324 A N A L Y T I C A L C H E M I S T R Y

d .A. I / I ? Nitroguanidine hydroclil oride

( ContJ.) 1 . 3,i 1 1 . 4 8 1 1 . 4 1 1 1 2; 1 1.10 I

I-Benzslirienesmirio- l-ijiethyl-2-nitt o-

guanidine

; 7 . 7 6 7 . 2 5 fi. A3 >

5 . 5 Q 2 5 . 2 8 2 4 . 4 8 4 4 2G :3 3.9( i 1 3.87 1

3 . 4 3 8 3 28 9 3 , 1 5 3 3.03 1 2 71 1 2 59 1 2 49 1 "39 1 "28 1 '1 '70 9 2 05 1 1 .98 3 1.93 9 1 .83 1 1 . 7 8 1 1 . 7 3 1 1 . 6 9 1 1 .FG 1 1. .58 1

3 . 9 5 IT,

3 A t i 9

Sitroaniinoguanidine

7 90 ! 5 . 7 2

~~~

Table I . (Continued) phcnonc, phenylnitroguanidine, l-acetan~ido-3-nitroguanidirir, l-aniino-l-methyl-2-nitroguanidine, niethylnitroguanidine, nit1.0-

d , A. I / I 1 d , A. I I 1 guanidine hydrochloride, nitroguanidine hydrobromide, l-hen- zylideiieamino-l-niethyl-2-nitroguanidine, nitroaniinoguaiiicliiie, ~-itroaniinog!lianidine Sitroguariidirte

(Cont?.) (Coni f . )

a . '3 1 1' OR , henzalaminoguanidine, and nitroguanidine. 4.77 4 7.01 > Data on nitroguanidine have been given elsewhere ( I ) , but

I > are presented here because a number of differences are observed

4 . 4 1 7 1.91 3 93 7 1 88 3 40 7 1 79 3 29 10 1 .A8 3 .05 4 1 . 6 2 1 2 91 G I . 5 3 > 2 . 6 6 4 1 . 4 0 1

9 . 4 7 3 1 . 2 6 2 . 3 4 3 1 29 1 9 . 2 8 2 21

I 1

. 2 13 4 1.10 2 05 1 1.08 1 . 9 5 ! 114.6-mm.-diameter camera, using CuK radiation filtered 1 58 The samples were rotated 1 ( 3 4 1 ,134 3 1 , i i l 3 Brnzal*irnino- 1 5 6 1 guanidine 1 . 3 3 3 15.50 .i Interplanar spacings and the intensities for these compounds

8 . 1 9 :j are given in Table I. Intensities were visually estimated 1 30 1

3 , 63 J 4 . 8 9 4 and are reported on a basis of 10 to 1, where 10 represents the

most intense line. 4 37 10 .$ . 10 7 Nitroguanidine

7 40 1 3 . 7 3 1 B.44 1 3 02 1 5 . Mi 1 3.49 1 .5.17 9 3.34 (3 4 . n2 2 3 19 4 4.22 7 3.09 R 3.64 5 2 . 9 3 1

2 . 9 1 8 2 . 5 9 2 . 6 4 7 2 . 2 5 3 2 52 3 2 .04 1 2 42 5 2 . 0 0 I 2 34 3 1 . 9 1 1

I 2 . 2 6 1 1 . 8 3 2 18 1 . 7 7 1 2 13 2 1 . 0 2 1

, I)et\yeen the present, data and those previously published.

"57 1 1 4 3 1 EXPEKI>IENT.A L

2 1 19 1 Samples were prepared by grinding small amounts in an agate J I 10 1 mortar, They were then mounted in thin-walled glass capil-

Film patterns were recorded in a Iaries of 0.3-mm. diameter.

through nickel foil ( A = 1.5418 A%.), d:ving exposure.

ACKXOW LEDGMENT

The materials for which data are given here were prepared by 3 . 3 0 8 2 . 7 9 1 I<. A . Henry, W. G. Finnegan, and Joseph Cohen. 3 08 10 2 . 6 4 L

LITERATURE CITED

9 (1) Soldate, A. XI., Soyes, 11. J I . , .%N.~L. CHEM. 19, 442 (1947).

_ _ _ _ _ _ _ _ _ _ ~ ~ RECEIVED for review September 9, 1953. Accepted November 14. 1955.

Determination of Hydrogen by Beta-Ray Absorption ROBERT B. JACOBS, LLOYD G. LEWIS, and FRANK 1. PIEHL Standard Oil Co. (Indiana), Whiting, Ind.

Despite the importance of the hydrogen content of liquid fuels, the petroleum industry seldom uses this property in process control because determining hydro- gen by combustion is slow and complicated. .4n instru- ment has been developed that determines hydrogen in 5 to 20 minutes by measuring the P-ray absorption of a sample. Replicate determinations on six hydrocarbons show- a standard deviation of 0,0370 hydrogen. and single determinations have an average error of 0 . 0 3 7 ~ hgdro- gen. Interference from elements heavier than carbon can be corrected by a simple factor.

ANY petroleum refining processes alter the hydrogen coli- M tent of the feed. Catalytic reforming removes hydrogen from paraffins and cycloparaffins to produce aromatics. Solverit extraction selectively removes aromatic hydrocarbons fronl lubricating oils and leaves paraffins of higher hydrogen content. Even simple distillation may separate a feed into fractions of widelv different hydrogen content. Thus, hydrogen content of organic liquids can be a useful index of process operation.

Because existing methods for determining hydrogen are s l o ~ , this property is seldom uqed. A skilled operator using conven- tional combustion techniques can analyze four to ten samples a day n i th an accuracy of 0.05 to 0.10% hydrogen. A method 14

needed to determine hydrogen rapidly with comparable accuracy by an unskilled operator.

An instrumental method that meets these requirements has been developed in t,his laboratory. The principle that hydrogen absorbs more @-rays than equal weights of other elements ( I ) underlies the operat,ion of the instrument. Thus, a liquid absorhs more p rays as its hydrogen content increases, This prinviplr has also been used by Smith and Otvos ( 3 ) .

The total energy loss, E , of p-rays in passing a dktanw, Z, through a liquid is approxiniately:

E = Z(LHPH + LCPC + ZLxPx)

where L refers to the energy losses per gram per square cni. and p to the specific gravities for hj-drogen, carbon, and other ele- ments (X) . The specific, gravity of hydrogen PH, is defined a$

where p is the density of the liquid and %H is weight per cent hydrogen. I n the instrument, the energy loss in t,he liquid is proportional to

PC + I ~ H P H + BKxPx

where K H and Kx are coilstants t,hat relate the difference.- in energy loss per collision of &rays with electrons of h\drogen and other elements to those with elect,rons of carbon. The espres5ion

V O L U M E 28, NO, 3, M A R C H 1 9 5 6

is evaluated by balancing the absorption in the liquid with that in a variable absorber and reading the value from a calibration chart prepared with known hydrocarbons. The instrument provides for simultaneous measurement of the specific gravity of the liquid. Per cent hydrogen is calculated from these two values.

Most hydrocarbon mixtures do not contain sufficient quantities of elements heavier than carbon to cause significant error8 through contributions by the term XKxpx. If the amount of the &her eloment is known, a simple correction can be applied.

APPARATUS

Essential features of the instrument are shown schematically

325

hration ourve must be constructed by analyzing hydrocarbons of known composition. Finally, if liquids containing elements other than carbon and hydrogen are to he analyzed, correction factors for these elements must he determined.

The volume of the specific gravity plummet is calculated from its weight in air, its weight in a pure hydrocarbon such a8 ben- zene, and the density of the hydrocarbon.

~I , ~~~~~~~~~~~~~ ~ . .~~. the arm of a. tor.& &%lance by apl$tinum wire. The 0-ray cell, C, is conical in shape. The axis of the eone 1388888 through the

,,TO BALAN

f ilOLTD

n CE ARM

E

A -VOLTAGE

' ' ELECTROMETER AMPLIFIER

Figure 1. Schematic diagram of instrument

The copper block maintains the liquid in both oells a t the same temperature. Calibration of the instrument thus does not depend on temperature. The block also serves a9 a radiation shield; less than 1 milliroentgen per hour can be detected outside the instrument.

The source of &rays (d), a 10-mc. deposit of strontium-90 on a mica sheet, A, emits two equal beams in opposite directions. One beam passes through 8. constant Invar absorber, B, and through the thin window, E, into an ion chamber. The opposing beam passes through the sample cell, C through a wedgeshaped ah- sarber, D, and then through the thin window, H, into a second ion ohamher. When the wedge is positioned so that the rate of absorption of &rays in the sample plus the wedge equals the rate in the standard absorber, &rays will enter both ion chambers a t the same rate. Two shutters bracket the source and cut off the flaw of 0-rays to the ion chambers when the instrument is not in use.

Became opposite voltages are applied to the ion chambers, the current.8 oppose each other. An electrometer tube amplifies the net ion-chamber current, which is read on a galvanometer or recording potentiometer. When the galvanometer shows no deflection, S m y s are entering both ion chambers a t the mme rate. The wedge position corresponding to this condition is the "null wedge position" and is read on a dial micrometer.

Figure 2 is a photograph of a commercid model being manu- factured under license.

CALIBRATION

Four calibrations are needed before the instrument can he used The volume of the specific gravity plummet must be

The constant K e must he evaluated, and a cali- routinely. determined.

Figure 2. Commercial model

YI ~ Y 5 1 . SEC-BUTYLBE//// 1 =- 1.740 ETHYLBENZENE ..

0 CUMENE ..' k 1.730 B E N Z E N E . . YI

I I I I ,920 ,925 ,930 ,935 , 9 4 0

1.700

pc + K"Pb

Figure 3. Evaluatio n of K E

ified example is show "nmstina hn+ loa- SA

Ka is evaluated graphically; a simp1 n in Figure 3. Because Kn is critical for a ._... I l.ll -_" ."~l I" for paraffins, four aromatic hydrocarbons, such a8 benzene,. cumene, ethylbenzene, and sec-butylhenzene, are used. For each, the null wedge position is plotted against (po+Kapnj forvalues of K a in the neighborhood of 1.800. The plot must be large enough to indicate wedge position clearly to 0,001 inch and (po+Kapa) to 0.0001 gram/ml. Attempts are made to draw smooth curves through the four points of constant RE; that value of Ka, to 0,001 inch, that gives the smoothest curve is used in all subsequent determinations.

The calibration curve is constructed by determining specific gravity and null wedge position for a series of hydrocarbons, cal- culating ( P O + Kepa) far each hydrocarbon, and plotting wedge position against(po + Kzapaj. Thescale of theplot should he the same as used in evaluating Kn. Phillips Pure Grade n-pentane, n-hexane, n-heptane, 2,2,4-trimethylpentane, eyelohesane, cyclo- pentane, benzene, toluene, ethylbenzene, eumene, and S ~ C -

hntylbenzene are adequate for the calibration. More accurate calibrations are possible for commercial instru-

326 A N A L Y T I C A L C H E M I S T R Y

I n the single-setting method, either the galvanometer or a recording potentiometer may be used. After the mechanical and electrical zeros are set, the shutters are opened. The wedge is then positioned so that the needle swings equally to the left and right of the zero. The null wedge position indicated on the dial micrometer is accurate within 0.002 inch.

While the wedge is being adjusted in either method, the spe- cific gravity plummet is weighed periodically. The final weigh- ing is made as the null wedge position is noted. For a complete determination, the first method requires 10 to 20 minutes; the second. about 5 minutes.

Table I. Precision and Accuracy for Six Hydrocarbons

% Hydrogen Standard Hydrocarbon Caled. Found5 Deviation

n-Heptane 2,2,4-Trimethylpentane Cyclohexane Benzene Cumene Ethylbenzene

a Arerage of 11 determinations.

16.10 15 .88 14.37 7 . 7 4

10.06 9.49

16.11 15.90 14.40 7.71

10 .06 9 .46

0 .031 0.035 0.032 0 .032 0.015 0.043

ments, which have wedges that taper linearly. points can be fitted directly to the equation

The calibration

Cc + KHPH = aW + b

where a and b are constants and W is the null wedge position. T o determine correction factors for other elements, liquids con-

taining these elements must be analyzed. Because most pure compounds fall beyond the range of the instrument, they must be diluted with aliphatic hydrocarbons. The specific gravity and null wedge position are determined for each mixture and used to calculate per cent hydrogen. The correction factor, F , for each element, X, is given by the equation

%H found - %H true %X

F x =

PROCEDURE

For each determination, the procedure consists of rinsing the sample cells, filling them with sample, measuring the specific gravity and the null wedge position, and draining the cells. Ten milliliters of liquid are sufficient to fill the cells. Two rinses with the sample are sufficient; no significant change in measured hydrogen content is noted between the third and fourth fillings. Viscous samples, such as lubricating oils, should be diluted with pure hrdrocarbons. The null wedge position may be determined by either of two methods. Interpolation is more accurate, but the single-setting method is faster.

~~ ~

W E D G E AT 1300" c + /

W E D G E AT 1.295"

T IME - Figure 4. Determination of null wedge position

The interpolation method emplors a recording potentiometer and is illustrated in Figure 4. Thecells are rinsed and filled, and the mechanical and electrical zeros are adjusted. The wedge is brought to approximate balance, and the shutters are opened. The galvanometer current Trill vary about an average value. The wedge is then moved to swing the galvanometer across the zero point to a new average current. The null wedge position is calculated to within 0.001 inch by linear interpolation.

If a series of samples that give similar null wedge positions is to be analyzed, the procedure may be shortened. Because the ratio of change in galvanometer current to change in wedge position is constant over limited ranges, i t can be determined for one sample and used with a single wedge setting for subse- quent members of the series.

CALCUL4TION

Per cent hydrogen is calculated from the specific gravity of the The specific gravity, p, of sample and the null wedge position.

the liquid is given by the equation:

- TY* - ws - I'

where U-4 and W s are the weights of the plummet in air and in the sample, respectively, and V is its volume. The value, d V , of ( p c + K H ~ H ) corresponding to the null wedge position is read from the calibration chart. Hence:

An independent method must be used to determine % S.

Table 11. Accuracy for Thirteen Binary nlixtures % Hydrogen

Calcd. Found Diff. n-Hexane + n-heptane 16.23 16.15 - 0 . 0 8 n-Heptane + toluene 11.97 11.97' 0 .00 n-Heptane + benzene 11.36 11.36 0 .00

13 .58 13.51 -0.07 2 2 4-Trimethyipentane + benzene 11.32 11.32 0 .00

2,2,4-Trimetliylpentane + cyclohexane 14.65 14.67 + 0 . 0 2 15.24 15.27 +0.03 15.57 15.59 +0.02

Cyclohexane + ethylbenzene 10.39 1 0 . 4 2 +0.03 11.32 11.38 +0.06 12.29 12.29 0.00 13 .31 13.31 0 . 0 0

2:2:4-Trimethylpentane + toluene 13.79 13.68 -0.11

RESULTS

Precision and accuracy of the instrument n ere studied by ana- lyzing individual hydrocarbons and known and unknown mis- tures. Reported results mere obtained by the interpolation method on the pilot model of the instrument. d gradual increase

130 13 5 14 0 I 4 5 15 0

% H Y D R O G E N . B E T A - R A Y M E T H O D

Figure 5. Comparison of &ray and combustion methods

V O L U M E 28, NO. 3, M A R C H 1956

in the meaqured hydrogen contents corresponded to a uniforni drift in calibration equivalent to 0.05% hydrogen per month. This drift was small enough that a daily check gave an adequate correction.

Six pure hydrocarbons were analyzed a t weekly intervals. The data are summarized in Table I, where the average measured per cent hydrogen was calculated after correcting for drift. The average values do not differ from the calculated values by more than 0.03% hydrogen. The average standard deviation for all si\ hydrocaarbona is 0.031 yo hydrogen; the average probable eiror is 0.021% hydrogen.

.$nalyses of thirteen known binary mixtures of hydrocarbons are given in Table 11. The average difference betn-een meawred and calculsted values is 0,03270 hydrogen; the standard devia- tion of the differences is 0.050% hydrogen.

Three sj-nthetic mixtures of hydrocarbons were also pirpared and analyzed. Table 111 summarizes the composition and anal-

327

Table 111. Accuracy for Three Synthetic IIirtures

Conipositiun of JIixture 2 2,4-Triinethylpentane n-Heptane n - H e x a n e Cyclohexane Met hylcyclohexane Methylcyciopentane Benzene To 1 u e n e Ethylbenzene Cumene Butylbenzene Isoprene

Hydrogen Calculated Found

_-__ 1 8 . 8 18.4 1 0 . 5 2 0 . 6 1 0 . 2 1 0 . 3 0 . 9 1 . 9 3 . 7 1 . 8 1 . 8 1 .1

100.0 __

Weight %

17.4 17 .2 0.0

10.9 0.0 0.0 1 . 8 7 . 8

1 8 . 0 21.8 4 2 0 9

100.0 __

~

1 6 . 8 ti . 5 3 . 2 1 . 1 0 0 0.0 2 . 3 9 . 8

23 .6 9.5 5 10 8 0 .4

100 0 __-

14 tit? 12.37 11 43 14 67 __ 12.34 __ 11 40 __

Difference 1 0 01 -0 .03 -0 03

Table IV. Correction Factors for Six Elenients Compounds Used 5 x

Kitrogen I’yridine + iso-octanec 2 . 4 1 Pyridine + iso-octane 4.62

AV.

Oxygen Acetone 27.55 Methanol 49.94 2-Propanol 26.63 1-Butanol 21.59 Ethyl acetate 1 cyclohexane 4 . 0 9 Ethyl acetate $ cyclohexane 8 17

Av. Sulfur

Thiophene + n-heptane 1.13

Thiophene + n-heptane 10.35 Thiophene + n-heptane 5.37

A r . Chlorine

Chlorobenzene + iso-octanec 4 . 8 4 Chlorobenzene + iso-octane 9 .00 n-Butyl chloride + cyclohexane 4.38 n-Butyl chloride + iso-octane 9.30 Chloroform + n-heptane 17.15

Bromine Hrornobcnzene + n-heptane 10.78 n-Butyl bromide + iso-octanec 10.18

A\,.

.4r. Lead

Tetraethyllead i- iso-octanec 0 . 9 5 Tetraethyllead + iso-octane 5 . 2 6 Tetraethyllead + iso-octane 7 . 0 0

.I\..

I” K“

0,029 0.025

0.027 1.022

0.047 0.051 0.053 0.054 0.066 0.056

0.054 1.044

0.266 0.245 0.254 0.255 1.208

0.213 0 .216 0.237 0.219 0.220 0.221 1 .181

0.5G9 0.563 O.56fi 1.4fi4

1 593 1 .609 1.641 1.614 2 . 323

K for carbon arbitrarily = 1.000. K for hydrogen = 1.820. b Per cent tha t causes a n error 3 t i k e s the standard deviation. 2,2,4-Triinethylpentane.

l’oleranceh

3 . 4

1 . 7

0 . 4

0 . 4

0 2

0 OR

ysis of each mixture. The errors observed were no larger than than those found for individual hydrocarbons or binary mixtures.

Seventeen liquid fuels were analyzed by both the @ray instru- ment and conventional microcombustion. The data are plotted in Figure 5 . @-Ray values represent single determinat,ions. Combustion values are averages of two determinat,ions on each sample; the average st,andard deviation for the duplicate values is 0.05570 hydrogen. The average difference betwern the two methods is 0.054% hydrogen, and the standard deviation of the differences is 0.066% hydrogen. Student’s t t’est phows that neither met’hod gives significantly higher or lower result3.

To determine the magnitude of errors caused by elements other than carbon and hydrogen, liquids containing pix elements tbat orcur in petroleum samples were analyzed. Table 11- summarizes the composition of the mixtures and indicat’es by the empirical correction factors, F , the error in per cent’ hydrogen introduced by each per cent’ of the other element. The theoretical K constant’s are derived from the correct’ion factors by the equation

Kx = 1 + F s ( K H - 1)

The tolerance is taken as the per cent of each element that will cause an error three times the standard deviat,ion, or 0.09370 hydrogen. Sulfur and lead are the only elements commonly found in petroleum stocks in large enough amounts t,o require cor- rections.

Precision and accuracy of the single-setting method with a gal- vanometer depend on the skill of the operator in visnally esti- mating the average position of the galvanometer needle. Limited experience of a few operators suggests a standard deviation of about 0.05% hydrogen.

DISCUSSIOR

The instrument meets the analyst’s requirements of speed, accnracy, and precision. A standard deviation of 0.03% hydro- gen represents a precision greater than that possible by conven- tional combustion techniques. The analyses of mixtures show that the accuracy of the determination is also satisfactory. Op- eration of the instrument is simple; a sample may be analyzed in les. than 20 minutes-a decided advantage over the combustion method. Because commercial instruments have larger 6-ray sources than the pilot model, they have greater precision.

Although the half life of the p r a y source is 25 years, the de- crease in intensity is about 0.1% per week. Because the instru- ment employp two opposing beams of p-rays, changes in intensity do not affect the calibration. Hoaever, the statistical uncer- taintv in measured hydrogen content increases as the source ages. The estimated replacement time is 25 years.

Corrections for elements other than carbon and hydrogen are the only factors that may limit the accuracy of the instrument. Satisfactory performance with hydrocarbons pointE to many poe- sible applications in the petroleum industry. Applications to hT-drocarbons in other fields have not been explored, brit the in- strument should find many further uees.

ACKNOWLEDGMENT

The authors acknowledge contributions to the development of the instrument by Evon C. Greanias, and are indebted to P K. Winter of the General Motors Research Laboratorie- for the chemical analyses of the liqnid fuelq.

LITERATURE CITED

(1) Hevesy, G., Paneth, F.. “Radioactivity,” p. 39, Oxford Univer-

(2) Jacobs, R. B., Greanias. E. C. [to Standard Oil Co. (Indiana)],

(3) Smith, V, N., Otvos, J . W., .INAL. CHEM. 26, 359 (1954).

RECEIVED for review August 8, 1955. Pre- sented in par t a t Symposium on Automatic rlnalytical Methods in t h e Petroleum Industry, 124th Meeting, .4CS, Chicago, Ill., September 1953.

sity Press, London, 1938.

U.S.Patent2,700,111 (Jan. 18, 1955).

Accepted December 5 , 1955,