Table VI. Recovery of Organic Nitrogen in Presence of Nitrates
Ethyl Alcohol Added, M1.
Organic Nitrogen Recovered,
0
5
%
98.35 98.02 99.34 97.80 98,02 99.04 97.95 99.36 98.74 98.94
controls. Results are shown in Table VI. RESULTS AND CONCLUSIONS
An average of 98.47% recovery of nitrogen was obtained with the modified method. Recovery tests from amino acids are not entirely satisfactory because samples of 100% purity are difficult to obtain and the hygroscopic properties of these compounds are marked. Although the expanded McKenzieWallace micromethod gives slightly more precise results than other methods tested, the ease and simplicity of a procedure in routine work must be
considered. The potassium acid iodate solution can be made more accurately than sulfuric acid but the high purity is expensive. Sulfuric acid can be standardized against reagent grade sodium carbonate and stored for several years without appreciable change in normality. Although high purity of reagents is desirable. it is not mandatory if reagent blanks are examined along with the samples. The mixed indicator usually gives a much sharper end point than methyl red alone. The boric acid solution used to absorb the ammonia is about 0.05X so that the pH a t the end point is well within the range of the mixed indicator. The modified Kjeldahl method, using sulfuric acid in place of potassium acid iodate, is preferred by the authors over other methods. The usual interferences in this method cause a pyrolytic loss of nitrogen due to the high acid-salt ratio which results. To determine to what extent this occurred, samples of primary effluent were examined in comparison with duplicate samples to which had been added sucrose, fat, and amino acid nitrogen. Results given in Table IV indicate that with excess sulfuric acid added there is not enough interference to be significant. There is, however, a greater spread in the per cent recovery, partially because it was cal-
culated on the relatively small quantity of amino acid nitrogen that was added to the primary effluent. The modified Kjeldahl method has further merit in that there is no bumping during distillation. This permits the solution to be distilled a t a faster rate. The effect of nitrates in concentrations usually found in water and sewage on the determination of organic nitrogen was insignificant. Less than 0.5% loss resulted. The modified Kjeldahl-mercuric sulfate method has been successfully used in the Sanitary Engineering Research Laboratory of the University of Florida as a routine procedure for 11 months. LITERATURE CITED
( I ) American Public Health Assoc., New
York, N. Y., “Standard Methods for the Examination of Water. Sewage, and Industrial Wastes,” 10th ed., 1955. (2) McKenzie, H. A., Wallace, N. S., Australian J . Chem. 7, 55-70 (1954). (3) Ogg, C. L., Brand, R. W., Willits, C. 0.. J . Assoc. Offic. Aar. Chemists 31, 661, 663 (194g). (4) Willits, C. O., Coe, 31. R., Ogg, C. L., Ibid., 32, 118 (1949). (5) Willits, C. O., Ogg, C. L., Ibid., 31, 565 (1948). ( 6 ) Ibid., 33, 100, 179 (1950).
-
RECEIVEDfor review October 3, 1956. Accepted January 28, 1957.
Automatic Recording Thermo balance CORNELIUS GROOT and
V. H. TROUTNER
Corrosion and Coating Operation, Reactor and Fuels Research and Development Operation, Hanford Afornic Products Operation, Richland, Wash.
b An automatic recording thermobalance was constructed from standard laboratory equipment a t a minimum of expense, to plot pyrolysis curves. The unbalance is detected by a photoelectric null-point detector and is restored by an electromagnetic restoring device. The balance recorded weight changes over the ranges of 50, 100, and 200 mg. with an accuracy within 3t0.3, k0.4, and h0.8 mg., respectively. The sensitivity was 0.3 mg. The balance required 10 minutes to register a full range weight change. Construction details and a discussion of the thermobalance are given.
A
measures the changes in weight of a substance as it is heated. Several systems for THERMOBALANCE
automatically recording changes in weight have been described (2). Duval has shown (1) how the curves of weight us. temperature (pyrolysis curves) can be used to study the composition of many substances. Because the recording thermobalance used by Duval was not commercially available a t the time of this work, an automatic recording thermobalance was constructed from standard laboratory equipment a t a minimum expense. Weight changes were detected by a photoelectric cell and measured with an electromagnetic restoring device. The temperature of the furnace, surrounding the material being studied, was increased linearly with time and the weight mas plotted as a function of time. The resulting curves of weight us. temperature were used to study the composition of hydrated aluminum oxide samples. This
report presents the construction details and a discussion of this thermobalance. The automatic recording thermobalance consists of four main systems: 1. The thermo system for holding and heating the sample. 2. The balance system for weighing the sample. 3. The automatic or control system to compensate the balance fo; changes of sample weight. 4. The recording system for recording the force needed to compensate the balance, and thus the weight change of the sample. The complete apparatus is shown in Figures 1, 2 , and 3. THERM0 SYSTEM
The thermo system (Figure 4) consists of a vertically mounted electric tube furnace, a quartz rod which supports the sample within the furnace, VOL. 29, NO. 5 , MAY 1957
835
4 Figure 1. Thermobalance
Figure 2.
and a controller for controlling the temperature of the furnace, A Wheelco 0" to l20O0 C. temperature controller (Wheelco Instruments Co., Chicago, Ill.), with a one-revolution-perday clock motor driving the set point, wae used to increase the temperatnre of the furnace at a constant rate. A limit switch on the controller shuts off the power to the power outlets and thus the entire apparatus when the heating cycle is completed.
Side view of thermobalance
BALANCE SYSTEM
The balance system (Figures 3 and 5) consists of a modified analytical balance. The hot convection currents from the furnace are kept out of the balance by placiog the balance below the furnace. This arrangement is the same as f i a t used by Duval. The quartz rod, upon which the sample is placed in the furnace, is supported by a yoke and counterbalance (Figure 6). The sample carrier is suspended from beneath the left knifeedge of the balance in place of the halance pan. The quartz sample rod extends up through a hole cut in the balance case and into the furnace. The right balance pan is replaced by a short brass pan, beneath which is suspended a bar magnet (Figure 7). The weight of the pan is adjusted to counterbalance the sample carrier on the other side of the balance. The sample carrier, counterbalance, and magnet load the balance to about half capacity, permitting a maximum sample of about 100 grams. The bar magnet is suspended within a hollow "work" coil. The action of the coil upon the magnet is used to rn turn the balance to its original null
836
ANALYTICAL CHEMISTRY
Figure 3.
Balance modifications
position following a change in weight of the sample. Deflection of the balance pointer is detected by a photoelectric cell arrangement (Figure 8). A black paper flag isgluedtothebalancepointer. A 6volt lamp and a cadmium sulfide miniature
phot.ocell are arranged on' opposlte sides of the flag. The lamp is masked with aluminum foil and the front of the photocell is masked with black tape. A narrow slit is cut in each mask so that a narrow beam of light strikes the photocell slit when the pointer Bag is
OUARTZ T u a E SUPPOR_T
A
61'' QUARTZ
TUBE VER INT
FURNACE M O U N T E VERTICALLY ABOV BbLANCE
Figure 4. system
Thermo
AUTOMATIC CONTROL SYSTEM
P
QUARTZ SAMPLE SUPPORT
The automatic or control system is shown above the recorder in Figure 1 and a schematic diagram is given in Figure 9. The control system responds to a signal from the photocell (indicating a decrease in sample weight) by increasing the current in the work coil to compensate for the decrease in weight. An output signal, proportional to the amount of compensation and thus to the change in n-eight, is fed from the control unit to the recording system. The values and dimensions given are for three operating ranges: 50-, loo-, and 200-mg. weight change (corresponding to positions A , B , and C, respectively, of switch SW1 in Figure 9). A 1-r.p.m. clock motor, M , is connected by a rubber belt to the 1000ohm, 10-turn Helipot, Rlo. The belt must fit loosely to allow manual resetting of the Helipot. The 10,000ohm relay, La, is operated by a thyratron, Vs, connected to the photocell, CZl. Potentiometer Rn adjusts the sensitivity of the relay circuit. The ganged rotary switch, SWI-SW~, is used to select the range of operation. The 200-ohm Helipot, Ra, is a calibration adjustment. The photocell, CL1, source lamp, W 2 , and work coil, Lp, are the components located in the balance case and are connected to the control unit by a multiconductor cable.
TO POWER OUTLET
H O L E IN TOP O F 0 A L A N C E C A S E
G
H O L E IN T O P
OF B A L A N C E C A S E E X T E N D I N G UP INTO
Figure 5. Balance modifications S A M P L E ROD S U S P E U S I G N YOKE AND COUNTER BALANCE
-
BLACK F L h G G 3 VOLT
LAMP CADMIUM SULFIDE MINIITURE PHOTOCELL
OJA?TZ CUP B n o O K T O AOLD SAMPLE Y FURNCCi
The control system performs as follows:
4
e
I
,(PAN WEIGHT
TUBE
'
9 3/4 COIL
1/4 X 1 l/4 A-NiCO 5 MAGNET IN LUCITE
HOLDER
OD
LUCITE FORM
MAGNET EXTENDS v e i h v COIL
U S E D TO
CARRIER
FINE HIRE QUAqTZ
not interposed. A state of balance is indicated when the flag interrupts the light beam to the photocell sufficiently to open a relay in the control system.
i
n
U
BRA 5 5 C 0 - Y T
E ? - IE , G M T
Figure 6.
Sample carrier
Figure 7. Weigh pan and magnetic balancer
A los3 of sample weight causes the balance pointer to be deflected from between lamp W 2and photocell CZl. The increased light striking CZl increases the current through it. The increased current in Cll causes thyratron T73 to become conductive. Current through V 3 energizes relay La, closing contacts SWS and SWs. Clock motor M turns the 1000-ohm potentiometer, RIo,increasing the current to the work coil, L2. With increasing current, the repulsion between L2 and the bar magnet suspended within it reduces the effective weight of the magnet and the balance returns to the null position. When the balance has returned to the null position, the flag on the pointer again reduces the amount of light entering CLl, reducing the current to V,, which then becomes nonconducting. Relay L1 is de-energized, preventing any further increase in current in Lo. The current in the work coil is measured by the recording system in terms of the voltage drop across Rs and Rs, Rs, or R?. A critically damped balance and a rate of coil current increase that is slower than the balance response are VOL. 29, NO. 5, MAY 1957
837
necessary to prevent overshooting of the null position. Ovardamping of the balance slows its response, while underdamping causes overshooting of the null position.
-
I
SWg
-
(M)----------
I I
I
I
I
I I
I
I f
RECORDING SYSTEM
I I
R3 R4
IOK IOW 6 5 K low 15K l o w 30K low
Rg
IO
Rl
Rz
Re
RT Re Rg
1
R10 R11 Ri2 R13
D WW 2ODWW 40R WW 200 Q HELIPOT 4 K WW
c3 c4 LI
500mmfd
smfd 4 5 0 v lOh)175mo
Le L3 VI Vp V3
1000 T U R N WORK COIL IOK REL4Y 5Y3 OD3
SW3 OPST S W q 1 / SWITCH
2DZI
5W7
TI
POWER TR4NSFORMER
F,
T2
FILAMENT TR4NS 6 3 V a ) l 4MP
OUTPUT TO RECORDER
When the balance is fully returned to null position, following a loss of sample weight, the force acting between the bar magnet and the work coil must be equal to the weight loss of the sample. For a magnet of given strength a t a given point in a given coil the force is
F
= kJ = W
where W = weight loss F = force I = current in the coil kl = constant of proportionality The current through the range resistor (R5,Re, or R?) and the calibration adjustment, Rs, is equal to I, and the voltage drop across this resistance (the output signal) is given by Ohm's laW where
V = IR V = voltage drop R = resistance
or because R is fixed for any given range, V = k,I where kz = constant.
Combining this equation with the force equation, we obtain
k
V=-2W=KW kl where K = constant.
Therefore the deflection of the recorder
838
ANALYTICAL CHEMISTRY
S P S T SWITCH
0 - 5 0 MV
I K HELIPOT
S O O K POT IMEG 1/2W IOOK I/ZW
CI i 2 0 m f d 4 5 0 V Cp 2 0 m f d 4 5 0 V
CII
M I N I 4 T U R E PHOTOCELL (C4OMIUM SULFIDE* I RPM CLOCK MOTOR 2 CIRCUIT-3POSlTlON R O T I R Y SWITCH SWg
IFw,
Wz W3
N O 4 4 L 4 M P (SOURCE)
NO 4 4
LIMP
(RELIYI
1
Figure
Photoelectric cell
T 2 MUST BE OUT OF P H 4 S E WITH 6 3 V WINDING O F T , I F V 3 W I L L NOT STOP FIRING, REVERSE 6 3 V CONNECTIONS O F T,
I
A standard chart-type 0- to 50mv. potentiometer recorder (Brown ElectroniK, Minneapolis-Honeywell Regulator Co., Philadelphia, Pa.) is used to record the output signal from the control system. An output signal of 0 to 50 mv. is obtained for each operating range: 0 to 50, 0 to 100, and 0 to 200 mg. The recorder has a linear scale in units of milligrams.
Figure 8.
NOTE
9.
Control unit
pen is directly proportional to the weight loss of the sample. Because the recorder chart is driven a t a constant speed and the furnace temperature increases a t a constant rate, the distance the chart has traveled is proportional to the change of temperature. mith the equipment used, the temperature changed from20" to 600" C. in 14 hours and 20 minutes and the recorder chart speed was 2 inches per hour. An inch on the chart therefore corresponded to 19.8" C.
calibration is necessary unless the range is changed. It is, however, desirable to check the calibration occasionally.
CALIBRATION A N D OPERATION
THERMOBALANCE CHARACTERISTICS
The thermobalance is calibrated in the following manner: The recorder and control system are turned on, The furnace and controller are off. With the 1000-ohm Helipot, Rl?, set a t zero coil current (Dosition X in Figure 9) and with SW7 open, the desired Gerating range-50 mg., for exampleis selected on the range switch, SWI-
The thermobalance described in this report has a sensitivity of 0.3 mg. and is accurate to *0.3, *0.4, and k0.8 mg. on the 50-, loo-, and 200-mg. range, respectively. It requires 10 minutes to record a full range weight change. The thermobalance used by Duval (1) measured the displacement of the balance to obtain the change in weight of the sample, but the authors preferred to measure the force necessary to return the balance to the null position. Because the balance and the recorder are both null-type instruments, this thermobalance has a constant sensitivity and is not affected by changes in the line voltage. For stability, the only components that must remain constant are the magnet, work coil, and range resistors. Use of an Alnico 5 magnet, a rigid work coil, and Fire-wound resistors gives a very stable system. The force between the coil and magnet must be repulsion rather than attraction in order to obtain stable operation. When the magnet is repelled by the coil, an equilibrium position exists. Any displacement from the equilibrium position increases or decreases the force
SWZ.. With the pointer flag deflected from between the light source, W,, and CZr, the sensitivity is adjusted with Rl1 t o where lamp W s is on (indicating that relay contacts SW5-SW6 are closed). The balance is adjusted by means of weights and rider (or chain) until the pointer flag sufficiently reduces the light beam to the photocell to just turn off Ws (indicating relay contacts SWt-SWe are open). A weight corresponding to the desired range (50 mg.) is added to the weight pan and SW7 is closed. When the balance is fully returned to the null position and Wtig is again off, the calibration adjustment, R8, is adjusted to give a reading on the recorder of the added weight (50 mg. in this case). The balance is now calibrated for the desired operating range and no further
When a sample is placed on the sample rod in the furnace, the balance must be adjusted by means of weights until W3is off (with SW, open and Rlo set a t zero current). SW, is then closed and the furnace is turned on. The operation then proceeds automatically until the entire unit is shut off by the limit switch on the temperature controller.
in a manner which returns the magnet to the equilibrium position. The control system alters the current in the coil, to bring the equilibrium position into coincidence with the original null position. If an attractive force exists between the coil and magnet, no equilibrium position exists, as any motion of the magnet brings it into a field that acts to increase the displacement. The repulsive force must be small compared to the weight of the magnet in order to keep the suspended magnet centered in the coil. If the force is
large, the magnet will “cock” with the coil. Fastening the magnet rigidly to the balance beam would solve this alignment problem and permit much larger forces and, consequently, larger changes of weight. This thermobalance was designed to follow weight loss only. Weight increases, as well as decreases, could be followed vc.ith the balance by the use of a reversible motor drive on the potentiometer. The motor-driven potentiometer would merely be reversed by the action of the light beam on the photo-
cell. The current in the work coil would then be constantly changing and a small oscillation to either side of the rest point would be recorded. LITERATURE CITED (1) Duval,. C., “Inorganic Thermogravi-
metric Analysis,” Elsevier, New York, 1952. (2) Gordon, S.,,Campbell,C., unpublished manuscript.
RECEIVEDfor review May 15, 1956. Accepted December 26, 1956.
Auto matic Recording Balance PAUL D. GARN Bell Telephone laboratories, lnc., Murray Hill, N.
,This automatic recording balance, designed and constructed for use in thermogravimetric and related studies, is an electronically controlled nullpoint instrument using a linear variable differential transformer as the sensing element. No closely regulated voltage sources or batteries are required for the automatic balance or weight recording. The recording circuit is a servo-system requiring no recalibration. Data obtained with this instrument, when correlated with differential thermal analysis and high temperature x-ray data, are useful in deducing the nature of solid state reactions.
S
investigators have constructed automatic balances to follow weight changes in thermogravimetric and other studies and have used a considerable variety of methods. Most of the automatic balances are equipped to record weight changes photographically or by pen and ink. A treatise on thermogravimetric analysis has been prepared by D u ~ ( 5l ) . I n general, the balances may be classed in two principal groups: those which measure the deflection of the balance caused by a change in sample weight ( 1 , 3,4)and those which operate at or near the null point by exerting a force (2, 13) or a t the null point by adding or removing weight (6, 8-10). The sensing elements in either case are usually optical or electronic. One type of sensing element, used in a t least two deflection-measuring balances (3, 12) is the linear variable differential transformer. This device consists of a hollow cylinder on which are wound three coils, a primary and two secondary EVERAL
J.
windings. When the magnetic permeability within the cylinder is symmetric about a mid-point, an alternating current in the primary induces alternating currents in the secondaries, which are equal in magnitude but out of phase by 180’. When a permeable core is introduced into the cylinder so that it is symmetric with respect to the midpoint of the cylinder, no voltage difference is detectable a t the secondary terminals. If, however, the core is moved along the axis of the cylinder, the induced alternating current in one of the secondaries exceeds that in the other. This unbalance signal may be measured or used to effect a return to a control point. As the secondary voltages are 180’ out of phase, the unbalance signal may be used to indicate direction as well as degree of displacement. In the work cited ( 3 , l a ) the output from the transformer was taken as a measure of the weight. DESIGN OF AUTOMATIC BALANCE
The automatic recording balance described is designed to operate a t null balance-Le., any deflection of the balance caused by a gain or loss in weight is detected and the balance chain is moved automatically, so that more or less of its weight is supported by the balance beam. The position of the chain is continuously recorded on a stripchart recording potentiometer. The balance assembly includes an analytical balance, a linear variable differential transformer, an amplifier and motor, and a gear assembly fitted with a slip clutch (Figure 1). The linear variable differential transformer is supported from the center post of the balance, so that the core, suspended from the right-hand pan hook, is a t the midpoint of the transformer when the balance is a t the null point. The con-
verter (chopper) of the amplifier was removed. The alternating current ordinarily used to drive the converter supplies the primary of the transformer. The signal from the secondary windings of the transformer is fed into the amplifier a t the converter socket. The 27r.p.m. motor associated with the amplifier is connected, through a 200 to 1 reduction worm gear and a slip clutch, to a drum from which the balance chain is controlled. A magnetic damper prevents oscillation of the balance. The voltage divider on the primary of the transformer is used to decrease the voltage input to a point a t which solenoid action is not troublesome. The voltage divider on secondary No. 2 and the potentiometer on secondary No. 1 are added so that the outputs of the two secondaries can be matched (zero signal) when the balance is a t the null point. This eliminates tedious mechanical adjustment of the core position. Recording Circuit. T o obtain a means of continuously recording the position of the balance chain, a precision potentiometer is driven by the same shaft that turns the balance chain drum. The drive mechanism is designed so that the potentiometer turns through 340 to 350” as the chain position is moved from 0 to 100 mg. The ordinary slide-wire of the recording potentiometer is disconnected. An auxiliary slide-wire, mounted on the same shaft, is supplied with the same potential that feeds the potentiometer on the balance. The difference signal taken from the moving contracts is fed into the recorder amplifier (Figure 2) SO that the amplifier and motor drive the slide-wire until a zero difference signal is obtained. As the recorder pen is driven by the same shaft, a continuous pen-and-ink indication and record of the balance chain position are obtained. After the two potentiometers are properly matched in position and voltage VOL. 29, NO. 5, MAY 1957
839
