An Electrolytic Resistance Method for Determining Carbon in Steel

Conductometric sensor for atmospheric carbon dioxide determination ... Conductometric Method for Determination of Carbon in Steel. E. L. Bennet , J. H...
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T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y mato seed gave a yield of about z j per cent of oil when extracted with ether and about 18 per cent of oil when the expeller was used. I n this connection i t might be of interest t o note t h a t the yield of oil obtained in Italy by pressing’the tomato seed is about 18per cent. SUNMARY

The constants have been determined for nine different authentic samples of tomato seed oil. It has been shown t h a t tomato seed oil contains a small amount of arachidic acid. BUREAUOF CHEMISTRY DEPARTMENT OF AGRICULTURS WASHINGTON, D. C.

AN ELECTROLYTIC RESISTANCE METHOD FOR DETERMINING CARBON IN STEEL1 By J. R. CAIN AND L. C. MAXWELL Received June 18, 1919

INTRODUCTION

The purpose of this study was t o investigate the accuracy, speed, and practicability of a method for determining carbon in steel, dependent in principle on passing the carbon dioxide produced by direct combustion. of the metal into a solution of barium hydroxide of known electrical resistance; after complete absorption of this gas the resistance is again determined and from the increase in this (due t o precipitation of barium ions) the percentage of carbon is deduced. This method is new in principle and i t is believed t h a t the principle can be applied generally in many cases where the substance being determined precipitates another substance from solution with resultant change in resistance. The assembly of apparatus for determining resistance is also new,2 and offers many advantages for technical work over the methods hitherto in general use for measurement of electrolytic resistances, which require the use of induction coils or high frequency generators, tuned telephones, balanced inductances and capacities, etc. Other new features are the application of the nomograph8 for the graphical representation of resistance data and the use of special conductivity cells with adjustable electrodes t o facilitate the manufacture of any number of such cells with the same cell constant. Much work has been done by others on electrochemical analytical methods. I n general, these fall into three groups in which an end-point is shown electrochemically by the following methods: ( I ) The unknown concentration is obtained from curves expressing a relation between cubic centimeters of titra1 Complete equipment for determination of carbon by this method may be obtained from Arthur H. Thomas Co., Philadelphia. 2 The elements of this were described by Weibel and Thuras. THIS JOURNAL, 10 (1918), 626. a The mathematical work in constructing the nomograph shown in Fig. 4 was done by Mr. H . M. Roeser of this Bureau at the request of the senior author, who suggested its application to electrolytic resistance data. A paper on this subject is in preparation by Mr. Roeser. References on the nomograph are: “Trait4 de Nomographie,” by M. d’Ocagne, GauthiersVillars, Paris; “Graphical Methods,” by Carl Runge, Columbia University Press, New York; “Graphical Interpolation of Tabulated Data,” by H. G. Deming, J . Am. Chem. Soc., 89 (1917), 2388; “The Nomon, a Calculating Device for Chemists,” by H. G . Deming, I b i d . , 89 (1917), 2137.

Vol.

11,

No. 9

ting solution and conductivity (or a related quantity) of the solution titrated;‘ ( 2 ) the unknown is obtained from curves giving the relation between cubic centimeters of titrating solution added and the corresponding electromotive forces of a cell composed of a normal electrode and an electrode not acted upon by the solution being titrated, the latter being the electrolyte;2 (3) special application of Method z used for determining hydrogen ion in acidimetry and alkalimetry and in precipitations from neutralized solutions.8 Such methods suffer by comparison with the present for the following reasons: ( I ) A curve has t o be plotted for every determination, which consumes much time; ( 2 ) the apparatus required t o determine carbon with a n accuracy of 0.01 per cent carbon would be too delicate and inconvenient of manipulation for every-day use; (3) the difficulty in some cases of fixing with sufficient definiteness the inflection or break in the curve denoting the end-point of the titration. Upon further comparing these methods with the present, i t is seen t h a t the latter dispenses with one operation common t o all the others, namely, the addition of successive portions of a titrating solution and the determination of the resistance a t each addition, resulting in additional time-saving. From an inspection of the chemical equation for the reaction underlying the present method, ’ Ba(OH)2 COZ = BaC03 HzO, i t is evident (when any given conductivity cell is used) t h a t the only factors which act t o change the conductivity of the barium hydroxide used for absorption are ( I ) the amount of carbon dioxide absorbed, which determines the disappearance from solution of the barium ion, and ( 2 ) the temperature. Since carbon dioxide precipitates barium without leaving reaction products in the solution t o increase the conductivity (such as would remain if, for instance, sodium sulfate were the precipitating agent for the barium) i t can be seen t h a t the present method should give the maximum possible change of resistance €or a given amount of barium removed-a condition tending to secure a high degree of sensitiveness.* However, the temperature coefficients of resistance of barium hydroxide solutions in the range of concentrations herein employed (Fig. 2) average nearly 1 . 7 per cent per degree, hence i t is evident t h a t the accuracy of t h e method will be largely affected by temperature if due correction is not made. I n developing this method it was deemed necessary: (I) To construct the curve showing resistance as a function of concentration of barium hydroxide solutions ranging from very concentrated t o very dilute and t o select the portion of this curve showing the maximum change of resistance for a given change of

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+

1 Harned, J . A m . Chem. SOC., 89 (1916), 252; Findlay, “Practical Physical Chemistry;” Ostwald-Luther, “Physikalisch-Chemische Massungen.” 2 Loomis and Acree, Am. Chem. J., 46 (1911), 585, 621 (a bibliography is also given); Hildebrand, J . A m . Chem. Soc., 8 1 (1913), 869; Kelly, I b i d . , 88 (1916), 341. a Hildebrand, LOC.cit. See also Weibel and Thuras, THIS JOURNAL, 10 (1918), 626, for another electrolytic method. 4 Compare the conditions in Harned’s work with Ba(0H)z solutions.

LOC.

cit.

Sept., 1919

T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

concentration; ( 2 ) t o devise an apparatus which, when the selected barium hydroxide solutions were used in it, would completely absorb the carbon dioxide a t the highest rates of passage of the gas current. The same absorption apparatus, in order t h a t the method might meet the requirements of convenience and rapidity, should permit resistance determinations t o be made without transfer of the solution t o another vessel; it should also be easy t o fill and empty; ( 3 ) t o determine the temperature coefficients of the barium hydroxide solutions in the selected range of concentration;l (4) t o prepare a chart enabling the operator t o read directly therefrom the percentage of carbon, all temperature corrections being incorporated; ( 5 ) t o design a n apparatus for the electrical measurements possessing the necessary robustness, reliability, simplicity, and protection from corrosion by the laboratory fumes. THE

RESISTANCE

OF BARIUM HYDROXIDE

SOLUTIONS

A curve was

prepared showing the relation between electrical resistance and barium hydroxide 170.

853

Baker’s analyzed barium hydroxide by diluting a stock solution of this with carbon dioxide-free water and determining their strength by titration against standard hydrochloric acid using methyl orange as indicator. The resistance measurements were all made in the same conductivity cell and a t practically the same temperature (27’ C.).l No great accuracy is claimed for these results, which are used only for establishing the form of the curve and selecting a portion of i t for more exact redetermination: The portion of the curve selected for use in this method , is t h a t between A and B. This region gives the maximum change of resistance per unit change of concentration consistent with the use of solutions sufficiently concentrated t o effect complete absorption of carbon dioxide under the conditions imposed. Comparing the resistance changes with changes of concentration of barium hydroxide corresponding t o I per cent carbon on different parts of the curve, i t is seen, for example, t h a t these changes in resistance are approximately six times as great on the portion AB as on the portion CD. The use of the most dilute solutions possible is, of course, also desirable from the saving in barium hydroxide effected. Solutions t o the left of A, even when used in very efficient absorbing vessels will not retain all the carbon dioxide except a t rather slow rates of aspiration.

440.

TABLE I-DATA FOR RESISTANCE-CONCENTRATION CURVE FOR Ba(0H) SOLUTIONS IN THE REGION 2 PBRCENT TO 30 PER CENT CARBONEQUIVALENT STRENGTX CELL CONSTANT = 0.715

Ba(0H)z per 200 cc. S o h . Equivalent1 Temperature Resistance Grams Per cent Carbon Deg C. Ohms 0.612 2.14 27.9 171.6 1.12 3.92 27.3 97.9 27.3 1.68 5.87 68.5 27.3 2.20 7.70 53.5 2.67 27.3 9.35 44.9 11.1 27.2 3.17 38.7 3.64 12.7 27.0 34.6 14.2 26.9 4.05 31.2 4.43 13.5 29.2 27.0 26.2 27.2 4.94 17.9 5.31 18.6 24.7 27.0 20.6 22.7 26.9 5.88 26.0 6.45 22.5 21.8 24.2 26.8 6.93 20.1 21.2 7.34 25.6 20.8 21.2 7.84 27.4 19.7 8.45 29.5 27.3 16.4 1 The method used in Tables I and I1 and elsewhere throughout this paper for expressing the strength of the barium solution in terms of “equivalent per cent carbon” was chosen for convenience in using the nomograph described subsequently. By “equivalent per cent of carbon” is meant the amount of carbon expressed as percentage on the basis of 2-g. samples being used, necessary t o precipitate completely all the barium ions from the solution concerned. This amount of carbon is calculated from the equation: COz = BaCOs HzO. For instance, in the first horizontal Ba(0H)z column of this table it is seen t h a t 2.14 “equivalent per cent carbon” corresponds to 0.612 g. Ba(0H)z; the former figure was obtained by solution of the proportion: Mol. Wt. Ba(0H)z : At. Wt. Carbon :: Wt. of Ba(0H)z in 200 cc. Soh. : Wt. of Carbon in Sample or 171.38 : 12.00 :: 0.612 : X whence X 0.0428 g. carbon or the “equivalent per cent carbon” = (x/2)100 = 2.14.

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TCMPCRATUUE 25’C. FIG. 1

concentration when the latter was varied from practically saturation t o nearly zero. The data in the literature being insufficient for this purpose, the determinations were made by Mr. Louis Jordan of this Bureau and are represented in Fig. I . The solutions for constructing the curve were prepared from J. T. 1 There is some possibility of placing the absorption vessel in a constant temperature bath together with a compensating cell in the other arm of the bridge. ‘This would remove the necessity for temperature correction. The method described herein, however, is believed t o be simpler.

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The portion of the curve selected for use by this method is again shown in Fig. 2 . The procedure used in determining the points on it was the same as for constructing t h e curve shown in Fig. I , except that 1 They were not corrected by any temperature coefficient since, as an inspection of Table 1 shows, they were made a t near enough t o one temperature t o give roughly the form of the curve, which was all that was desired.

T H E J O U R N A L OF I N D U S T H A L A N D E N G I N E E R I N G C H E M I S T R Y

854

more care was taken t o secure accurate readings and the results were corrected by the coefficients given under the heading “Temperature Coefficients.” Table I1 gives the d a t a used in constructing this curve. TABLE11-DATA

FOR

RESISTANCE-CONCENTRATION CURVEOF Ba(0H)z

SOLUTIONS IN THE REGION4 PER CENT TO 5 PER CENT CARBONEQUIVALENT STRENGTH

CELL CONSTANT = 0.715 Observed Ba(0H)z per Resistance Equivalent Temperature 200 cc. Soln. Per cent Carbon Deg. C. Ohms Grams 100.9 1.164 97.4 1.198‘ 94.6 ,1.239 1.242 95.8 91.4 1.280 87.6 1.286 85.3 1.301 86.8 1.303 86.0 1.346 82.7 1.385 84.2 1.403 82.8 1.460

Corrected Resistance Ohms 98.49 96.08 93.26 92.81 90.42 90.01 88.92 88.79 86.06 83.90 83.94 80.11

THE ABSORPTION APPARATUS

The essential features desirable in the absorption apparatus are: (I) It must retain all carbon dioxide when gas passes a t the rate of 300 to 400 cc. per rnin.; ( 2 ) i t must permit resistance measurements t o be made without transfer of the solution t o another cell; (3) temperatures of solutions should be easily read t o 0.1’; (4) the cell should be easy t o clean and fill with fresh solution; ( 5 ) all cells should be built with the same cell constant,’ or the latter should be capable of adjustment t o one value for all, so t h a t the chart or set of tables may be used for all cells. Fleming2 and others have shown t h a t the combustion of a sample of steel and the sweeping out of the products of combustion from the apparatus where combustion tubes of the usual length and bore are used, can be accomplished in j t o 6 rnin.; this requires passage of the oxygen a t the rate of about 300 t o 400 cc. per min. Soda lime has been found t o absorb all the carbon dioxide a t this and higher rates. However, barium hydroxide solutions of all concentrations are much less efficient absorbents t h a n soda lime. The absorbing efficiency increases slightly with the concentration, but even when the most concentrated solutions were used i t was impossible t o absorb completely all the carbon dioxide in t h e usual types of gas absorption vessels, nearly all of which were tried. The method of testing these forms of apparatus was * t o burn a sample of steel, having a gas meter before the furnace t o control the rate of passage of the oxygen, and t o attach after t h e tube being tested a second absorption vessel containing a little clear barium hydroxide solution; a test was not considered satisfact o r y if the second tube showed any cloudiness. Satisfactory absorption was secured in a vessel similar t o t h a t described b y Weaver and Edwards,3 with suitable modifications, the details of which were developed b y Mr. S. M. Hull, formerly of the Chemical Division of this Bureau. For use in the present work, electrodes were sealed into this absorption tube in the middle reservoir. I n order always t o secure the same cell constant, the area and distance apart of the elec1 The “cell constant” is not a constant at all, as Washbourne and others have shown, but for want of a better term this expression has been used throughout this paper. 2 Iron Age, 93 (1913). 64. 8 THISJOURNAL, 7 (1915), 534.

Vol.

11,

No. g

trodes were originally adjusted before sealing into t h e reservoir. This was found t o be a n extremely uncertain and difficult operation, and this difficulty as well as the general fragility of the apparatus led t o its abandonment and the search for something simpler. After the completion of investigations described in another paper,l i t was found t h a t by burning the sample a s therein described (namely, by placing it on a preheated boat, allowing boat and sample t o further preheat for a minute in the furnace and then admitting oxygen a t 300 t o 400 cc. per min.), it was possible to completely burn a 2-g. sample in 1‘/2 t o 2 min. Since the first few hundred cubic centimeters of oxygen combine with the iron, there is produced a much greater partial pressure of carbon dioxide t h a n is secured where a sample burns slowly and this makes i t possible t o use a very simple absorption tube, such as t h a t shown i n Fig. 3, for the carbon dioxide. 100. 0.

8 7. 6.

5. 4.

3. & I d

I.

ao.

a. 8.

Y

5-

*

$

7.

g

6

0

S

4. 3. 2. I.

00.

so

41

*2

43

4,+

46

4.’

4,a

4.9

5,O

52

CELL C O N 5 T A N T 0 . 7 1 5

TCMPERATURC 25°C. FIG. 2

I t was also found simpler t o build a cell whose electrodes could be adjusted t o secure a given cell constant than t o attempt t o obtain the same result with the electrodes sealed in a fixed position in the cell. This fact and the use of the special method of combustion described led t o the design of the apparatus shown in Fig. 3. This is not fragile and t h e glass parts can b e built by any glass blower of ordinary skill; i t meets all the listed requirements of the absorption apparatus. The adjustment of the cell constant is made b y moving the electrodes up and down after loosening t h e stuffing box; a marked change takes place as they approach the meniscus. Initial approximate similarity can be attained b y the glass blower with comparative ease. Once the electrodes are set in the proper position, this is maintained by cementing with DeKhotinsky cement. The electrodes are platinized and t h e cell constant is determined as described under “Operating Suggestions.” A certain amount of adjustment of the constant of the cell can be secured by removing or adding platinum black t o one or the other of t b electrodes during the platinizing operation by t h e use of a n auxiliary electrode. Some cells in long use a t t h e 1 Cain

and Maxwell, THISJOURNAL, 10 /1918), 520.

T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY

Sept., 1919

FIG. 3-DETAILLS

TEMPERATURE COEFFICIENTS

The solutions for determination of these coefficients were prepared and standardized as already described. The conductivity cells used for the work were kept in a thermostatically controlled chamber where the temperatures were maintained within 0.01O C. Resistances of barium hydroxide solutions equivalent t o approximately 4.0, 4.25, 4.50, 4.75. and 5.oper cent carbon were determined at 20°, 2 5 O , and 30°. The experimental values and the corresponding calculated values for a! and p are shown in Table 111. These values for a! and /3 were calculated by substituting t h e values for temperature and resistance from Table I11 I I 4-art251 4- /3[t-25I2, for t ir, the equation = Rf R26 and solving for a and p. FOR

DIRECT

READING

n

OF ABSORPTIONVESSEL

Bureau have differed in cell constant by 0.04 without affecting the accuracy of results. Actually, it is easily possible t o adjust cell constants within 0.005, and this practice is t o be recommended.

METHODS

855

OF C A R B O N

PERCENTAGES

Several methods were tried for graphically representing the relation between carbon percentages and the corresponding observed temperature and resistance measusements. At first tables were constructed in which temperature corrections were calculated and applied for every tenth of a degree and every tenth of a n ohm, but these were found to be too cumbersome.

Other tables were then constructed giving the temperature corrections only, with the idea of adding or subtracting these each time a reading was made. This condensed the tables very considerably b u t increased the amount of calculation necessary. A series of curves was then constructed showing the relation between resistance and per cent carbon, a curve being constructed for every tenth of a degree. Such curves are convenient t o use only if they are plotted on an inconveniently large scale; even then i t is very fatiguing t o use them for many readings a t a t i me. TABLE111-DATA Concn of

FOR

TEMPERATURE COZFPICIENTS OF RESISTANCE

Ba(0H)S Soln.1 Per cent C. Temperature Approx. Deg. C. 5.0 20 25 30 4.75 20 25 30 4.50 20 25 30 4.25 20 25 30 4.00 20 25 30

Resistance Ohms 175.25 124.02 114.37 142.18 130.36 120.16 149.07 136.62 125.91 156.18 143.07 131.89 165.18 151.28 139.48

0.01674

x 10-4 0.2687

0.01680

0.3505

0.01686

0.3085

0.01687

0.1652

0.01687

0.0898

a

,t~

* 200 cc. of the solution contained barium hydroxide approximately equivalent t o the carbon in a 2-g. sample with 5 per cent C, or 100. cc. of the solution contained barium hydroxide approximately equivalent to the carbon in a I-g. sample with 5 per cent C; or approximately 0.7125 g. Ba(0H)a. The other solutions contained barium hydroxide approximately equivalent t o 1%. samples with 4.75 per cent, 4.50 per cent, 4.25 per cent, and 4.00 per cent, respectively.

856

T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

FIG.4

Vol.

IS, No. g

Sept., 1915

T H E J C U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y

After a little study it was found t h a t the p term could be omitted from the equation for correcting temperatures t o 2 5 O , provided this equation is applied only between the temperatures I j O and 35'; a t higher or lower temperatures the correction for the /3 term is appreciable. If the laboratory temperature is not within these limits, the stock solution in carboy F of Fig. 6 must be brought within this range by placing it in a bath of cold water. The equation = R ~ [ I a(t25) P ( t 2 ~ ) ~ (I) then reduces

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Rt

t o RY5= R ~ [ I a(t - 2 5 ) ] , in which CY = 0.01681, this value being taken from Table 111. After trials of other forms of curves, it was found that the parabolic form answered the requirements of fitting the observations sufficiently well and of permitting the construction of a nomograph; the equation to the curve shown in Fig. 2 was calculated on the assumption t h a t i t was parabolic, using the methods of least squares and the observations shown in Table 11. This equation was found t o be C2 - 13.589 C 63.191 - 0.2478 R25 = o or, C2-- 13.589 C 63.191 -0.2478 [I a ( t - 2 5 ) ] = o whence, ( 2 ) C = 6.791/2d0.99~2R(o.5798 .01681t-67.11), C being the equivalent per cent of carbon in the sense already explained. From Equation 2 the nomograph shown in Fig. 4 was prepared. This nomograph can be used for all cells having cell constants not too different from 0.715, which was the cell constant of the cell used when data for the curve shown in Fig. 2 were obtained. As has been shown, the form of cell used (Fig, 3) allows the electrodes t o be adjusted always to secure this result. The use of the nomograph is very simple. A straight edge is made t o connect the observed values for temperature and resistance and the intersection with the third (middle) ordinate gives the concentration of the solution in terms of carbon percentage; this may be termed the first concentration. After the combustion is ended a similar set of readings is taken and subtraction of the second concentration reading from the first gives directly the per cent of carbon if a 2 - g . sample has been taken; or, if a I-g. sample has been used, the result is multiplied by two. The scales can be read to 0.005 per cent C, 0.05' C., and 0.05 ohm. It was found by comparison of chart readings in a n u a b e r of cases with the resistances computed by Equation I t h a t the error of the chart is less than 0.005 per cent carbon.

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857

tained with the high frequency generators and tuned telephones, provided t h a t inductances and capacities were used as directed by recent investigators. Such refinements are inconvenient in a method such as the present, intended for works use, and there are two other important objections from the same standpoint: ( I ) the difficulty of detecting a minimum in the telephone when working in a noisy place such as the usual works laboratory and the fatigue of the operator wha would have t o make possibly 80 or 90 determinations in an 8-hr. day, and ( 2 ) the liability t o deterioratior; of a high frequency generator when operating continuously 2 4 hrs. per day as might happen in technical use. These considerations led to the development of a method of measuring electrolytic resistances by the use of commercial 60- or 25-cycle current. It was found after trial of the Weibel' galvanometer, a vibration galvanometer and of a direct current galvanometer operating off a crystal rectifier placed in the secondary of a transformer (the cell being in t h e primary), that the Weibel galvanometer offered a very satisfactory zero instrument for such measurements of electrolytic resistance as had t o be made in this work.

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APPARATUS F O R D E T E R M I N I N G E L E C T R I C A L R E S I S T A N C E

With the cooperation of the Leeds and Northrup Company, different forms of apparatus for measuring electrolytic resistances were built and tried. Two forms of high frequency generator in connection with tuned telephones were used; also one or two forms of induction coil as sources of alternating current, and an interrupter such as is used in wireless investigations; the latter was quite unsatisfactory, as might be expected, due t o polarization. Good results were ob-

FIG.J-DIAORAM OR BRIDGEU S E D

FOR RESISTANCE &fEASUREMENTS

The bridge shown in Fig. j was accordingly constructed by Leeds and Northrup. The resistance coils and galvanometer are enclosed in a hermetically sealed box so that they are efficiently protected from corrosion. There are three dials-tens, units, and tenth ohms, respectively-and a fourth which adds IOO to the readings of the others, when this is desired. An accuracy of 0.1 ohm in the readings is all t h a t is necessary, although much better than this can be done, This instrument has been in use several months and has been very satisfactory. A perfect minimum i s always registered by the galvanometer, provided t h e electrodes are well platinized. PROCEDURE FOR DETERMINING CARBON

A nichrome-wound furnace is used for heating. Porcelain or glazed quartz tubes may be employed; they should be fitted with a sheet nickel sleeve to protect internally from fused iron oxide which is thrown off from the burning steel. The two absorption vessels are filled to the 2 0 0 cc. mark with barium hydroxide solution from the stock bottle F (Fig. 6 ; see also Operating Suggestion I). Oxygen or air freed from carbon dioxide is passed for a few seconds t o mix the solutions and their temperatures and resistances a r e 1

E. E. Weibel, Bureau of Standards, Scienlij5c Paper, 297 (1917).

8.58

T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

then recorded. I n the meantime the combustion boat filled with alundum sand has been preheating in the furnace, which for this work should be maintained continuously a t 1 0 5 0 ~t o I IOO', preferably the latter temperature.' This is a n extremely important point, for if the temperature is too low or the oxygen is not pure or is not admitted a t 300 t o 400 cc. per min. after the combustion starts, the rapid combustion essential t o successful absorption cannot be secured. The boat is removed from the furnace and when a t a low red heat the sample is distributed on the alundum2 and the boat replaced in the furnace and left t o preheat, without oxygen passing, while the next sample is being weighed. Oxygen is now passed a t the rate of 300 t o 400 cc. per min. for the next two minutes; then the stopcock, Fig. 6, is turned t o the position, which admits carbon dioxide-free air; this should pass a t t h e same rate as the oxygen. During this combustion period, if directions have been followed, all carbon dioxide will have been removed from the furnace, but some still remains in the large bulb of the absorption apparatus. The air removes this. The advantage of If this 1 The melting point of gold is a convenient temperature check. metal i s not melted the temperature is too low. See cited paper by Cain and Maxwell. 8 Experience has shown that no loss of carbon occurs unless the sample contains cxtremeiy fine particles; with most steels these can first be removed without causing an error in the carbon determination. This point should always be tested, however, in burning new steels.

VO~.*II, No. g

the use of air a t this stage is obvious: a saving of oxygen is effected and the furnace is immediately available for burning the next sample. While air is passing through the first tube (requiring 1 l / 2 t o 2 min.) the combustion of the next sample proceeds as already directed, using the second absorption tube. The second reading of resistance and temperature on the first tube then follows, and if the solution is not too dilute it can be used for absorbing the carbon dioxide from other samples; otherwise, a little is allowed t o flow into the reservoir G, and the tube is filled t o the mark again with fresh solution. Of course i t is a n economy in time for the operator, wherever possible, t o choose conditions (weight of sample, carbon content of same, etc.) so as t o get the maximum number of determinations for a single filling, since in this way the second resistance and temperature readings ,serve as initial values for the next combustion and so on. The solution should not be used when i t is more dilute than corresponds t o 4 per cent carbon (i. e., 99.5 ohms a t 2 5 ' ; see nomograph, Fig. 4, for the limiting resistances corresponding t o other temperatures) , since its absorptive power a t rapid rates of passage of the oxygen is then less, and some carbon dioxide may be lost. The data relating t o combustions should be recorded as obtained. It is convenient t o use a tabular form for record showing: ( I ) Designation of sample; ( 2 ) weight taken; (3) cell used; (4) initial temperature;

T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

Sept., 1919

( 5 ) final temperature; (6) initial resistance; (7) final resistance; (8) initial concentration; (9) final concentration; (IO) carbon percentage in sample; (11) remarks. There is ample time for entering this information while other operations are going on. A very good way is t o enter “final temperature” below “initial temperature,’’ and “final resistance” after “initial resistance” for each sample, since this relates these quantities in an easy manner for reading t h e nomograph. The speed of the method naturally depends on the skill of the operator and t h e ingenuity displayed in arranging a cycle of operations which secures the best speed under his working conditions. Operators a t the Bureau when working on a series of Bureau of Standards’ analyzed samples averaged one determination per 4 l / 2 t o 5 min. The accuracy of results is shown in Table IV. TABLEIV-RESULTS

BY

ELECTROLYTIC RESISTANCEMETHOD

B. S. Value for

B. S.

Standard Sample Used lla

Wt. Used Grams 2.0 114. 2.0 2.0 12b 2.0 1 .0 14a 2.0 1 .o 2.0 16a . . . . . . . 0 . 5 0.5

....... ...... ........ ......

Carbon (Ay. by Direct Combustion) Pes cent 0.223 0.409

-

0.813 0.990

1 .o

21a

.......

Sugar..

2.0 2.0 2.0

.......

0.617 Gram 0.00421 0.0042 1 0.00421

Value by Electrolytic Resistance Method 0.21 0.21 0.41 0.41 0.42 0.81 0.82 0~.~~ .80 1 .oo

1 .oo 1 .oo 0.98 0.62 0.62

Gram 0.0046 0.0042 0.0040

ANALYST Maxwell Swindells Swindells Maxwell Cain Cain Maxwell Maxwell Maxwell Maxwell

OPERATING SUGGESTIONS

I--Stock barium hydroxide solutions are conveniently made in one t o two carboy lots by adding solid barium hydroxide t o the carboy nearly filled with water (agitating with air) until the equivalent strength approaches 5 per cent carbon; subsequent additions can then be made by adding a saturated barium hydroxide solution. Of course, i t is not necessary t o make up exactly t o the equivalent of 5 per cent carbon; a n approximation t o this is all t h a t is desired. The strength of the solution is determined from time t o time during the standardization by running a portion of the solution into the cell and measuring its resistance. If a set-up like t h a t shown in Fig. 6 is used in measuring t h e resistance, i t is not necessary t o remove the carboy from the shelf or t o break any of the connections during these operations. If it is desired t o economize in the use of barium salt, the waste solution in reservoir G can be brought up t o strength as described, after first decanting it off from the barium carbonate t h a t has settled out. A still further economy can be effected by drying and calcining t o oxide the precipitated barium carbonate; this oxide can then, of course, be used again. 2--The cell constants should be checked from time t o time. This may be done ( I ) by burning standard samples, or (2) by determining the resistance of a N / l o solution of pure potassium chloride. This soluiion should be prepared on the day it is used,

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since i t has been found t h a t stock solutions change during the course of this work. Table V shows the resistivities a t various temperatures of N / I O potassium chloride solutions. The cell constant is computed from the formula R = NS, or N = R/S, where R is the observed resistance, N the cell constant, and S the resistivity, taken from Table v, for the same temperature. If a change in cell constant has taken place i t is most probable t h a t the electrodes need replatinizing. Directions for this are given below. If Method I is used, any marked deviation from the correct carbon value of the standard steel may be due t o a change in the cell constant, and this should then be checked by Method 2 , unless the error in the carbon determination is suspected t o be due t o some other cause. I n the present work no deviation of cell constants has been observed until after several months’ use, and then the change is sudden and erratic. When the cell constant changes it should be brought back t o the original value by the methods already described under the heading “The Absorption Apparatus.” 3-If the absorption vessels are not t o be used for some time they should be cleaned with hydrochloric acid (not over 2 t o 3 per cent HC1) followed by distilled water. Extreme care should be taken t h a t none of the hydrochloric acid or chlorides enters the reservoir for waste barium hydroxide solution, if this is t o be used again. 4-To platinize the electrodes, the cap carrying them (Fig. 6) is removed from the cell and they are first cleaned with sulfuric acid and chromic acid mixture followed by distilled water. Then they are immersed in a vertical position in a solution made of I O O g. water, 3 g. chloroplatinic acid, and 0 . 0 2 t o 0.03 g. lead acetate. Current is passed through the solution by connecting the electrodes t o three dry batteries in series; the current is passed for 5 min., reversing every half minute. Finally, an auxiliary platinum electrode is introduced and current passed with this as anode for another 2 min., after which the electrodes are washed thoroughly with distilled water and are then ready for use. TABLEV-SPECIFIC RESISTIVITY OF N/10 RCl SOLUTION AT VARIOUS TEMPERATURES (FROMLANDOLT-BORNSTEIN PHY~IKALISCHCHEMISCHE TABELLEN,4TH ED.,PAGE: 1 1 17). Temperature Resistance Deg. C. Ohms 15.0 95.42 93.28 16 0 17.0 91.32 18.0 89.37 19.0 87.49 20.0 85.69 21.0 83.96 22.0 82.30 23.0 80.71 24.0 79.11 25 n 7. .7. .64 . 76.16 26.0 27.0 74.79 28.0 73.42 29.0 72.10 70.82 30.0 31.0 69.59 32.0 68.40 33.0 67.20 34.0 66.09 64.98 35.0 SUMMARY

I-This paper describes the fundamental principles of a method for determining carbon dioxide by absorbing i t in barium hydroxide solution and measuring

T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Val.

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the resistance change of the solution in relation t o its concentration. 11-A suitable absorption vessel with electrolytic resistance cell incorporated is illustrated and described. HI-Resistance measurements of barium hydroxide solutions varying in concentration from 3.08 g. Ba(OH)2 per 1. t o 4 2 . 2 5 g. Ba(OH)2 per 1. were determined approximately, and determinations accurate within 0.01ohm were made of the resistances of twelve different solutions varying from 5.820 g. Ba(OH)2 per 1. to 7.300 g. Ba(OH)2 per 1. IV-Temperature coefficients of resistance of these twelve solutions were determined in the range 2 o . 0 0 ~ t o 30.00’ C. V-Based on the measurements of resistance of the barium hydroxide solutions, solutions with concentrations varying between 5 . 8 2 0 g. Ba(OH)2 per 1. and 7.300 g. Ba(OH)2per 1. were chosen as most suitable for this method. VI-Special resistance-measuring apparatus was developed which simplified these measurements by dispensing with tuned telephones, high frequency generators, and balanced inductances and capacities. VII-A convenient nomographic method of applying necessary temperature corrections to resistance measurements was developed. VIII-The method permits of accurate determinations of carbon in steel (i. e., within 0.01per cent carbon), by the direct combustion method in 4Ij2 t o 5 min. ACKNOWLEDGMENT

The authors desire t o acknowledge the work of Mr. H. E. Cleaves, former member of the Chemistry Division of the Bureau, who carried out some preliminary measurements, and the assistance of Messrs. Silsbee, Agnew, and Isler of the Electrical Division of the Bureau, who cooperated effectively in the selection of a suitable alternating current galvanometer. The Leeds and Northrup Company also cooperated fully by constructing and loaning electrical measuring apparatus for experimental work and by finally building in practical form the apparatus that was developed. B U R ~ AOF U

STANDARDS

WASHINGTON,

D. c.

THE DETECTION AND ESTIMATION OF SMALL AMOUNTS OF CERTAIN ORGANIC NITRO COMPOUNDS WITH SPECIAL REFERENCE TO THE EXAMINATION OF THE URINE OF TNT WORKERS By ELIASELVOVE Received March 31, 1919

I n connection with the sanitary control of munition establishments where trinitrotoluene1 was employed, i t appeared desirable to have a convenient The trinitrotoluene which is ordinarily employed is the 2, 4, 6 variety. The specimen of this compound which was used in this work was obtained by several recrystallizations from a mixture of alcohol and benzene of a very pure sample which was kindly sent to this laboratory by the Bethlehem Steel Company, of New Castle, Delaware. A melting point determination carried out in this laboratory, by Dr. J. M. Johnson, on a similarly recrystallized product showed the melting point to be 80.9O C. (corr.). 1

11,

No. g

and fairly rapid method for t h e detection and estimation of small amounts of the trinitrotoluene in air. I t was thought that the qualitative color test with alkali, which forms the basis of the so-called Websterl test for T N T in urine, might be developed into a quantitative method. As this test is ordinarily carried out, however, the color obtained is very fugitive and hence cannot well be utilized for making quantitative determinations colorimetrically. The best one can do is t o follow the procedure of Webster, i. e., adopt some arbitrary scale of one’s own, such as “trace, I, 2, 3, 4, and intense.” It was primarily with the object of making this color test more nearly quantitative t h a t this work was undertaken. The Webster test t o which reference has been made is carried out by first extracting the acidified urine with ether and then treating the extract with an alcoholic solution of potash. The latter is prepared, according to Webster, by dissolving 4 t o 5 g. of caustic potash in IOO cc. absolute alcohol or by mixing I O cc. of a saturated solution of caustic potash with go cc. of alcohol. Since these directions allow of a considerable variation in the concentration of the alkali and since in order to standardize the conditions it appeared desirable to fix more definitely the concentration of the alkali, some experiments were carried out with this object in view. An ethereal2 solution of 2 , 4, 6-trinitrotoluene was prepared containing 0.01 mg. of the trinitrotoluene in I cc. of the solution. A number of alcoholic3 potash solutions were also prepared containing different concentrations of the alkali. Each of 5 cc. portions of the ethereal solution was then mixed, a t room temperature, with 5 cc. of one of these alcoholic potash solutions. The results obtained are given in Table I. The results given in Table I show that the concentration of the alkali is a very important factor in controlling the stability of the color produced by the action of the alkali on the trinitrotoluene in the Webster test. They also show that by reducing the concentration of the alkali used by Webster to about t h a t used in Expt 3, we can obtain sufficient stability for the color to render it possible to utilize it for quantitative purposes. I n order t o apply this alkali-trinitrotoluene color reaction t o the detection and estimation of trinitrotoluene in air, it appeared best to substitute alcohol as the solvent in place of the much more volatile ether. I n order, therefore, t o determ’ine the favorable concentrations of the alkali t o employ when alcohol is used as the solvent for the T N T , experiments were carried out with varying concentrations of the alkali, using for each experiment 5 cc. of a solution of the trinitrotoluene in absolute4 alcohol, containing 0.01mg. of the trinitrotoluene in I cc. of the solution, I British Med. J., 2 (1916). 845. 2 The ether used was the ordinary ether of the quality described in the U. S. Pharmacopoeia. a Absolute alcohol or the ordinary alcohol which has been redistilled over lime was used in all this work. It was found that by using such an alcohol, instead of the ordinary 95 per cent alcohol, even the more concentrated alcoholic potash solutions were practically colorless when fresh. 4 The specific gravity of the alcohol used was 0.788 at 2 5 “ C.