A Tyndallometer for Colored Solutions HUGO P. KORTSCHAK E x p e r i m e n t S t a t i o n , H a w a i i a n S u g a r Planters’ A s s o c i a t i o n , H o n o l u l u , T . H .
M
~
X E S the turbidity of solutions is measured, it has alnays Table I.
b e m a draivback that, solutions of different color could not
1w compared direct.ly. T o make such comparisons, it has been
ry to make separate color determinations and apply an chnipirical correction. Faced with the problem of comparing the turbidities of sugar cane juices, i t was found that the C. &- H. colorimeter ( 1 ) could etisily he converted into a tyndallonieter whose readings are r i c ~ l yindependent of the color of the solut,ion. The colorimeter is immediately available for its original purpose, the only rhanges being removal of the harrier between the ttvo solution ( ~ ~ l lsubstitution s, of a rigidly mounted mirror for the front cell, anti covering the hole through xhich light is admitted to the frollt photoccll.
RdR
Relation between Turbidity and Suspended >ratter
Colora
Turbidity
2.0 15.7 28.6 38.7 47.9 54.7 60.8
5 34 56 70 99 125 137 152 185 200 217
c
0 10 20 30 40 50 60 70 80 90 100 a
66.0
70 5 74.3 77.5
$6 light absorbed through 35 mm
Suapended Matter. from Turbidity G 1100 ml 0 0 0 0 0 0 0 0 0 0
0027 0048 0061 0089 0113 0124 0139 0170 0184 0200
Suspended hlntter G 1100 mi.
’
0’0620 0 0040 0 0060 0 0080 0 0100 0 0120 0 0140 0 0150 0 0180 0 0200
of solution.
F beam. h still stronger light source viould have been a definite advantage. The colorimeter is calibrated to read in terms of light absorption: Scalc reading = Lp-l - LP-z LP- 1 a-here Lp-1 and Lp-2 represent the amount of light actuating the photocells. Thus a scale reading of 0 is obtained when the same amount of light reaches both photocells, and very turbid solutions, producing a Tyndall beam stronger than the filtered transmitted beam, gave readings which were off the scale. This was corrected by using a less dense filter, one which allowed 1.2% of the transmitted light to pass being sufficient for the most turbid solutions encountered.
I 1
Figure 1.
M.
Optical S j h l e r n
As complete absence of a Tyndall beam would give a reading of 1000, the turbidity figure should theoretically be obtained by subtracting the scale reading from 1000. However, because the instrument does not exclude all stray light, i t was necessary to use the value found for distilled viater as a n arbitrary reference point. For this reason, small negative values were sometimes obtained when working with carefully filtered solutions, showing that for very accurate work a better standard must be used. Table I illustrates the results that can be obtained. -43 3 5 solution of raa cane sugar, containing 0.020 gram of suspended matter in 100 ml., mas diluted with a 3 3 5 solution of purc (ConA$.$) sugar. Assuming a linear relationship between turbidit)- and the concentration of the suspended matter, which could be expected onlv with complete compensation for color, the concentration of the suspended matter was calculated a t each dilution by the equation
Thc optical system, shoivn in Figure 1, is self-explanatory. I i g h t passing through the solution, S, actuates one photocell, 1’-1. h portion of the Tyndall beam is reflected by the mirror, Ill, into the second photocell, P-2. The scale reading of the instrument indicates the intensity of the light reaching P-1, as eompared with t h a t reaching P-2. This reading, therefore, is now a niwsure of the rat,io of t,he intensities of the transmitted and wattcrcd light.
If we may assume that the intensity of the Tyndall hcam (for a givcn Concentration of suspended material) varies with the color of the solution in the same manner as does the intensity of the transmitted beam, this ratio will he independent of the color. This assumption is not strictly valid, of course. It can be sho\r.n that, it becomes valid, for monochromatic light, when the rc.11 holding the solution becomes infinitely narrow. This eondition may be approximated by using a narroT3- beam of light, passing as closely as possible to the side of the solut,ion from which the Tyndall beam is taken, so that the intensity of the scattered light is only slightly lessened by passing through the solution. K h e n the C. 8: H. colorimeter was converted, the greatest difficulty was found t o be the extreme weakness of the Tyndall beam. I n order to obtain usable readings, it is necessary that, srattered light reaching the front photocell be between 1 and 100% of the transmitted light which actuates the rear photocell.
Suspended matter
=
turbidity - turbidityo turbidity,oo - turbidityo
x
0.020
LITERATURE CITED
(1) Holven, A. L., and Gillett, I. li., Facts About Sugar, 30, 169 (1935).
Correction
This was attained by introducing into the path of the transmitted light a neutral filter, F , which allowed only 0.3% of the total light to pass. The very low absolut,e light intensit,ies reaching the photocells caused poor scnsitivity, and t,he intensity of the light was therefore increased by the use of a lens which concent rated all the light from a Pulfrich photometer head into a narrow
I n the article on “Colorimetric Determination of Cobalt Using Nitroso-R Salt” [ANAL.CHEM.,19, 505 (1947)] the last line immediately preceding the center head “Discussion” should read: properly diluted nitroso-R salt solution was used as the blank. H. H. WILLARD
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