The Turbidimeter in Paint Manufacture E. J. DUNN,JR., National Lead Company, Brooklyn, N. Y. HE principles of turbidimetry or nephelometry
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rapid means of measuring the average relative fineness of samples of a single pigment-that is, one sample of white lead against another sample of white lead, or one sample of titanium o x i d e against another sample of titanium oxide. One pigment cannot be compared directly against another for relative fineness by this test. The turbidimeter can be calibrated into units of fineness by microscopical sizings of a number of samples sufficient to cover the fineness r a n g e . Then the obscuring power can be read in terms of fineness. if the calibration has been made properly. Suspensions for the determination of relative fineness of fine pigments such as white lead are usually made up of water or light-bodied linseed oil. First 0.125 gram of white lead, 0.05 gram of gum arabic, and 0.05 gram of saponin are accurately weighed out, then transferred to an agate mortar, and two drops of distilled water added. The mixture is rubbed up until a semi-dry tacky paste is formed, when one more drop of water is added. Incorporation should be kept up for about 10 minutes, adding more water if necessary to keep t h e paste tacky, The paste is diluted to 200 cc. in a v o l u m e t r i c flask, shaken thoroughly to a uniform s u s p e n s i o n , and poured into the long arm of the turbidimeter tube to eliminate air bubbles in the calibrated arm. P r e s s u r e i s applied to the plunger and t h e height of suspension necessary to oblitera t e the filament noted. The s a m e p r o c e d u r e is followed when using oil, but no a d d i t i o n a l FIGURE1. THE TURBIDIMETER agents are required. Pigments which have individual particles large enough to be broken down by the incorporation in the mortar are made up as follows: Of the pigment, 0.2500 to 0.3000 gram is weighed out accurately and transfered to a 600-cc. beaker, one drop (25 mg.) of linseed oil added, and the mixture briskly brushed over the entire bottom of the beaker for a t least 10 minutes. If the mixture becomes too dry, another drop of oil should be added. It is important to use a stiff-bristled brush for this test, A small brush, approximating 0.25 inch (0.63 cm.) in diameter, with the bristles cut to about 0.37 inch (0.90 cm.) to increase the stiffness, is a very good size. This
The turbidimeter is being used to estimate the relatiue fineness of pigments and powders, to determine the relative degree of incorporation or dispersion of a pigment in a vehicle, to determine the relative obscuring power of one white pigment to another, and to judge the merits of grinding mills. A relation has been established between obscuring power and hiding power and between obscuring power and tinting strenglh, whereby the turbidimeter test is used to give both the hiding power in square feet per pound and tinting strength on the basis of white lead as 100. The iurbidimeter is also used to determine ;f the pigment or powder consists mostly of colloidal part d e s .
have been used a great many years, and slight modifications in their application have been made to adapt turbidimeters to specific uses. Applying the use of the turbidimeter to the paint and pigment industry has provedveryhelpfulin judging the merits of pigments and paints. The t u r b i d i m e t e r used in these l a b o r a t o r i e s and shown in Figure 1 is a modification of the one described by Vogt ( 3 ) . The fundamental principle of the apparatus is the obscuring of the lamp filament by g r a d G a l l y i n c r e a s i n g the height of the column of supension which is mounted directly over the filament, The turbidimeter consists essentially of a right-arm Kennicott-Sargent colorimeter tube mounted over a single-strand straight-filament galvanometer-type bulb, which is a 3.5- to 4.0-volt bulb in series with a 60-watt 120-volt bulb on a 120-volt circuit. A slit diaphragm is mounted over the bulb to allow only direct light from the filament to pass up through the suspension, whereby most extraneous diffuse light is eliminated. The plunger held by a friction clamp is used to force the suspension from the long arm to the calibrated arm of the tube and hold it constant while recording readings. The small rectangular box shown in Figure 1is placed over the calibrated arm to exclude extraneous light when obtaining the end point. Obscuring power is usually expressed in square centimeters per gram and is calculated as follows: Let W = weight of pigment in grams V = total volume of suspension in cc. H = height of suspension in cc. V Obscuring power = - = square centimeters per gram WH
This figure multiplied by the specific gravity of the pigment gives the obscuring power on the volume basis-that is, square centimeters per cc. The following precautions should be adhered to when making observations: Vehicles being used for the suspensions should always be filtered. The end point should always be obtained by raising the level of the suspension until the filament just disappears, and not by lowering the level of the suspension until the filament reappears. The point of disappearance is sharp and does not require prolonged observation. Three or four readings should be taken from each suspension,the average being used as the end point. Large variations in room temperature should be avoided or corrected for when comparative results are desired. It is well t o regulate the concentration so as t o have the end point between 2.5 and 4.0 cm. However, concentrations may be varied 100 per cent with practically no change in derived obscuring power.
It is a well-established fact that the properties such as tinting strength and hiding power of pigments vary materially with changes in particle size. The measurement of the turbidity of a suspension has proved itself to be an excellent,
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ANALYTICAL EDITION
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treatment breaks down agglomerates without affecting the individual particles. Then 20 cc. of linseed oil are added, the mixture stirred until the suspension is uniform, diluted to 200 cc., stirred to uniformity, and the obscuring power determined. These tests give the ultimate obscuring power of the pigment. I
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poured into a 600-cc. beaker, and all the paint transferred from the 100-cc. beaker to the 600-cc. by repeated washings and brushings with more oil. It is then diluted to 200 cc. with linseed oil, stirred until the suspension is uniform, and the obscuring power determined. Comparing this result with the ultimate obscuring power of the pigment indicates the degree of dispersion in the paste or paint. For practical purposes, this treatment preserves the relative dispersion in the sample, because agglomerates are not readily broken down by mixing with a pliable-bristled brush in a large proportion of oil. It has been the writer’s experience that both good wetting by the vehicle and good mechanical action are necessary to break down agglomerates. A study of the obscuring-power measurements of fineness and degree of dispersion can also be used to judge the ability of paint mills to grind and incorporate pigments in oil. The quality of the finished paste or paint from any mill can be determined by these tests, and also, the relative fineness of dry grindings may be checked. Hallett (1) established a relation between hiding power and tinting strength based on the results of twenty different white pigments. Obscuring-power determinations were also made of these same paints used for the work on hiding power. Graphs have been made plotting the relation between obscuring power and tinting strength, and between
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obscuring power and hiding power, so that the simple obscuring-power test of the paint or pigment can be used to compare one pigment against another, to obtain both hiding power in square feet per pound and the tinting strength on the basis of white lead as 100. The following formulas may be used to convert obscuring power in sq. cm. per gram to hiding power and tinting strength: obscuring power - 140 Hiding power = 17.75
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Figures 2 and 3 more clearly show the relations and the basis for converting one to the other. Another aid in judging the merit of a pigment is the indication of the presence of sub-microscopic particles. Stutz and Pfund (2) show the effective use of light filters to indicate colloidal fineness. Large particles are more opaque to red than green, and conversely small particles are more transparent to red than green. Therefore, when the obscuring power is lower for red than for green light it indicates the presence of colloidal material. As the value for the red light becomes increasingly less, there is evidence of greater fineness-that is, the average diameter for the sample is probably much smaller than for a product which has only a
April 15, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
slight difference between red and green. Therefore, if both a red and a green filter are used in obtaining the obscuring power, the results can be used to show the presence of colloidal particles. The obscuring-Power test for a Paint Or Paste requires aPProximately 15 to 20 minutes, and for a dry Pigment about 35 minutes. A simple control test of this nature which nets so much information after being calibrated seems invaluable in the paint industry. The new Burgess-Parr turbidimeter just put on the market the Same principle for measuring turbidity as the one used in developing this work. If this new turbidimeter is properly diaphragmed and a straightstrand galvanometer-type bulb mounted below the base instead of on top of the base, thereby minimizing extraneous
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light, then both turbidimeters will be on exactly the same basis and will give the same obscuring-power results, ACKNOWLEDGMENT Many thanks for helpful suggestions are due G. 0 . Hiers under whose supervision the work was developed. LITERATURE CITED (1) Hallett, R.L.,Proc. Am. floc. Testing Materials, 30,895 (1930). (2) Stutz, G. F. A., and Pfund, A. H., IND. ENO.CHEM.,19, 51 (1927). (3) Vogt, W. W., I n d i a Rubbe? W o r l d , 66,347(1922).
RECEIVED September 2, 1931.
Analysis of Hydrocarbon Gases H. S. DAVISAND J. P. DAUGHERTY Research and Development Department, Vacuum Oil Company, Inc., Paulsboro, N. J.
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This procedure for the analysis of hydrocarbon gases is based primarily on the fractionation of a liquid condensate from the gas. However, not all the gas is hue$ed, and liquid air is not necessary. Solid carbon dioxide and acetone provide all the refrigeration required. The analysis of the separate fractions is
tube. Further, it will cause t r o u b l e in the s u b s e q u e n t fractionation of the liquid condensate b y f r e e z i n g i n the lines and also by p r e v e n t i n g sharp fractionations between the cuts. The conditions of condensation must be so regulated that f r a c t i oThe gases. n a t iearlier o n Of the workers liquefied acaccomplished largely by methods of standard gas the uncondensed gas practically analysis. Diagrams of the procedure and Of the reaches e q u i l i b r i u m with the complished this by repeatedsimnecessary apparatus are given. The results from liquid condensate during their ple distillations. However, in the analysis of a synthetic mixture qf pure hydroseparation. This m e a n s t h a t recent years this laborious and carbons are shown and an illustration of the the internal temperature of the relatively ineffective method is g r a d u a t e d r e c e i v e r B must being replaced by distillation practical application of the method is given. remain constant and that the through low-temDerature fracgas must not be passed in too tionaGng columns ( I , 7). The present method is also based primarily on the fractiona- fast. A mush of solid carbon dioxide and acetone will easily tion of a liquid condensate, but not all the gas is condensed. maintain a temperature of -78" to -80" C.in the Dewar Liquid air is not required, and solid carbon dioxide and vessel C. However, if the condensate is formed too rapidly, its heat of condensation may raise the temperature of the acetone provide all the necessary refrigeration. The apparatus and manipulation which are described were liquid several degrees above that of the refrigerant outside. developed largely from experience gained in the analysis of Uncertainty on this point can, of course, be avoided completely by immersing a thermometer in the condensate itself. many gases in this laboratory. The general procedure will be plain from Figure 3. The For the apparatus shown in Figure 1, a rate of gas flow equal original gas is partially condensed by slow passage through a to about 0.05 cu. ft. per minute was found to be good practice. It is essential that the condensation set-up be gas-tight. receiver maintained at a constant temperature near -80" C. If the original gas is uniform in composition and if the tempera- This can easily be accomplished with reasonable care' since ture and pressure of condensation are maintained constant, it is subjected to only slight pressures, but tightness should then the compositions of the condensing liquid and of the un- never be taken for granted. Before each test, an open-tube condensed gas will not change with time. The uncondensed manometer should be attached and air blown in to a pressure gas is measured and analyzed as described in more detail of about 2 inches (5.08 cm.) of mercury, and shut off. The below. The liquid condensate is measured and then frac- apparatus should hold this pressure for at least 5 minutes. tionated by means of a low-temperature column into a gas cut, FRACTIONATION OF LIQUIDCONDENSATE a propane cut, a butane cut, a pentane cut, and residue. It must be borne in mind that the condensate contains large These separate cuts are analyzed by the methods indicated in Figure 3 and described in the text. The quantity of each proportions of propane and other substances which a t ordinary constituent, preferably expressed in moles, having thus been temperatures are gases and even at -78" C. are highly volaobtained, the composition of the original gas is easily calcu1 Leaks are most frequently found where a glass tube passes through lated. a cork stopper. It is often difficult to find & cork borer of the exact diameter ETHODS for the complete analysis of hYdrocarbon gases have already been described by others. In general they involve the cornplete iiquefaction of the gases by means of liquid air or liquid nitrogen. T h i s is followed by
PARTIAL CONDENSATION OF GAS The gas must be thoroughly dried by passing through tube A , Figure 1, filled with dehydrite. Moisture, unless eliminated, will freeze out in receiver B and may stop up the inlet
required. The following procedure, first shown to one of the writers by Professor A. A. Morton, is recommended: Bore the hole slightly smaller than required and then gently enlarge i t t o the required size by a tight roll of fine sand paper (a round file i8 bad for this purpose). Joints made in this way with good stoppers hold pressure well. If desired they can be covered with collodion.
