., 22, 896 (1950). (3) Muller, R. H., IKD. I;SO. C H E \ f . , r l N 4 L . ED., 18, KO. 5 , 24 (Adv. Sect.) (1946). (4) Reilley, C. N., Cooke, W.D., and Furman, N.H., A V I L .CHEY., 2 3 , 1 2 2 3 (1951).
RECEIVED for review June 9, 1953. .4ocepted September 8, 1953.
Determination of Polystyrene in Styrenated Alkyd and Styrenated Epoxy 1IELVIN 11. SWAN3 Paint and Chemical Laboratory, Aberdeen Proning Ground, Md.
A quantitative method for determining polystyrene was needed for quality control and acceptance of quick-drying styrene-modified alkyd resins. The styrenated epoxy resins were investigated because of their related modification and their possible future interest to the Ordnance Department. Two procedures for determining the polystyrene have been developed. One is applicable to either the alkyd or epoxy resins. The other, applicable to alkyds only, allows the subsequent determination of the oil acid content of the alkyd resin. Composition requirements of styrenated alkyds can be established and analytical control exercised. The extent to which monomer styrene is converted to polystyrene in the preparation of styrenated resins and oils can be determined.
0
F T H E synthetic resins, the alkyds probably powess the widest range of applicability, especially for automotive
equipment. They are unusually tough and durable and possess excellent solubility, compatibility, adhesion, and gloss retention. Compared to other synthetic resins they are relatively slow-drying. To incorporate the alkyd resins in a quick-drying enamel for ammunition finishing @), they have been modified with styrene. At the present time no analytical control is exercised over the styrene content of these enamels and no composition requirements have been specified, because of the lack of an analytical procedure for determining the styrene. By applying the method of analysis to ten commercially available styrenated alkyds, i t has been found that the polystyrene content varies from 26 to 44% of the nonvolatile portion of the solutions. The composition of some of these resins is shown in Table I. Polystyrene is very resistant to the action of alkalies in alcoholic and nonaqueous media. Because phthalic anhydride in alkyd resins is separated and determined by saponification, styrene, if present, would be found in the filtrate from the phthalic determination. Upon removal of all solvents, the dried
productq, consisting mostly of excess potassium hydroxide, polystyrene, drying oil soaps, and polyhydric alcohols, can be treated with Si% methanol and the styrene separated, dried, and weighed. The product is insoluble in water but slightly soluble in absolute methanol and in aqueous alkali. However, it separate8 quantitatively in filterable form from a solution of 87% methanol, even in the presence of excess alkali. -4s styrene-modified epoxy resins are of current interest, they were tested by the procedure described. It was found that epoxy resins resist saponification by any medium that can subsequently be evaporated to dryness. By controlled fusion with potassium hydroxide pellets, the epoxy resin could be decomposed and the styrene left unchanged. The same technique can be applied to the styrenated alkyds, but not as conveniently as by the method reported here in detail. Interference of certain unsaponifiable resins can be prevented by using the fusion technique. ANALYTICAL PROCEDURE
Method for Styrenated Alkyd Resins. To determine the polystyrene in styrenated alkyds, the phthalic anhydride should first
