Determination of Specific Gravity of Dry Paint
Pigments IRVIN BAKER AND GEORGE MARTIN Chemical Laboratory, Norfolk Navy Yard, Portsmouth, Va.
IFFICCLTY has been experienced in getting accurate, in determining the specific gravity reproducible of d r y paint pigments by the pycnometer bottle method when using oil, water, kerosene, or alcohol as a wetting medium ( I ) . The errors encountered were: evaporation of liquid from glass joints of the pycnometer bottle, incomplete
D
removal of surface-adsorbed air, and moisture condensation on the exterior of the pycnometer bottle. This procedure requires a very long treatment under vacuum, presents a hazard due to possible collapse of the desiccator under high Vacuum, and is too complicated for routine tests for specific gravity of paint pigments.
previously dried a t 105" C. for 2 hours, For pigments of low specific gravities, such as blacks and blues, weigh about 1.5 grams; for opaque Jvhite pigments, about 12 grams; for red lead, about 25 grams. Add sufficient kerosene to the pigment to wet it entirely and leave about 0.6 cm. (0.25 inch) of clear liquid above the pigment. Place the tube containing the pigment and stirrer in another beaker of kerosene and heat for 0.5 hour on a hot plate at a temperature of 65" to 70' C. with frequent stirring to remove air and other occluded matter and entirely wet the pigment. (This produces a small change in the specific gravity of kerosene and a negligible change in the final
~ v e ~ ~ ~ ~ ~ , ' " ; bath, 2 , fill ~ with i ~ kerosene ~ ~ ~ ~ ~ ~ ~
to about 0.6 cm. (0.25 inch) from the top, and centrifuge at about 2000 r. p. m. for 0.5 hour. Remove the tube and carefully fill with kerosene. Place the tube and its contents again in the original 600-ml. beaker and place in the water bath a t 25" C. When the temperature of the entire contents, inside and surrounding the is 25' C., weigh the tube as before. The increase in weight T~~~~1. sPECIFIC G~~~~~~~ OF T~~~~~~~ D~~~~~~pIGIIENTS tube, represents the weight of the pigment in kerosene. The difference between the weight of the pigment in air and the weight in kero7. \'ariation Variation sene represents the weight of kerosene displaced. Accurately Sperific from from Sample Gravity Average Average determine the specific gravity of the kerosene at 25" C. (It is +o.ooi 0.18 not necessary t o determine the specific gravity of kerosene after 1 3.859 3.849 -0.003 0.0s testing each of several pigments.) The specific gravity of the 3.850 -0.002 0.05 pigment is calculated from the formula:
A\..
3.861 3.850 3.856 3.853 3.852 3.854 3.854 3.851 3.852 3.853 3.853
+0.001 0.003 +0.001 +0.001 -0 . 0 0 2 -0.001 0.00 0.001
0.03 0.05 0.10 0.03 0.07 0.03 0.03 0.05 0.03 0.00 0.03
Av.
3.861 3.856 3.853 3.858 3.884
-0.003 +0.002 -0.001 i-0.004 0.0025
0.08 0.05 0.03 0.10 0.06
AV.
2
3
-0.001 -0.002
i-0.004
Specific gravity of pigment = weight of pigment in air weight of pigment in air-weight of pigment in kerosene X sp. gr. of kerosene
TABLE 11. SPECIFIC GRAVITYOF TITANIUM-CALCIUM PIGMEKT Specific Gravity
Sample 1
3.197 3.201 3.199 3.207 3.206 3,2065
AY.
A rapid, accurate, reproducible method for the determination of specific gravity of such materials is presented here.
2 AV.
Using heat and a high-speed centrifuge, pigments may be deaerated and wetted and their specific gravities determined rapidly with a minimum of equipment and skill, and with a n accuracy of 5 parts per thousand.
Variation from
.iverage
7.
Variation from Average
-0.002
0.07 0.07
0.002 +0.0008 - 0.0005 0.0005
0.07
+o.ooz
0.02 0.02 0.02
Discussion The method outlined is direct and easily performed, and involves no detailed, difficult procedures. The errors of the pycnometer method have been eliminated since evaporation or moisture condensation is prevented by the fact that the apparatus is \&ked both times submerged in the same liquid at the same temperature. Air and other occluded gases are removed more thoroughly by the double process of heating and centrifuging than by the single process of \Tacuum treatment. The accuracy of the results, as noted in Tables I and 11, is 5 parts per thousand or better.
Procedure Fill a 600-ml. Berzelius beaker with kerosene oil. Prepare a suitable specific gravity tube by reducing a test tube 3.5 cm. in diameter t o a length of 7 cm. At opposite sides of the tube and close t o the top, punch two small holes through which a fine &tinum or steel wire can be attached to act as a handle, SO that the t'ube may be suspended from the hook above the analytical balance Pan- Prepare a small glass stirring rod, slightly longer than the tube,. Wash the tube and stirrer with cleaning solution followed by an alcohol xash, heat, and cool in a desiccator t o remove moisture. Insert the stirrer in the tube, place in the 600-ml. beaker of kerosene, and bring the entire contents to 25" C. in a thermostat-controlled water bath. Wipe the kerosene from the suspending wire, support the beaker over the balance pan, and weigh the tube and stirrer completely submerged in kerosene at 250 c., after makillg certain that t,he temperatureinside the test tube has had sufficient time to come t o equilibrium with the surrounding temperature. Remove the tube from the keroaeiie bath and weigh into it, by difference, from 1.5 to 25 grams of the paint pigment, depending on the specific gravity of the pigment, the pigment being
Literature Cited (1) G a r d n e r , H.
"Physical and Chemical E x a m i n a t i o n of P a i n t s Varnishes, Lac:yuers, arid Colors", 8th ed., p. fi93, Washington, Instit.iit,e [ i f F'aiiil :111d Vnriiisli Ilesrarcli, 1937. A\.,
THEviews presented in this article are those of the writers and should not be construed a s the official v i e w of t h e Navy Department.
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