Carbon Train for Control Analysis LOUIS SINGER, HOWARD J. STARK,
AND
JOHN A. KRYNITSKY, Naval Research Laboratory, Anacostia, D. C.
0
train described, the quartz plu was 2.34 cm. (0.906 inch) in outside diameter and the inside ofthe combustion tube was 2.5 cm. (1 inch). REMOVAL OF SULFURGASESAND WATERVAPOR. Following the combustion tube is an absorption tube, B , of about 50- to 60-cc. capacity which is attached to the combustion tube by means of a short piece of heavy-walled rubber tubing. It is half m e d with 20-mesh zinc to remove sulfur gases ( 1 ) and the balance is packed with Anhydrone. ARRANGEMENT OF TRAIN. Considerable time may be saved by setting up the carbon train for easy manipulation. Following the three-way stopcock (Figure 2) is a 0.47-cm. (0.188-inch) copper tube, E , rigidly secured by means of a table clamp, F , rod, G, and clamp holder, H , and extended into and close to the base of the balance. With this arrangement it is necessary to hold the balance door only slightly o en during xeighing. This narrow opening does not sensibly a&ct the weighing. The copper tube in the balance terminates in a one-hole rubber stopper, C. The tube extends half way into the stopper, leaving the other half to accommodate the weighed absorber, B. The combination of copper tube and stopper should be rigid enough to act as the only support of the weighed absorber. Using this support, the absorber can be handled with one hand. A sketch of the weighed absorber, including dimensions, is seen in Figure 3. Another absorber of the same dimensions, also filled with Ascarite and Anhydrone, is used as a counterpoise ( A , Figure 2). This counterpoise is necessary, as the effluent gas, oxygen, exerts an increased buoyant effect on the weighed absorber ( B , Figure 2) over that of air. The absorbers may be interchanged when the weighed absorber is exhausted. Both are left unstoppered during the entire time of an analysis. Weighing with the absorber open introduces no error. METHODOF WEIGHING. Since stress is placed upon speed, i t
F THE methods now being used for the determination
of carbon in steel, the combustion method has proved most satisfactory from the point of view of accuracy and general applicability. On occasion, however, as for some purposes of control work, greater speed is demanded than t h a t afforded by the ordinary combustion method. It is the purpose of this paper t o show how, by a n adaptation of the ordinary combustion train, more rapid analyses may be obtained. Practically all texts of quantitative analysis include discussion of the ordinary combustion method for carbon in steel and a description of a carbon train. This being the case, only those modifications of the train will be discussed which serve to speed u p the analysis.
Modifications of Train HASTENING THE SWEEPIXG PROCESS. A large part of the time taken for an analysis is used in the sweeping process, in which the gaseous products formed in the combustion of the sample must be swept out of the combustion tube and absorbed. This sweeping period may be considerably shortened by reducing the volume through which the gas must be passed. Such volume reduction is effected by inserting in the exit end of the tube a quartz plug (A,.Figure l), made by sealing off both ends of a piece of quartz tubing. The outside diameter of the plug should be only slightly less than the inside diameter of the combustion tube. I n the
r
7
I
115
FIG.4
INDUSTRIAL A N D ENGINEERING CHEMISTR Y
116
VoI. 13, No. 2 The time that elapses after droduction of the boat into le combustion tube, before Lpid ignition sets in, varies mewhat with the composion of the steel, the size of the mple, and its form (shot, ~Alings,eto.). In the ease of drillings, for plain carbon steels, rapid ignition sets in within 30 seconds after introduction of thesample. Using the Bureau of Standards Samples, which are millings, hetween 30 and 60 seconds elapse before rapid combustion sets in. In the case of shot, which is the form in which the control chemist
. .
SET-UPOF CARBON TRAIN
is necessary to use a. fast method of weighing; accordingly, the single deflection method (2)mas employed. This method is rapid and does not involve B loss in sensitivity as when using a magnetic damDer. The authors used a chainomatic balance of ordinary senshivity (0.25 division per 0.1 mg). BOATSAND COVERS. For combustion of the sample an Alundum boat 8.75 em. (3.5 inches) long, 1.25 cm. (0.5 Inch) wide, and 0.78 cm. (0.31 inch) deep is used. The boat is provided
Prior to uae, ~
a large number of boats snd covers are burned-out
~~~~
~
~~~
~
0.500-gram steel sample together with appro-at& 6.5 &am of tin shot. which momotes rapid combustion of the sample. The eomb&ion tube is kept &t a temperature of 1150" to 1200" C., using B furnace with globltr heating elements. P~RTWTCATZON AND METERING OF GAB. The followine devices are used for the regulation and pkifielttian of oxyge;: a gas regulator for the oxygen tank (an Air Reduction Company com~
~~~
after ignition once starts. At the end of this time, the tube must be flushed for 30 seconds. After flushing, the oxygen is exhausted into the air and the absorber is immediately weighed. Thus the absorber is ready for weighing 2.5 minutes after introduction of the sample into the combustion tube. The total time of an analysis uill depend on the skill of the operator. HONever, using the carbon train described and Neighing by single deflection, au analysis can be made in 4 minutes. I n the manipulation of the train, severd features represent a departure from conventional practice. No attempt is made to regulate the gas flow during the analysis. The flow rate is set a t the start of a series of analyses and the only change is the path the gas takes, as controlled by the three-way stopcock (D, Figure 2). During a combustion, the gas is passed through the absorber; while the absorber is being weighed the gas is allowed to escape into the air. The absorber is a t all times kept in the balance case, which minimizes any error in weighing due to temperature difference between absorber and balance. The absorber is left Unstoppered at all times and weighings are made as rapidly as possible.
Discussion of Results
~
Table I contains the results obtained using Bureau of Standards samples. These inolude two aIIoy steels which are diffioult to burn and a cast iron. With the exception of
to remove moisture. It is t ~ l s oadvisable to provide the combustion tube with a clear glass window. A simple and inexpensive device is the glass Ttube of Figure 4, which fits into the rubber stopper of the combustion tube. A bulb is blown on the glass T-tube. so that the operator may see the boat in the combustion tube.
TABLE I. DETERMINATION OF CARBON Bureau of Sample NO.
Stsndarda Certified Value
Procedure for Analysis .. -.... .~
~~~~~.~~ ~~~~
~
~~~
fidflow rate. At the beginning of a series of determinahons the weighed absorber is set in place on the train in the rubber
Carbon Present
Carbon Found
Error
%
%
% +0.01 Nolle
lld
+om -0.02 -0.01
12d
+O.Ol
100 130
None Nolle +0.01
72a
NOIN
-0.01 -0.02 +0.01
14b
k i g h e d the three-way stopcock (D, Figure 2) is turned so that oxygen from the combustian tube exhausts into the air; Neighing while a stream of gas is directed into the balance would not be advisable. The absorber is now taken from the stopper, suspended from the stirrup of the balance, and weighed. It is then replaced on the train and gas is passed through it by proper adjustment of the stopcock. One-half gram of tin is placed in the boat, the cover is put in place, and the boat and contents are pushed into the combustion tube. The boat is pushed as close as possible to the quartz
72
+O.Ot Nolle
15b
Nome -0.02 None
130
121 CR 18%,
hi 8%)
126 (Ni 36.4%)
Alloy Steels 0.057 O.OS7 0.034 0.034
0.06
0.06 0.03 0.03
0.07 0.07 0.03 0.04
+0.01 None
2.91 2.88
+0.05
10.01 +0.01
cast Iron ..... ~ . .
~~~~
~~~~~
~
~~
~
~~
1150" and 1200' C. in every case the sample burned rapidly.
Rapid burning is attended by a stream of dowins oxides from the sample
5g
2.86 2.86
2.86 2.86
+om
Av. -0.01
February 15, 1941
ANALYTICAL EDITION
the cast iron, which showed in one determination an error of SO.05 per cent, the greatest error for any determination was 0.02 per cent. Most of the analyses show either an error of 0.01 per cent or no error at all. The certificate values of the second column are rounded and listed as percentage of carbon present in column 3. It is Obthat are 'Ompared with the these latter tained, because the magnitude of the weight change in the
117
absorber restricts expression of results to ti$fo significant figures.
Literature Cited (1) Bright, H. A., and Lundell, G. E. F., Bur. Standards J. Reseamh,
5 , 943 (1930).
(2)
Brinton, P. H. M. P., J . Am. Chem. SOC.,41, 1151 (1919).
OPINIONBexpressed in this article are those of the writers and do not necessarily reflect the ideas of the Naval Service.
Simple Tests to Indicate the Condition of an Analvtical Balance J
LEONARD C. KREIDER, Bethel College, Pr'orth Xewton, Kansas
I
T OFTEX falls to the lot of a teacher of quantitative
analysis or an industrial chemist to determine whether a balance is in satisfactory weighing condition. This paper presents a simple and rapid method of balance testing that is sufficiently exacting for most purposes. The fundamental principles upon which this brief series of tests is based are well recognized, but the method of application is believed to be new. A recent paper (1) gives valuable additional information from the instrument repairman's point of view. The following points should be considered: 1. The general condition of the balance must be acceptable. 2. The rest point should be constant for any particular load when the mass on each of the pans is equal. 3. The balance must be of the proper degree of sensitivity. (Sensitivity is understood in this paper to be numerically equal to the deflection on the pointer scale caused by the addition of a 1-mg. load to a single pan of the balance.) 4. The balance must give weighings that are closely reproducible. 5 . The balance must have lever arms of nearly equal length.
These points are intimately interrelated-for example, a n imperfect knife-edge may cause the balance to perform poorly in respect to points 2, 3, and 4. Point 1 is not readily tested quantitatively, but a careful inspection will usually suffice. Make sure that the beam releases, pan rests, rider carrier, chain weight devices, and other moving parts are in good mechanical condition. See that the knife-edges are separated from their bearing plates by the beam lift to a gap of about 0.1 mm. and that when the beam lift is released all three knife-edges make contact with their plates over the whole edge gently and simultaneously. This is essential to the life of the edges. Metal parts well finished and free from corrosion are desirable, but do not necessarily indicate an accurate balance. The information sought in points 2 and 3 can be obtained quantitatively by the following method: Place the balance on a firm support in a part of a room where a fairly constant temperature prevails (away from radiators, open windows, and other drafts, out of direct sunlight, and removed from other hot light sources). Level the balance with the setscrews provided in the base. If the balance has recently been moved from another location, open the door of the case and allow at least an hour for the balance to attain room temperature. Bfter the balance has been brought to the same temperature as its environment, determine the data required to construct a table similar to Table I. For this purpose select two sets of analytical weights, W1 and WB. I t is convenient, but not necessary, to have the sets agree within fairly narrow limits. They need not be calibrated. If two sets of weights are not available, one can make shift with only one set-for example, if the data a t 20 grams' load are to be determined, one could call the two 10-gram weights together W1 and the 20-gram weight W2.
Table I records data obtained in applying this method to a typical student balance. The principle involved is that of double weighings first devised by Gauss. TABLEI. DATAFOR STUDENT BALANCE Weight of
WI and Wv2 (Each) Grams 0 10 20 50
Rest Point", A
Rest Pointh, B
Average Rest Point, C (A B)/2
9.0 9.0 9.5 9.1 8.6 10.2 9.7 9.5 100 9.3 11.1 WIon left pan Wz on right pan. b Ws on left pan: WI on right pan. c W I on left pan, WI 1 mg. on right
+
+
9.0 9.3 9.4 9.6 10.2
Rest Point",
Sensitivity, E (B D)
6.9
2.1 1.8 1.4 1.0 0.8
D
7.3
7.2 8.5 10.3
-
pan.
Constancy of the values in column C would satisfy point 2. If the rest point in C should shift by as much as two or three pointer scale divisions between loads of zero weight and 100 grams' weight on each pan, the balance would not be acceptable for determining absolute mass values, but might prove acceptable for certain types of gravimetric analysis where the determination of small differences in mass only is required. The balance tested (Table I), where the rest point shifts 1.2 scale divisions between zero weight and 100 grams' weight load on each pan on the basis of a sensitivity of 0.8 at 100 grams' load, would cause an error of 1.5 mg. in determining a 100gram load. This amounts to a deviation of only 0.0015 per cent, which would be negligible for most work. It is worth while to test the effect of changing the position of the masses from the centers of the pans to the edges and see if the value of the rest point is thereby changed. Defects of the end knife-edges may sometimes be detected by this method, whereas they may remain unnoticed when the masses on the pans are perfectly centered. The point of rest should also be checked by using swings of small amplitude and then swings of considerably greater amplitude. Difference between the two values indicates worn, nonparallel, or otherwise faulty knife-edges. If the balance is to be used where the requirements are only moderately exacting (point 3), the sensitivity (column E ) should have a numerical value of at least 2 and preferably 3 or 4 a t zero load on the balance pans. The sensitivity of a balance should remain nearly constant or should decrease slowly and regularly with increasing load on the balance pans. The fall in sensitivity is usually due to a difference in level between the middle and the end knife-edges, and may be caused by bending of the beam under the load, by wear of the knife-edges, or by not sharpening them uniformly. I n general, i t is not safe to use a balance for loads that reduce the sensitivity to less than 40 per cent of the value with zero load.
