Electronic Method for Bubble Frequency Measurement A. 0.NEWMAN and
B. J. LERNER
The University o f Texas, Austin, Tex.
T
H E measurement of bubble size is of basic importance in the determination of interfacial area in absorption and reaction kinetics studies involving t n o-phase gas-liquid contact in bubbling flow. LIost often, the only n-a) bubble size data can be obtained is indiiectly from measurement of the gas flow rate and the bubble frequency. Determinations of bubble frequencies have been accomplished by simple counting a t low frequencies ( 5 ) ,by mechanical counting ( I ) , by stroboscope (3, 4),by sound recording ( 7 ) , and by high speed photograph ( 2 , 6) a t high bubble frequencies. Davidson (3)has pointed out that optical methods such as that in which the stroboscope is used are subject to inaccuracy a t high bubble rates because of bubble coalescence above the orifice. I n addition, the necessary transparency of liquid and apparatus is sonietimes not obtainable, so that the need exists for a reliable nonoptical method. Accordingly, a precise mechanico-electronic method has been developed which is applicable t o these situations over a wide range of bubble frequencies. With the apparatus and technique described below, bubble frequencies of 3.5 to 50 bubbles per second were measured with a precision of 1% of the indicated bubble frequency. Khen a bubble is formed and released from a submerged orifice, the liquid impact on the gas column below the orifice causes a percussive shock which is transmitted t o the orifice material and the surrounding liquid. The vibrations of this shock wave are in the audible range of sound frequencies, and when bubbles are being formed at a constant rate, the bubble generation frequency is made faintly audible by the sound attending release. The method employed here for the determination of bubble generation frequency includes microphone pickup of the bubble release “noise” impulse, amplification, and comparison on an oscilloscope screen of the period of bubble percussion with a sine wave of known frequency. I n order to folloF the stability and nature of the bubble generation, it was convenient to pass the amplified bubble signal through a loudspeaker. This allows immediate detection of any irregularity in bubble formation and permits adjustment of the bubble rate without visual checking of the orifice. The application given above yields the “uncoalesced” bubble frequency a t the orifice. By insertion of the microphone into the gas space above the liquid, it is possible to follow the bubhle
bursts at the liquid surface. Comparison with the orifice frequency then yields a quantitative measurement of the extent of coalescence. EQUIPMEZIT
The apparatus layout is presented schematically in Figure 1. The microphone used for picking up the bubble percussions was a DeArmond guitar microphone, which n-as found to be ideally suited for clamping on the flat surface below the orifice plate. The signal is amplified by a Masco audio amplifier, Model ME-8, and then fed to a U. S. Army Signal Corps loudspeaker. No. LS-3, from the 500-ohm output of the amplifier. The amplified bubble signal is also applie‘d to one pair of the fixed contacts of a double-pole, double-throw (D.P.D.T.) switch. The other pair of the fixed contacts of the snitch is connected t o the output terminals of a Hewlitt-Packard audio-frequency generator, Model 200-D. The moving sn-itch contacts are connected to the vertical input and ground terminals of a DuMont Type 208-B oscilloecope. This arrangement permits observation of either the bubble signal or the sine wave a s a function of the internal linear time-base of the oscilloscope. Some difficulty was experienced v, ith 60-cycle interference from power lines. Powerstats, and motors This n as adequately eliminated by the arrangement of condensers and resistor between the vertical input of the oscilloscope and the double-pole, double-throw switch as shown in Figure 1. The values of the condensers and resistor are apparently not very critical, and the sizes used R ere determined by a trial and error procedure in order to produce the best oscilloscope trace. The 0.25-microfarad condenser also prevented the accumulation of appreciable direct current voltage on the vertical input tap of the oscilloscope. OPERATIOR
Cathode-ray O s c i l l o
Figure 1.
Schematic Diagram of Equipment
After sufficient warming-up and stabilization time, the gain of the amplifier is increased until a comfortable sound level is heard from the loudspeaker. The double-pole, double-throw switch is then thrown to apply the bubble signal to the vertical input of the oscilloscope, and the vertical and horizontal gains of the oscilloscope are adjusted to give a suitable trace on the oscilloscope screen. For the formation of a single bubble per gas emission from the orifice, the percussion trace consists of a short region of high amplitude, followed by a comparatively long region of decreasing amplitude. The oscilloscope sweep frequency is adjusted until one peak is seen to drift slowly from right to left across the screen. When this adjustment has been made as closely as possible, the synchronization signal amplitude is increased slightly until the pattern appears stationary. The doublepole double-throw switch is then thrown to connect the audiofrequency generator to the oscilloscope vertical inuut. Without a n v change in the setting of thk oscilloscope con’trols,, tYhe frequency of -the sine n-ave is varied until one stationary cycle appears on the screen. The frequency of the sine wave as given by the generator reading is then equal to the frequency of bubble formation. The accuracy of the bubble frequency measurement is directly dependent on the accuracy of the sine-wave generator dial. In this case the frequency of the generator output differed by more than 10% from the dial reading, so that recalibration of the dial was necessary. F i t h the aid of the oscilloscope, the generator was calibrated against the laboratory power line frequency of 60.0 cycles per second. The line frequency was supplied to the horizontal input of the oscilloscope through a 2-megohm series resistor, while the generator output was connected to the vertical input. After the generator and oscilloscope had warmed up for 45 minutes, the generator dial &vas varied through settings of 60, 30, 20, 15, 12, and 10 cycles per second. At these frequencies, the calibration was made by means of Lissajou’ figures, acs c o p e 2 cording to the procedure given by Rider and Uslan (8).
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ANALYTICAL CHEMISTRY
The lower limit of bubble frequency that: can be measured by this method is set by the least sweep frequency of the oscilloscope used, and by the image-retention time of the oscilloscope screen The DuMont 208-B oscilloscope has available sweep frequencies down to 2 cycles per second, but its screen does not have s u k cient persistence for frequency measurement below 4 cycles per second. A special cathode-ray tube can be obtained with longer persistence, but it is simpler to determine bubble frequencies below 4 cycles per second by actual bubble count. The upper limit of frequency measurement depends on the nature of the percussive trace, which is in turn a function of the components of the bubble noise. I n general, a high-pitched bubble sound permits the measurrment of higher bubble frequencies, because at the higher sweep frequencies required, the vertical displacement of the electron beam remains well-defined. Obviously, any sweep frequency above the lower audible limit of 20 cycles per second would obliterate the lower-toned components of the bubble shock. The pitch of the bubble sound is dependent on the size and shape of the reaction vessel, the properties of the liquid, and the size and thickness of the orifice device. Preliniinary consideration of the effect of these factors on hubblr sounds should yield a design suitable for the application of thii technique. The electronic technique has been applied in a study of bubble "twinning"-that is, the formation of a bubble pair per gas emission from the orifice. This phenomenon was earlier notcd bv Budge ( I ) , who attributed i t t o too large avolumeof gas between the orifice and the flow control valve. This particular bubble habit is easily followed on the oscilloscope. since the stntion:irv
pattern for pair formation consists of two regions of peak amplitude spaced closely together, followed by a comparatively long region of decreasing amplitude. This pattern is distinguishable from that of single-bubble formation at a bubble frequency twice the sweep frequency, because the latter gives two widely separated peak-amplitude regions. Studies of the twinning process with the oscilloscope indicates that a t low flows, bubbles of approximately equal size arc formed, while at higher gas rates, tlisproportionation takes placc. ACKNOWLEDGMENT
The interest and encouragement of the Monsanto Chemical Go., Texas City Division, are gratefully acknowledged. LITEH4TIJRE CITED
(1) Budge, E. A., J . Ant. Che7n. SOC.,53, 2451-3 (1931).
(2) Datta, R. L., Kapier, D. H., and Newitt, D. M., Trans. Inat. Chem. Engrs. ( L o n d o n ) , 28, 14-26 (1950). (3) Davidson, L., Ph.D. thesis i n chemical engineering, Columbia Universitv. New York. 1951. (4) Eversole, W.-G., Wagner, G. H., and Stackhouse, E., I n d Eng. Chem., 33, 1459-62 (1941). (5) Maier, C. G., U. S. Bur. Nines, Bull. 260 (1927). (6) Pattle, R. E., Trans. Inst. Chern. Engrs. (London),28, 32-i (1950). (7) Remy, H., and Seemann, W., Kolloid-Z., 72, 3-12, 279-91 (1935). (8) Rider, J. F., and Uslan, 8. D., "Encyclopedia on Cathode-Ray Oscilloscopes and Their Uses," pp. 427-75, New York. John F. Rider Publisher, Inc., 1950. RECEIIED for review July 31, 1053. Accepted October 30, 1953
Boron Determination in Soils and Plants Simplified Curcumin Procedure W. T. DIBLE', EMlL TRUOG, and K. C. BERGER University o f Wisconsin, Madison,
Wis.
F
OR the determination of boron in soils and plants, t v o colorimetric procedures are commonly used. One is based on n change in color from pink to blue when quinalizarin reacts with boric acid in concentrated sulfuric acid. This procedure, conimonly called the quinalizarin method, is widely used and has been described in several places (1, 2 ) . It gives reliable results, b u t has a disadvantage in t h a t the color must be developed in very strong sulfuric acid, the strength of which must be carefully controlled. Recently hIacDougall and Biggs (9) have shown t h a t by increasing the concentration of the quinalizarin, ordinary reagent grade sulfuric acid may be used, with a permissible variation of 1 % in strength. This partially overcomes the disadvantage mentioned. The other procedure is based on the rose-colored rosocyanine produced when a n acid borate solution containing curcumin is evaporated to dryness. I t is commonly called the curcumin method, and was adapted to plant and soil analysis by Saftel (1f). Curcumin, 1,T-bis (.i-hydrosy-3-niethoxyphenyl)-1,6-heptadiene-3,5-dione, is a natural dyestuff obtained either from the rhizomes of Czcrc~c7mtznctora or by synthesis. Upon evitporation to dryness of a n acidified solution containing boric acid and curcumin, a red reaction product soluble in alcohol is formed in amount proportional to the amount of boric acid present. This product has been identified by Schlumberger (13) as an isomeric form of curcumin and was named rosocyanine for the rose color of the acid form and the blue color of its metallic salt. The 1 Present
address, Potash Division, International hfineritls and Chemical Corp., Chicago, Ill.
mechanism of the reaction has not been clearly defined, hut Hafford ( 7 ) suggests that the rosocyanine is probably formed by a loose combination of the boratc with one of the hydrouyl groupfi of the curcumin molecule. Cassel and Gerrans (3') first outlined a colorimetric method in which a solution of oxalic acid, curcumin, and boric arid is evaporated to a dry residue, which is then taken up with ethyl alcohol. The steps involved w r c numerous and tedious, : i d Saftcl (11) and later Hafford ( 7 ) refined the method by improving the color development technique and adopting propos:tls by Gooch (6), whereby solutions containing boron are conc(mtrated by evaporation to dryness in the presence of excesh c-:rlcium hydroxide to prevent volatilization of boric acid. The curcumin method has a n advantage over the quinalimrin method in t h a t thp color reaction in the former is more sensitive to small amounts of boron and proceeds without the use of a strongly corrosive reagent like strong sulfuric acid; its disadvantage as commonly carried out lies in a multiplicity of timeconsuming evaporations and filtrations. Accordingly, the possibility of simplifying the curcumin method for plant and soil analysis was investigated. and a simplified procedure was evolved which requires 0111)- one evaporation and filtration after a water solution of the test sample is a t hand. Details of this procedure, comparative results obtained with i t and the quinalizarin procedure, and related matters are here presented. REAGEVTS
Standard Boron Stock Solutions. Dissolve 2.8578 grams of boric acid (reagent grade) in 1000 ml. of distilled water. This
