however, a corresponding sacrifice in wave height. Figure 13 demonstrates the maximum resolution capabilities of the instrument. The two waves are those of thallium and lead with halfwave potentials differing by only 40 mv. The normal polarograms show no indication of an inflection point between the two waves, while the derivative curve clearly shows the composite nature of the wave. For quantitative determination, it would be necessary to consider the effect of each wave on the height of the other, but qualitatively the resolution is very definite. This instrument has been used for many derivative polarograms, including several which are irreversible (nickel in potassium chloride and iodate are examples of the latter). The derivatives of these irreversible waves are normal in shape but are somen-hat broader than would be expected. This is a distinguishing feature of this method of derivative polarography as compared to alternating current methods which show no waves a t all or greatly reduced sensitivity on irreversible waves. The instrument has found its greatest application in rather dilute solutions (10-6 to l O - 4 M ) in which maxima are seldom a problem.
Figure 13. Regular and derivative polarograms of solution of lead and thallium (with diode filter)
The dual dropping-electrode technique without synchronization gives well-defined derivatives, but will probably have limited application because the sensitivity is limited and the use of t\To electrode systems is awkward. The use of a tachometer as a differentiating device falls short of satisfactory performance because of mechanical defects of the available equipment. The use of resistor-capacitor differentiating networks, proposed by Leveque and Roth (IO),can be made to give very satisfactory polarographic n-aves. By adding parallel-T and diode filters and using a suitable current amplifier, this technique is satisfactory for analytical use, even though the shape of the waves still deviates from the theoretical equation.
SUMMARY
LITERATURE CITED
The use of the parallel-?' filter has made possible several techniques of recording derivatire polarograms.
(1) hirey, L., Smales, A. A,, Analyst 75,
287 (1950). (2) Barker, J. C., Brit. Patent 709,826 (June 2, 1954).
. .
Breyer, B., Gutman, F.. Hacobian S.,Australian J . Sei. Research 1 3 558 (1950). Cambridge Instrument Co., Ltd., 13 Grosvenor Place, London, S W, 1. Endand. Tech. Bull., Sheet 313. Hanim,-R. E , , Asn~.CHEJI. 30, 350 (1958). Heyrovskg, J., Chetii. / i 3 f y 40, 222 (1946); dyalust 72. 22cl (1947). Kellev. M. r.."FlshPr. D. J.. . ~ A L .
CH&. 28, 1,130 ( l n & j , ' (8) Ilelley, hl. T., Fisher, D. J., Southm-ide Chemical Conference, l l e m phis, Tenn., Ilecemher 1956: I S A Journul, to be ptibli.shed. (9) Kelley, 11. T., AIiller, H. H., .%XAL. CHEJI. 24, 1895 (1952). (10) Leveque, 3f. P., Roth, F., J . chitn. phys. 46, 480 (1949). (11) Lingane, J. J., Williams, R., J . A m . C'hem. SOC.74, 790-6 (1952). (12) Semerano, G., Riccoboni, L., Gazz. c h h . ital. 72, 297 (1942). RECEIVED for review September 2 j 2 1957. ilccepted December 26, 1957. Division of Analytical Chemistry, Beckman Award Symposium Honoring Ralph H. lluller, 131st Meeting, ACS, Miami, Fla., rlpril 1 0 5 i .
Analysis of Explosives by Nonaqueous Titration ROY D. SARSON Explosives Division, Olin Mathieson Chemical Corp., Mount Braddock Works, Mount Braddock, Pa.
,The value of nonaqueous titrations as applied to explosives and to the use of automatic differential titration with a view to reducing analysis time was determined. Trinitrotoluene, dinitrotoluene, pentaerythritol tetranitrate, and hexogen are titrated as acids in methyl isobutyl ketone. Nitroglycerin, nitrocellulose, mononitrotoluene, and ammonium nitrate are titrated as acids in dimethylformarnide. Inorganic nitrates are titrated as bases in glacial acetic acid. Trinitrotoluene, dinitrotoluene, and mononitrotoluene are differentially resolved by titrations in methyl isobutyl ketone and dirnethylformamide. Extraction of explosive mixtures with methyl isobutyl ketone separates nitrobodies from inorganic nitrates. Nonaqueous titration of explosives with indicators and the
932
ANALYTICAL CHEMISTRY
automatic differential titrator results in an accurate and time-saving means of quantitative analysis.
titrations with particular respect to explosives and their ingredients have been investigated. Ammonium nitrate and sodium nitrate are differentially titrated as bases in acetic acid-chloroform solvent. ii'itroglycerin, nitrocellulose, mononitrotoluene (MITT), and ammonium nitrate are titrated as acids in dimethylforniamide and ethylenediamine. Dinitrotoluene ( D S T ) , trinitrotoluene (TNT), hexogen ( R D X ; hexahydro 1,3,5-trinitro-s-triazine), and pentaerythritol tetranitrate (PETN) are titrated as acids in dimethylformamide and methyl isobutyl ketone (4 methyl-2-pentanone). J I N T , DNT, and T N T are OXAQUEOUS
differentially resolved n ithin certain limits. The insolubility of inorganic nitrates in methyl isobutjl ketone provides a means of separation and subsequent direct titration of some organic nitrates. Through the use of both indicators and the automatic titrator, routine analyses can be carried out with considerable saving of time and n i t h accuracy equal to present methods. The nitrometer has been the standard device available to the explosive cheniist for the determination of nitrate explosives. Although capable of excellent precision, this device iq somewhat cumbersome and requires maintenance. The titanous chloride method has largely supplanted the nitrometer for the differential analysis of explosive mixtures. This method is capable of good precision but, because of the neces-
means of a switch the cathode is grounded, which allonw the relay to operate on positive pulse of the second derivative curve. The plate voltage for the differentiator, taken from the Fisher Titrimeter, is regulated, which contributes to the stability of the control unit. If 12AX7 (duotriode tube) was substituted for the amplifier tube, a direct current heater voltage was not necessary. The input grid of the 12AX7 is connected across the cathode bias resistor of the 6J7 cathode follower in the Titrimeter. The standard Titrimeter burets were replaced n-ith 50-ml. straight burets. Small-bore rubber tubing connected these to special long buret tips. Spring-held solenoids on rubber tubing shut off the titrant flow electronically. Teflon microcontrol valves inserted after the solenoids controlled the titrant flow a t 6 to 15 nil. per minute. d later modification used hypodermic needles attached to LuerLok microtips, which with KO.20 to 24 needles gave titrant flow rates of 8 to 20 ml. per minute. The input iinpedance of the chloroform solution titrations is very high and the special balanced cathode follower circuit (Figure 2) was used instead of the Titrimeter connections. When using this unit, the cathode of the 6AK5 tubes is reversed according to the polarity of the potential break. The bias on the thrayton mas later made variable so that the triggering point could be set. With large potential breaks the bias can be
sarily small sample size and rapid oxidation of the titanous chloride solution, accuracy is impaired unless all precautions are followed. This method requires daily standardization, aliquoting, and refluxing procedures. A survey of the literature of nonaqueous titrations revealed methods appliT N T ( g ) , amcable t o hexogen (4, monium and sodium nitrate ( I ) , and ammonium nitrate (3). This laboratory has extended this to include the analysis of nitroglycerin, nitrocellulose, M E T , D N T , and PETK. The automatic titrator described by Malmstadt and F e t t (6, 6) has been modified and set up for routine analysis. With appropriate precautions, the accuracy of these nonaqueous methods approaches that of titanous chloride and niuch less time is required. EQUIPMENT AND GENERAL PROCEDURE
The automatic titrator described by Malmstadt and F e t t has been combined into one unit (Figure 1). This unit is connected t o the acid and alkali burets with switch control for either titration. For titration with perchloric acid, the potential break is initially a negative voltage and the thyratron is biased negatively. For titration with alkali solution, the potential break is initially a positive voltage and by 112 V I
1
v2
l / Z VI
I
1
C6
-7 I
- R Y r- 1 1
I
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Figure 1.
Automatic differential potentiometric titrator
R 1 . 1 000-ohm, 1 -watt R2. 0.5-meg., 1 -watt R3, R5, R6. 1 .O-meg., 1 -watt R4. 500-ohm, 1 -watt R7. 200-0hm, wire-pound pot. RE. 7000-ohm, 1 -watt C1. 2500-mfd., 12-volt d.c. C2, C4. 0.02-mfd., 400-volt d.c. C3, C5. 0.5-mfd., 400-volt d.c. C6. 8.0-mfd., 150-volt d.c. C7. 500-mfd. 20-volt d.c. C8, C9. 4 0 - 4 0 mfd., 600 volt d.c. SW1. 5p.d.t. S W 2 . 110-volt 0.c. s.p.d.t., push to make S W 3 . 1 IO-volt 0 . c . d.p.d.t.
B1. 3.0-volt battery RYl. 1 1 0-volt s.p.a.t., a x . relay RY2. 6-volt d.c. s.p.s.t., normally closed RY3. 1 1 0-volt a x . s.p.s.t., normally open SE. 8-volt a x . selenium rectifier 1. 1 1 0-volt indicator lamp S o l . 1,2. 1 1 0-volt ax. continuous duty solenoid V1. 1 2 A X 7 V2. 2 D 2 1 V3. 5 V 4 G V4, V5. OB2 CH. Filter choke 5-1 0 henrys T1. Power trans. 1 2 0 volts a t 2 0 ma. (Thardason 2 6 R 3 2 or equivalent) T2. Power trans. 2 8 5 - 0 - 2 8 5 volts a t 60 ma.; 5 volts a t 3 amps.; 6.3 volts a t 3 amps.
increascd, which reduces the chance of noise impulses giving false triggering. Titration with indicators such as crystal violet and azo violet require little explanation. I n order to have the same normality factor, standardization is carried out potentiometrically with indicator present. APPARATUS AND REAGENTS
Automatic differential potentiometric titrator unit. Platinum-40% rhodium electrode. Caloinel electrode, deeve. modified, wturated potassium chloride in niethanol. Perchloric acid, 0.3.Y. Diqsolre 33.0 nil. of TO to 72% perchloric acid (reagent grade) in l liter of p-dioxane. Tetrabutylammoniuni hydroxide, 0.1.V. Dissolve 40 granis of tetrabutj-lainnioniuni iodide in 90 nil. of absolute methanol. Add 20 granis of silver oxide (fine) and agitate vigorously for 1 hour. Test for iodide in the supernatant liquid, adding more silrer ovide until the test is negative. Filter and make u p t o 1 liter with dry benzene. Crystal violet, 1% in acetic acid. .Izo violet, saturated solution of piiitroplieii~lazoresorcinol in benzene. EXPERIMENTAL PROCEDURE
Perchloric Acid, 0.3N. Standardize this titrant against potassium hydrogen phthalate in glacial acetic acid using crystal violet indicator a n d the glass-calomel fiber electrodes. T h e potentiometric end point is t h e greatest change in millivolt reading for t h e addition of a given volume of titrant (usually 0.2 ml.). Correct for titrant overshoot, encountered when using the automatic titrator, by standardization using the platinum-40% rhodium and caloniel electrodes. Tetrabutylammonium Hydroxide, 0.1N. Standardize this titrant against benzoic acid in diniethylformamide or niethyl isobutyl ketone using azo violet indicator and t h e glass-calomel (modified) electrode system. Substitute the platinum-40% rhodium for the glass electrode when using the autcmatic titrator. Where ethylenediamine is the solvent, neutralize it first and use azo violet indicator. Add the standard sample and titrate to the same blue color as the blank. Determination of Ammonium and Sodium Nitrate. Separate organic or estei nitrates from the inorganic nitrates by estraction nitli hot methyl isobutyl ketone. Both ammonium and sodiuni iiitiate are insolub!e in this solrent. Titrate the organic or ester nitrate according to the appropriate titration procedure. The ammonium and sodiuni nitrate can be dissolred in glacial acetic acid, aliquoted, and titrated differentially with perchloric acid in a 10 to 1 ratio of anhydrous chloroform. Hoxever, t o determine the ammonium nitrate content niore readily, dissolve the inorganic nitrate portion in diniethylformamide and titrate colorimetrically m-ith 0.1N alkali titrant using azo violet VOL. 3 0 , N O . 5, M A Y 1 9 5 8
933
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1 O,OOO-ohm, 0.5-watt 200-ohm, 2-watt wire-wound pot. (to adjust, ground both grids and set for 0.0 volt a t cathodes) 0.1 -mfd. 450-w.v.d.c.
6AK5
g I 2 3 O 4 5 6 7 8 9 1011 O 12 1314 d _._
M L . PERCHLORIC ACID-DIOXANE
indicator. Sodium nitrate does not titrate in this basic solvent. Determine total inorganic nitrates by dissolving the explosive sample in glacial acetic acid and titrating with 0.3N perchloric acid using either crystal violet indicator or the automatic titrator. Determination of Organic or Ester Nitrate. Titrate the organic nitrate contained in the methyl isobutyl ketone extract either potentiometrically or with the automatic titrator using tetrabutylammonium hydroxide. When the solution contains nitroglycerin, reflux before titration. Titration with azo violet indicator is possible only for hexogen in this solvent. For titration in dimethylformamide and ethylenediamine, separate solid explosives from inorganic nitrates by water extraction. I n the absence of inorganic nitrates, titrate nitroglycerin and nitrocellulose colorimetrically in ethylenediamine or potentiometrically in dimethylformaniide. For accurate work determine blanks for both solvents and diluents. Determination of Organic Nitrate Mixtures. Differential titration is possible only for T N T , D K T , and M N T , although hexogen and T N T can be separated by selective solvent and titrated separately. Potentiometric titration in dimethylformamide gives two potentiometric breaks. The maxima represent the T N T and the total D S T , respectively. If M N T is also present, titration in dimethylformamide will result in the sum of T N T , DNT, and LINT. For second titration of the same mixture in methyl isobutyl ketone, h l X T does not titrate but T N T and D N T titrate differentially. RESULTS
As this investigation was made with plant control analysis in mind, most of the samples titrated were the commercial products as used in the plant. The 934
ANALYllCAL CHEMISTRY
Figure 3.
Perchloric acid-dioxane titration
Curves displaced vertically A. Total ammonium nitrate and sodium nitrate with 75 ml. of acetic acid E . Total ammonium nitrate and sodium nitrate with 75 mi. of methyl isobutyl ketone C. Titration using chloroform (no addition after ammonium nitrate end point) D. Titration using chloroform (methyl isobutyl ketone a d d e d after ammonium nitrate end point)
purity of these samples was determined by other analysis methods. The initial work was done with indicators and potentiometric plots, because the automatic titrator had not been completely evaluated. The work of Malmstadt and Fett (5) with regard to electrodes and that of Pifer, Wollish, and Schmall (8) with regard to differential titration were repeated with variations which resulted in no significant change in observations. Attempts to increase the accuracy of ammoniumsodium nitrate determination by increasing the sample size were unsuccessful because of the poor solubility of these nitrates in glacial acetic acid. Figure 3 shows the increase in magnitude of the sodium nitrate potentiometric break due to the reduced normality of the perchloric acid-dioxane titrant. It also s h o w the effect of adding methyl isobutyl ketone to the solution for total nitrates after the ammonium nitrate end point has been recorded. Some difficulty was encountered with poor end points in the chloroform-enhanced ammonium nitrate determination. Because of the insolubility of the acetic acid aliquot in chloroform, the solution was mixed vigorously for 10 t o 15 minutes after the addition of the first increment (approximately 2 ml. before the potential break) of perchloric acid-dioxane titrant. This procedure
results in an improved potentiometric plot. To reduce the analysis time, the increment of titrant ITas added to the sample aliquot and the solution was mixed for 2 to 3 minutes. The chloroform was then added and the titration was continued. Ammonium and sodium chlorides are present either as actual ingredients in some explosive mixtures or as an impurity in the organic nitrates. Pifer and Wollish (7) indicate that these chlorides are acidic in glacial acetic acid. However, the rather intense heating required to effect solution of the inorganic nitrates in acetic acid apparently volatilizes the hydrogen chloride, driving the reaction to the basic side. When titrated, using crystal violet indicator, and heated to boiling after reaching each end point, the reaction is driven to completion and the chlorides titrate to 1 0 0 ~ of o actual value. Because of the relative insolubility of these chlorides in glacial acetic acid, large amounts cannot be titrated. The method of adding mercuric acetate to convert the chloride into undissociated mercuric chloride as stated by Pifer and Wollish (7) was attempted. The addition of two or three drops of a saturated solution of mercuric acetate did not materially affect the end point, but the addition of 1.0 ml. of the mercuric acetate solution reduced the ammonium nitrate value by some 5%.
800 -
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12
14
16
18
20
22
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ML. T.B.A.H. T I T R A N T
ML. T B A H TITRANT Figure 4. and DNT
Differential titration of mixtures of TNT A. B. C.
TNT 0.3 gram, DNT 0.1 gram TNT 0.3 gram, DNT 0.2 gram TNT 0.3 gram, DNT 0.3 gram
If the explosive mixture is relatively insensitive, grinding to pass a 100-mesh screen will result in solution in glacial acetic acid with very little heating. When the explosire is shock or friction sensitive, solution is effected by addition of a small amount of water, followed by glacial acetic acid and finally sufficient acetic anhydride to remove the 11ater. The determination of ammonium and sodium nitrate in an explosive mixture by dissolving the sample in glacial acetic acid, aliquoting, and titrating differentially (or using one titration for ammonium nitrate and a second larger sample titration for total nitrate) is not completely satisfactory. The make-up and aliquot procedure which requires rinsing with hot acetic acid results in noxious fumes and may be subject to sample loss. The small aliquot sample size and titration volume for ammonium nitrate are inherently inaccurate and occasionally the potentiometric break is not of sufficient magnitude for accurate end point determination. The revised procedure results in larger titration volumes and a sharper potentiometric break for ammonium nitrate. The azo violet end point is sharp and potentiometric titration shows a n endpoint break of 100 mv. for 0.2 ml. of tetrabutylammonium hydroxide. The differential titration of various mixtures of D N T and T N T (Figure 4) is accurate down to 10% D N T content
Figure 5.
Titration in dimethylformamide Curves displaced vertically
A. B. C. D.
E.
Hexogen F. Nitroglycerin G. TNT H. DNT and M N T
MNT PETN Nitrocotton DNT
using about 45 ml. of 0.1N tetrabutylammonium hydroxide titrant; below this value the curves blend together. This separation might be further increased by doubling the sample size and titrating with 100-ml. burets. RlKT and D N T do not show a separate potential break when titrated as a mixture in dimethylformamide. Figure 5 shows complete potentiometric plots of the various explosives tested in dimethylformamide. The titration of nitro bodies in ethylenediamine is rather practical but, because of carbon dioxide absorbency, noxious fuming, and compressed titration range, this solvent is used only when necessary. Nitroglycerin and nitrocellulose are saponified in this highly basic solvent a t room temperature. Titration of nitroglycerin in cold dimethylformamide results in drifting initial millivolt readings. Apparently the addition of the basic titrant results in gradual saponification. Heating the dimethylformamide solution gives smooth titration curves having two distinct end points. The second potentiometric break in both nitroglycerin and nitrocellulose titrations represents the total number of nitro groups present. The first end point represents one of three in the case of nitroglycerin and two of ten in the case of dodecanitrocellulose. Figure 6 shows the titration of nitroglycerin in dimethylformamide, pyridine, and ethylenediamine. As ni-
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M L . T . B A H T TRANT Figure 6. Titration of nitroglycerin, 0.2 gram, in various solvents Curves displaced vertically
A. B. C.
Ethylenediamine Pyridine Dimethylformamide
VOL. 30, NO. 5,
MAY 1958
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M L . T.B.A.H. T I T R A N T Figure 7.
Titration in dimethylformamide
Curves displaced vertically A. Nitrocellulose 0.1 gram, TNT 0.2 gram 6 . TNT and hexogen (no separation) C. Nitroglycerin 0.1 gram, TNT 0.3 gram D. Nitroglycerin 0.2 gram, TNT 0.3 gram
troglycerin and nitrocellulose exhibit two potential breaks, it seemed possible that a mixture of each with T N T might be differentiated. Figure 7 shows a very slight potentiometric break for the first nitroglycerin end point in a inixture with TKT. Xitrocellulose and T K T mixtures similarly show a slight break for the first nitrocotton end point. Neither of these potentiometric breaks is of sufficient magnitude or reliability for accurate analysis of explosive mixtures. Mixtures of nitrocellulose and nitroglycerin (not shown), T S T and hexogen (Figure 7 ) ,and nitroglycerin and T N T (Figure 8) show only one potentiometric break representing the total content of both ingredients. The curve
Table I.
Mixture Composition, % Analysis Std. dev. Composition, % Analysis Std. dev. Composit'ion, % Analysis Std. dev. Composition, 5 ilnalysis Std. dev. Composition, % Analysis Std. dev. Composition, % Analysis Std. dev. 936
XHJO) 70.00 69.68 0.33 70.00 69.74 0.39 50.00 49.80 0.32 60.00 59.76 0.21 T O , 00 69.88 0.27 70, 00 69.78 0.22
ANALYTICAL CHEMISTRY
ML. T.B.A.H. T I T R A N T Figure 8.
for a mixture of two explosive ingredients is flatter than the curve of each one separately. When this project was almost conipleted, it was noted that methyl isobutyl ketone might be an improved substitute for dimethylformamide ( I ) . Figure 8 shows complete potentiometric plots using methyl isobutyl ketone. I n each case the potentiometric breaks are steeper and better defined. The separation of T K T and D K T remains a t the same ratio. Both nitroglycerin and nitrocotton exhibit only one potentiometric break in
Analysis o f Explosive Mixtures
XaK08
TXT
20.00 19.89 0.21 20.00 19.91 0.2i 20.00 19.86 0.21 20.00 19.88 0.23 10.00 9.92 0.19 10.00 10.22 0.29
10.00 10.15 0.07
15.00 15.11 0.12
Titration in methyl isobutyl ketone
Curves displaced vertically A. Nitrocotton E Nitroglycerin and TNT 6 . DNT F. PETN C. TNT and DNT G. Hexogen D. Nitroglycerin H. TNT
DST
SG
Hexo!Zen
PETS
10.00 10.21 0.09 15.00 14.86 0.10 20.00 20.21 0.06 20.00 19,56 0.70
20.00 19.81 0.11
methyl isobutyl ketone. Although check titrations are obtained Kith nitrocellulose in this solvent, it is not sufficiently basic to saponify the explosive completely. The potentiometric and azo violet end points are rather transient unless the solution is refluxed. The potentiometric end point corresponds to only one half of the nitro groups present. Further addition of the alkali titrant with azo violet indicator results in no further potentiometric break, but the masked azo violet end point occurs a t the titration point for all ten nitro groups. The highly basic titrant tetrabutylammonium hydroxide no doubt saponifies the nitrocellulose while the titration is in progress. Methyl isobutyl ketone is not sufficiently basic to titrate MKT. This a l l o w separation of L I S T , DNT, and T N T . Two separate titrations in dimethylformamide and methyl isobutyl ketone resolve all three nitrotoluenes. For the results indicated in Table I, the nitro body was separated from the inorganic nitrate by extraction with hot methyl isobutyl ketone. The ammonium nitrate content was determined by colorimetric titration of the residue in dimethylformaniide. The total inorganic nitrate was determined by colorimetric titration of a second sample in acetic acid. The nitro body in the methyl isobutyl ketone extract (except nitroglycerin) mas titrated potentio-
metricall!-. Kitroglycerin determination in methyl isobutyl ketone results in lo^ value-lformaniide, and nitrocellulose and nitroglycerin were titrated in ethylenediamine. The results in Table 111. were obtained by dissolving the T S T in methj 1 isobutyl ketone and the inorganic nitrate in acetic acid. Aliquots were taken from the volumetric solutions and titrated (the T S T a t 10 ml. per minute and the inorganic nitrate a t 20 nil. per minute). The synthetic mixtures were made up on a percentage basis using C.P. grade ingredients n itli the exception of nitroglycerin ( 16 40% nitrogen) and nitrocellulose ( 12.52Tc nitrogen) hich were assayed hj- means of the nitrometer. The analysis percentage value is the mean of f i e~samples.
ACKNOWLEDGMENT Table 11.
Analysis of Single Explosive
Titration, RlNT Nitrocellulose Xitroglycerin
0' /O
Purity, Std. % Dev.
99.75 97 83 99 01
100 00 0 . 1 4 98.11 0 . 8 1 99.46 0.77
The author wishes to acknowledge the cooperation of IT.C. Crosby in carrying out this work, and the communication from Picatinny Arsenal, which resulted in this investigation. LITERATURE CITED
Table
111.
Reproducibility matic Titrator
of
Auto-
TST Titration, RII.
Total Inorganic ?-itrate Titration, 111.
14 60 14.60 14.65 14 01 14.65 14.60 14.62 14.65 14.65
23.88 23.90 23.87 23.82 23.82 23.85 23,86 23.82 23.80 ~~
~~
(1) Bruss, U. B., Kyld, G. E. A., ANAL. CHEW29, 232 (1957). (2) Caldin, E. F., Long, G., J . Chem. SOC. 1954,3737. (3) Fritz, J. S., ASAL. CHEY. 24, 306 (1952). ( 4 ) Kave, S.RI., Ibid., 27, 292 (1955). ( 5 ) Mdmstadt, H. V., Fett, E. R., Ibid., 26.1348 11954). (6) Ibid.,' 28, 1412 (1956). (7) Pifer, C. IT., Wollish, E. G., Ibid., 24, 519 (1952). ( 8 ) Pifer, C. W., Wollish, E. G., Schmall, AI., Ibid., 26, 215 (1954).
RECEIVEDfor review Spril 15, 1957. Accepted December 11, 1957.
Nonaqueous Titration of Zinc Rapid Method for Zinc in Lubricating Oils THOMAS L. MARPLE, GEORGE MATSUYAMA, and LORENZO W. BURDETT Research Departrnenf, Union Oil Co.o f California, Brea, Calif.
b The reaction of dithizone with various heavy metals was studied in benzene-methanol solution. It was found that this reaction can be made the basis of photometric titrations of some of these metals, and a method for the determination of zinc in lubricating oil was developed. Less than O.lyo of zinc in oils can be determined with an accuracy of 1 to 270. Alkaline earth and alkali metals commonly present in oils do not interfere. Among the common heavy metals, only lead and mercury behave as zinc in the titration procedure. The presence of the other heavy metals can b e detected b y a change in shape of the titration curve.
T
HE COLOR reaction between diphenylthiocarbazone (dithizone) and various hea1-y metals has long been utilized for the colorimetric determination of micro quantities of these elements. The usual procedure relies upon the extraction of the nietal dithizonate complex from water into a nonaqueous phase, followed by the colorimetric measurement of the coniplex in the latter medium. Vallee ( 9 ) eliminated the extraction procedure by the use of a monophase water-
glycol mixture for the determination of zinc. Pflaum, Popov, and Goodspeed (6) used a 50% aqueous dimethylformamide medium to determine copper with 2,2'-biquinoline. A monophase colorimetric procedure is especially appealing for the analysis of petroleum products which are immiscible with water and are therefore usually ashed or extracted for the determination of metals. Of particular interest in the work reported here was the possibility of improving the accurac>- by a direct titration of zinc in petroleum products using an organic medium. Acid-base titrations in nonaqueous media have been used for a long time (8). Nonaqueous media have made possible the titration not only of materials immiscible with water but also of acids and bases too \Teak to titrate in aqueous solutions. T'ery little has been done, however, towards the direct titration of metals in nonaqueous media. Brummet and Holln-eg ( 2 ) utilized an acid-base titration in nonaqueous medium for an indirect potentiometric titration of copper, cobalt, and nickel in a medium consisting of 80% benzene and 20% methanol. Four chelating agents n ere used : dimethylglyoxime dithizone, 8-quinohno1, and l-nitroso-2-
naphthol. The titration with standard sodium hydroxide solution of the hydrochloric acid liberated in the reaction of the metal chloride with chelating agent was used to determine the metal content. Because of the acid-base titration involved in the procedure, buffers which are generally used to make organic reagents selective for particular metals could not be used. Titrations of solutions of the pure chlorides only were reported, and the suitability of the procedure for the analysis of the metals in the presence of other materials w s not established. Gerhardt and Hartmann (3) reported a back-titration procedure for the determination of metals in lubricating oils with disodium (ethylenedinitri1o)tetraacetate (EDT-4). The sample was dissolved in acetone, and treated with excess aqueous standard EDTA solution, which was back-titrated with an aqueous standard magnesium chloride solution. Because of the high stability of metal-EDT$ complexes, this method is not selective and most metals except the alkali metals are determined together. The work reported here n a s undertaken to develop a direct titration procedure for determining zinc in VOL. 30, NO. 5, M A Y 1958
937
