A S A L Y T I CAL EDI TI O S
248
1-01. 2. Yo. 3
T h e Measurement of Turbidity' C. D. Ingersoll and R. E. Davis THESPRECKELS SCGAR PRODUCTS LABORATORY, YOSKERS, K. Y.
KE of the fundanieatal problems of filtration is t o
0
obtain filtrates which are brilliantly clear and also to express numerically the degree of clarity attained. Heretofore filtrates have been classified largely by such qualitative t e r m as "slightly turbid," "clear," "brilliantly clear," etc., because no satisfactory method for the exact measurement of turbidity has been devised. The problem is one of long standing in the sugar-refining industry, and various attempts have been made to devise meanr of measuring either the clarity of the filtered liquor> obtained or the amount of suspended matter in the liquors. This suspended matter is often so fine that no sign of turbidity is noticeable to the nsked eye, arid yet every sugar refiner i> acquainted with it. detrimental effects in boiling, on
Tk--
SCALE ON SIDE
ards, each series having a different representative color h a & so that the house pressmen and xiperintendents can follonthe work of their presses. Kopke ( 1 ) devised a rough turbidimeter for use with r a x sugm liquors. A glass plate on which a standard alyhabrt i; etched is immersed into a sugar liquor: the depth a t ivliicli the alphabet becomes unreadable is taken as a measure of tlie turbidity. Here, as in the Horne arid Rice instrument, the color of tlie liquor interferes with the measurerneiit. The most serious attempt at a proper turbidimeter has liccii made by Paine, Balch, and co-workers at the Carhohytlrate Division of the Bureau of Chemistry and Soils. They 1iai.e been working on a n instrument which has as its ba& the transmission of light through turbid solutions. The intensity of the transmitted light is measured by a spectrophotometer and is compared with the intensity of light transmitted through the same solution after the removal, by proper means, of the material causing the turbidity. Theory
I
K h e n a beam of light is passed through a turbid wliition, the well-known Tyridall cone is obtained. This cone is (liw to the reflection of light' by the small particles suspentled in the solution, and the intensity of the reflected light then coilstitutes a measure of the total reflecting surface. Thin total reflecting surface is the bum total of the individual particle surfaces. Xorking on the assumption that the hrightne~sof the coiie i h proportional to the number of particles premit i i i the liquor, and hence to the turbidity of the liquor, and that the eone could be produced at a zero depth from the surface of the liquor in order to eliminate color interference, tlie apparatus herein described was constructed. Apparatus
Figure 1-Turbidimeter A-Light source B--Spectrometer cell C-Nessler tube (modified)
D-Light filter C-Leieling bulb F--Scale rod
the quality of product obtained, and on the scale it produces in evaporators, pans, etc. The instrument to be debcribed is an attempted solution to this problem of turbidity measurement and constitutes a means of numerically expressing with considerable accuracy the relative amount of suspended colloids present in liquors. Previous Work Practically all of the turbidimeters devised heretofore fall short in desirability in that they measure the color of the liquor as well as its turbidity. In 1924 Horne and Rice ( 2 ) suggested a form of turbidiscope which had as its basis the Tyndall effect and was designed particularly for use by sugarhouse operators for rough control of their liquors. I t has been found very suitable for this purpose J+hen used in conjunction TTith a series of accurately adjusted standards. The Spreckels Sugar Refinery a t Yonkers, N.Y., is at present using this apparatus in conjunction with six series of turbidity standPresented before the Division of Sugar 1 Received February 17, 1930. Chemistry a t t h e 79th Meeting of t h e ilmerican Chemical Society, Atlanta, Ga , April 7 to 11, 1930.
The apparatus (Figure 1) consists of a light source, A , spectrometer cell, B , containing the liquor under examination, modified Sessler tube, C, nionochromatic light filter, D, aiid leveling bulb, E , mounted on a scale rod, F . A 6-volt, 5-ampere microscope lamp is uied as a light source, the beam passing frGm it through (1) a 1-nim. aperture, (2) a light screen of 6250 A. doniinant wave-length transmittal, arid (3) a second aperture of 1 mm. diameter, t o t'he spectrometer cell (4 X 4 X 1 cm. deep) 25 em. away. The light icreeii i* a S o . 29F Wratten filter. Directly over the spectrometer cell is a niodified Kessler tube containing cone-free potawiurii dichromate solution, saturated a t 20" C. By a short side nipple sealed on a i the base of the tube and a leveling h i 1 h j the depth of the dichromate solution is adjusted in making the turbidity determination. The depth of the dichromate solution is measured by noting the height of the solution ill the leveling bulb above the hottoiii of the Sessler tube. The ~vholeapparatus is supported on a black base resting on lei-elirig screws so that it' can be leveled at any desired height. The lamp is enclosed in a black box. The apparatus is preferably used in a constant-temperature dark room, where errors due to temperature variation. arid stray light rays are eliminat'ed. The dichromate solution is prepared by saturating coiiefree water with potassium dichromate at 20" C. This concentration was chosen because the apparatus is used in a constant-temperature room which is kept a t 20" C.
IAYDUSTRIALA X 0 ESGIiYEERING CHEMISTRY
July 15, 1930 Method
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a pure sucrose solution which was almost cone-free. The
A beam of monochromatic light is passed through the solution under exaiiiiiiatioii and, theoretically, at zero depth from tlie surface of the liquid. The operator then observes the beam of light through a vertical tube having a n optically flat bottom, aiid adjusts tlie depth of the non-t,urbid dichroinate solution until the Tyndall cone is eliminated from i-iew. The depth of dichromate solution necessary to effect this blanking-off of the Tyndall cone is considered a measure of the tnrhidity of the solution under examination.
dilution of sugar liquors with cone-free water does not, change the shape of the curves. In measuring t,he turbidity of house liquors it is preferable to use cone-free water as a diluent because blank determinations are necessary if sugar solution:: are used. The turbidity of house liquids is expressed in centimeters of dichromate a t a definite dilution. If the liquors are not too turbid, they need not be diluted. With tlie present apparatus turbidities greater than 36 cni. of dicliroinate cannot be measured. Discussion
C E M VE ~
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c o , . c m r ~ m m N ~ e a rr / owcou6 t ~ E E S U G MS O I U ~ I O N c
Figure 2-Turbidity
of S u g a r h o u s e L i q u o r s of Various Concentrations
The object of paqsing the beam of light horizontally through the d n t i o n at a theoretical zero depth is to elininlate color of the liquor influencing the resultq. Zero depth IC ohtainetl by placing a definite 1-olmne of the wlution to lie examined in tlie qpectronieter cell and raising or lonering the cell until the Iieain is just below the position of qiirface refraction of the beam. Thiq point is very clearly defined. K i t h the apparatu. used in this laboratory 10 cc. of solution nere placed in the cell. The accuracy of the instrument i i limited b y the ability of the operator to see that point nhere the cone beconieq invisible. Practice has been productive of a considerable degree of accuracy in the case of the seT era1 obsen ers ~ h have o used the instrument. T a b l e I-Turbidity SCGAR PER LIQVOR 100 cc. S O L N .
cc 1
DEPTHO F DICHROMATE SOLUTIOS Raw liquor
Vallez press liquor
Cm. 5 4 7 8
Cm.
3 5 10
10 4
15
15.6
20 25 30 35 40 45
of S u g a r h o u s e Liquors
13 0 18.1 20.6 22.8 25.4 28.1 30 4
6.3 8.2 10.2 12.1 14.0 1.56 17.8 19.8 21.9 24.2 26.0
Shriver press liquor
Cm 4.4 6.3 8.2 10.1 12.1 14.0 16.1 18 0 20.0 21.9 23 8
Results
The apparatus has been used to study the turbidity of a number of sugarhouse liquors. The results obtained from three types of such liquors are giren in Table I and shown graphically in Figure 2. The house liquors were diluted with
If a n assumption that the intensity of reflected light ih proportional to the number of particles present in the solution is correct, one n-ould expect the dilution of the turbid liquor rvith a non-turbid diluent to diniiniih the inteiisity of reflected light in direct proportion to the dilution of the liquor. 111 other words, when turbidity is plotted against the concentration a straight line should result. Since the curves are not straight throughout, but show a definite break a t the lower right-hand ends, it is evident that other fxctors are acting. Since it is difficult to visualize and evaluate these factors which influence the intensity of the reflected light, it is not, possible to state definitely why the intensity is not proportioid to the concentration of the turbid liquor. A possible explanation has been suggested. -1cone-free solution should, of course, give a zero reading. Klien a feir- particles are present, tlie distance between them is sufficiently great so that there is little or no interference betlyeen them, .rr-liich results in a Tyntlall cone rclatively high in intensity in proportion to the nunilier of particles present. For low turbidities, therefore, a relatively deep layer of dichromate is required for extinction. As the nuiii1)er of particles iiicreases, the distance between them cliniinislies and there occur greater interference and secondary reflections, etc., so that the amount of light to be obseri-et1 in the Ses>ler tube become. less in proportion to the number of particles present than in the first instance. The concentration a t which interference becomes noticeable is undouhtedly the breaking point of the curve. This point should T-ary somewhat for different liquors, depending on the size of part'icles present in the product under ObPerJ-ation. When this rea,soning is applied to the more concentrated solution?, one should expect a further dropping off in intensity of light, resulting in a curve rather than a straight line, unless the combined masking aiid multiple-reflection effect happens to be a constant factor under the conditions. AUthoughthe present method was designed particularly in connection with refinery house liquors and press filtrates, it may be adapted to any filtrate where it is desired to measure small amounts of turbidity which are not visible to the naked eye. Thus i t takes in the whole field of colloids not retained on filt'ers and not visible t o the naked eye. An interesting fact which came up during the work vias the extreme difficult,y of obtaining distilled water of zero turbidity-i. e., cone-free water. Such a diluent was necessary for the preparation of liquors of standardized turbidity for use both in testing out the present apparatus and in Horne and Rice turbidimeter adaptation used in the plant. The present' authors have no doubt that when this method is applied to what have heretofore been considered brilliantly clear liquors varying and measurable turbidities will be found. Literature Cited (1) Facts About Sugar, 23, 177 (1928). ( 2 ) Horne and Rice, IND. ENG.CHEM.,16, 626
(1924).
