Table 111. Iron Content, x , us. Ball Milled Samples XI
Xz
lR1
3.670 3.689 3.700 3.710 3.720 3.726
3.677 3.691 3.702 3.713 3.725 3.731
0.007 0.002 0.002 0.003 0.005 0.005
s1 and
/RI = 0.004 xP: iron atom ratio (Y3.00All.25Fe.012).
d2: Sample_number factor = 1.128.
Std. dev. (Rid2)= 0.0036.
iron in the garnet was measured for the standards (Table 11). The deviation from the linear curve was estimated to be maximum r0.006 iron atom fraction. The equation for the least square curve is given below:
W W
=
=
0.444
+ 6.632
iron atomic fraction
z = IV../IY
An example of the analysis of six samples using this calibration curve is presented in Table I11 and plotted in Figure 2 a s a function of the number of revolutions each sample balimilled. The average range of duplicate sample is 0.004 atomic fraction unit. These samples represented time-function samplings from a 16 gallon ball-mill which contained 35 Ib of garnet powder. The ball-mill was revolving at a regular rate, and each successive sample had a slightly higher iron content than the previous sample because of the picking up of iron from the steel balls and ball-mill. The results indicate that the x-ray fluorescence analysis was capable of discriminating between the iron contents of successive samples. Thus this analytical technique could be used t o calibrate the iron content of the garnet batches. ACKNOWLEDGMENT
The author expresses his appreciation to R. H. Meinken, H. M. Cohen, and H. Basseches for their encouragement and guidance and to E. L. Soronen for his technical assistance.
RECEIVED for review October 19, 1966. Accepted February 21. 1967.
Determination of Acidity in Petroleum Products by Thermometric Titrimetry Charles J. Quilty
Rock Island Arsenal, Rock Island, Ill. 61201 EXISTINGMETHODS for the determination of acidity in petroleum oils are not entirely suitable for routine operation or they are time-consuming. The color-indicator method ( I ) is virtually useless for highly colored or thermally degraded materials. Although the potentiometric method (2,3) is more versatile, it seldom gives a very pronounced peak at the equivalence point, and often requires excessive time. In the method described, these disadvantages are overcome by utilizing thermometric titrimetry ( 4 ) . EXPERIMENTAL
Reagents. Isopropyl alcoho! (99%) was used as both titration solvent and titrant solvent. Potassium hydroxide (reagent grade) was prepared as a 0.2N solution as stated in ASTM D664-58 (2), arid stored in chemically resistant bottles. The solution was standardized by potentiometric titration of weighed quantities of potassium acid phthalate. Apparatus. The experimental work was performed on a Titra-Thermo-Mat thermometric titrator (American Instrumeni Co., Silver Spring, Md., Catalog No. 4-8350). The po:arsium hydroxide reagent was dispensed by a Meniscomatic buret with constant delivery speed of 600 p1 per minute ~~~~
~~
TAN
=
(11
(56.1) (Nliori)(mIKol%)/W
where w denotes the weight of sample in grams.
~~
( l j ASTh4 Designation D974-64: “196.5 Book of ASTM Standards.” Part 17, Baltimore, 1964, p. 437.
( 2 ) ASTM Designation D664-SS, “1955 Book of AST,M Standards.” Part 17, Baltimore, 1964, p. 312. (3) I.. L y k k e n , P. Porter, M. D, Ruliffson. and F. D. Tuemmler, T W b , EN(,. C H E M . ANAL,ED., 16. 2i’9 i1944). (4) M’esley M., Wendlandt, ”Tnerma! Methods of Analysis,” Interscience. New York. 1964. p;%27: -96.
666 *
(American Instrument Co., Catalog No. 4-2302). The unbalanced potential of the thermistor bridge, AE, which is proportional to AT ( ” C), was recorded on a high-impedance, 1-mV stripchart dc potentiometer (Model G14, Varian Associates, Palo Alto, Calif.). All titrations were carried out a t room temperature ( 2 5 i 2” C). Procedure. The sample, preferably 0.5 t o 1.0 gram of oil, was weighed to 0,0001 gram in a clean, oven-dried 30-ml tall-form beaker. After addition of 20 ml of isopropyl alcohol from a volumetric pipet, the sample beaker was placed in the adiabatic titration chamber. The instrument heater was used to bring the titration solution to within approximately 0.2” C of the temperature of the titrant. The titration was then carried out. A linear back-extrapoiation ( 5 ) was used to correct for heats of dilution and to determine the volume of potassium hydroxide required to neutralize the sample of oil, Blank determinations revealed that there was no detectable acidity in the titration solvent. The total acid number (TAN) of the oi! in mg of KOH per gram is obtained from Equation 1 :
ANALYTICAL CHEMiSTk‘
RESULTS AND DISCUSSION
In new and used oiis, the constituents considered :o have acidic characteristics include organic and inorganic acids, esters, phenolic compounds, lactones. resins. salts of heavi (5) Josepii Jordan,.[. Ch~n:.Editc., 40, ( l ) , A5 i 19631.
-
Table I. Potentimometric and Therometric Titrations of Lubricating Oils‘
Potentiometric Total acid no., mlKo&ram 3.28 1.77 2.31 2.53 2.76 0.15 1.00
... ... ...
...
o
New oils Thermometric Average total acid no., dKOH/@am Std. dev. 3.15 0.63 1.32 1.72 1.36 0.30 0.76 0.67 1.59 0.70 0.88
0.23 0.08 0.10 0.00 0.45 0.02 0.29 0.07 0.01 0.01 0.12
oils _ _Used_
_
Thermometric
AHd, kcal/mole
-25 -28 -12 -13 - 15 -1 -15 -8 -12 -8 -4
Potentiometric Total acid no., mlKoH/gram
Average total acid no., dKOH!gram
4.34 2.13 1.96 2.59 3.33 0.34 1.02
3.16 0.92 1.08 1.55 0.95 0.38 0.95 6.94 3.17 1.33 0.92 2.18 1.57 0.32 0.80
...
3.65 2.24 2.16 2.68 3.45 0.63 2.53
Std. dev.
_
_ ___
kcal/mole
0.06 0.12 0.04 0.21 0.38 0.08 0.04 0.13 0.19 0.21 0.00 0.44 0.19 0.10 0.01
-26 -13 -8
-8 - 12
-6 -9 - 19 -27 - 14 -8 -6 -5
-5 -12
All figures based on from two to five determinations. 2 kcal/mole).
* Approximate values only (estimate f
metals, and addition agents such as inhibitors and detergents
(6). The accuracy and precision of the method depend upon the acids present m d the materials with which they are associated. The accuracy, though not the precision, may be impaired by the nonreactivity of some acids and the limited solubility of some materials in the solvent. Enthalpy changes less than 1 kcal/niole make a titration exceedingly difficult and decrease precision and accuracy. The oils used in this investigation were commercial products qualified under various government specifications (7-10). These samples were representative of both light and heavy oils; the kinematic viscosities of the oils a t 100” F ranged from 6 to 300 centistokes. Heats of neutralization ranged from - 1 to -28 i 2 kcalimole. Satisfactory curves were obtained whenever the temj?erature of the titration solution and the titrant were carefully equalized. The effect of having unequal temperatures a t the start of the titration was previously discussed by Linde et. a/. (ZZ). A comparison of values obtained by thermometric and potentiometric methods is given in Table I. No relationship or agreement has teen found between acid numbers obtained by the thermometric method, and those obtained by the potentiometric method. Discrepancies of a factor of two between analyses based on different methods is common with the type of oil samples used in this investigation. The results ( 6 ) “Significance of ASTM Tests for Petroleum Products,” Special Technical Publication No. 7-B, Baltimore, 1956, pp. 64-7. (7) Federal Specification VV-L-800, “Lubricating Oil, General
Purpose, Preservative, (Water-Displacing, Low Temperature),” 11 March 1964. (8) Military Specification MIL-L-3150A, “Lubricating Oil, Preservative, Medium,” 1 April 1964. (9) Military Specification MIL-L-l407B, “Lubricating Oil, Low Temperature, Weapons,” 30 July 1964. (10) Military Specification MIL-L-21260, “Lubricating Oil, Internal-Combustion. Engine, Preservative,” 8 February 1954. (11) H. Linde, L. B. Rogers, and D. N. Hume, ANAL.CHEM., 25, 404 (1953).
Table II. Thermometric Titrations of Phenol and Benzoic Acid Solutions4 Amount Amount taken, found, pmoles/gram pmoles/gram
Weak acid Phenol ( K A = 1.3 X 1O-lo) Phenol Phenol
121 46
Benzoic acid ( K A = 6.3 X lo+) Benzoic acid Benzoic acid
119
42
6
I1
48 45
53 51
44
50
a All samples were dissolved in a lubricant base oil which had been slowly percolated through an alumina column.
reported here are analogous to a comparison of colorimetric and potentiometric methods by Lykken et a/. ( 3 ) . Table I also illustrates the precision attainable by thermometric titration. The precision is competitive with conventional methods and is not limited by either sample color or size as are other methods. No problems in the application of this method have been encountered in approximately six months of usage in this laboratory. Titration of the phenol and benzoic acid samples was performed t o assess the accuracy and limitations of the method. The samples were dissolved in a purified lubricant base oil which was free of detectable traces of acidity. Phenol and benzoic acid were selected because they represent weak acids which would normally present a very difficult problem of quantitative analysis. Table I1 shows that semiquantitative results are obtained even in very dilute solutions of weak acids using the thermometric method; whereas, acids having K-, I IO-9 are not detectable using potentiometric or colorimetric methods. The wide applicability and rapidity of this method make it VOL 39, NO. 6, MAY 1967
667
~
particularly suitable to quality control in both industrial and government laboratories. W a s i k s k i et ul. (22) have developed a direct injection technique which might be especially n-. applicable to process control in the petroleum i This technique is especially convenient in that unstandardized mgentsmaybeused.
(12) 1. C Wasilewski, P.T.S. h i,and Joseph Jordan, CmkL, 36,2131 (1964).
ANAL
ACKNOWLEDGMENT
The author thanks Ralph L. LeMar for his suggestion to utilize thermometric titrimetry for petroleum products and Joseph C. Wadewski for his interest and for providing a variety of useful information. RECENEDfor review November 14, 1%. Accepted March 6,1%7. The opinions or assertions contained herein are not to be construed as official nor do they reflect the views of the Department of the Army.
Simple and Accurate Calibration Model for Hydrogen-Content, E3eta-Particle Backscatter Gauges Robin P. Gardner Meawemetu and Controls Laboratory, Research Triangle Iirstitute, Research Triangle Park, N. C. 27709
IN PREVIOUS WORK on beta-particle gauging (I, 3,investigations showed that backscattering of beta particles is almost independent of the sample mass density. 7”hese experimental observations seem to contradict the fact tbat tbe cross section for the nuclear scattering of beta particles depends directly on sample mass density. Qualitative explanations of this phenomenon have been put forward ( I , 5), but these cannot be confidently used as a h i s for a caiibration model for beta-particle backscatter gauges. The calibration models that have been used involve the experimental determination of “effective atomic numbers” or “&e-ctie stopping powers” for each elemental constituent in the samples. This approach, while accurate, is tedious to employ and the parameters so obtained have meaning only for a specificexperimental arrangement. It is the purpose of the present paper to derive a model that explains the apparent anomaly between experimental okrvations and the theory of beta-particle scatkring. The derived model is found to be simple and accurate enough for use as a calibration relation for beta-particle backscatter gauges. The use of “effective atomic numbers’’ or “effective stopping powers” has been avoided. The model is checked against experimental data from the literature. MODEL DEWVATION
The technique of identifying independent probabilities that describe separate parts of the path of beta particles that are emitted from the source, scattered by the sample, and eventually detected is used to derive the present d e l . The derivation is based on the single nuclear scattering of beta (1) C. L. De Ligny, R. Lwerin& and H. D e H o w , Rec. Tmt. Chim., 84, 503 (1965). (2) R. D. Evans, “The Atomic Nucleus,” McGraw-Hill, New York, 1955. (3) R. P. Gardner andJ. W.Dum, XU.A s n ; C m . , 37,528 (1965). (4) R. P. Gardner and K. E Robens, Ibid, 38,923(1966). (5) F. R Gray, D. K Chrey, and H‘.H. Beamer,Zbid...31,2065 (1959). (6) Ibid-,32, 582 (!%o).
