1 OL. 10, \o. 9
INDLSTRIAI, AYD EUGIUEEKI\G CIIEAIISTRY
518
nere added liefore titrating. For thi. purpose 10-ml. pipet. nere used and the n ater titration n as made from a 10-ml. buret graduated to 0.05 ml. If the alcohol solution contains mole than about 15 per cent of n a t e i , lien calibration c u n e s vi11 have to be e~ta1)liAxlusing a smaller quantity of carbon tetrachloride. bince this titration is very sensitive to temperature, three calibration curves (Figure 1) mere established a t 20°, 25", and 30" C. Interpolation between theye cur\ es permits the titration to he made a t any ordinary room temperature. The volume of water required to titrate is the abqcissa, n hile the ordinate is the volume of n a t e r contained in a 10-ml. portion of alcohol qolution. The fact that the curves are parallel straight lines makes the interpolation for intermediate temperatures very simple. The results obtained by thi. method are accurate to nithin * 2 per cent. The curves ale valid, of course, only for ethyl alcohol and carbon tetrachloride. Othei curve' may be established for other alcohols and organic liquids iitle
Water (ml Required To Titrate lOml Alcohol Solution t lOml CC14 FIGURE 1 C k L I B R A T I O X (?IRVES
Literature Cited arious concentrations of water in alcohol ameared to lie 1)v the establishment of an empirical calibration curve.' This curve was obtained by titrat'ing 10-ml. portions of alcohol solutions containing accurately known quantities of wat'er. To each 10 ml. of &coho1 solution 10 ml. of carbon tetrachlo\
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(1) Bridgeman, 0 C , and Querfeld, D , Bur. Standards J Reseajch. 10, 693 (1933). (~~ ~ I c K e , v v , Bur, Standards, B u l l , 9,344 (1913), Chem, So,.,, 48, lg2g (33 ~ i ~hf,i AI., ~ and ~ Hicks, , J. J, (1926). June 23, 1938.
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Elimination of Surface Tension Effects in Specific Gravity Measurements C. H. 31. ROBERTS, Petroleum Rectifying Company of California, Long Beach, Calif.
IN
THE measurement of specific gravity of water and aqueous solutions of high surface tension by means of specific gravity balances, considerable uncertainty may be introduced owing to surface tension effects that act on the mire supporting the plummet. Similar effects may be encountered with analytical balances, in determining specific gravity by the displacement method. The effect of nonuniform wetting is to introduce a resistance to motion of the plummet, making i t impossible t o obtain accurate values. If the balance point is approached by raising the plummet, specific gravity values are lower than those obtained when the motion of the plummet is in the opposite direction. The magnitude of these effects is indicated by the following measurements on distilled water, taken on the Becker chainomatic balance: Plummet Motion Downward Upward
Observed Specific Gravity a t 23.0' C. 0,9999 1.0000 0,9990 0.9989
Specific Gra\;ity at 20°/20 1.0004
1.0005 0.9994
0.9995
Khereas the above pairs of values differ materially, their average gives a good value for water, although this is more or l e v fortuitous, as reproducible wetting of the plummet n-ire
does not always occur. As an example of this effect, the following measurements on a dilute salt solution are given : Plummet Motion Downward
Observed Specific Gravity a t 23.0' C. 1,0008 1.0013 1,0009
Specific Gravity a t 2Oo/2O0 1.0013 1.0018 1,0014
Such effects as those illustrated above make i t apparent t h a t accurate values of specific gravity are difficult to obtain and require tedious repetition of measurements. I n instructions for use of specific gravity balances, the writer is not aware that any precautionary warnings as to surface tension effects are given, or any provision made for assuring reproducible wetting of the wire. I n order to avoid these difficulties and to secure rapid and accurate measurements of specific gravity, the writer has adopted the simple expedient of adding a minute amount of a water-soluble, surface-active material to the liquid whose specific gravity is to be determined. Since these liquids may include various brines or other aqueous solutions, as well as &tilled water, the addition agent must be of a type that is nonreactive with the components of such solutions. A class of materials t h a t admirably meets these requirements consists of the sulfonated or sulfated higher alcohols, typified by sodium lauryl sulfate. There are, of course, a number of similar
ANALYTICAL EDITION
SEPTElIBER 15. 1938
and equivalent materials that may be used for the purpose and that evert a large lowering effect on surface tension when used in v r y small quantities. These addition agents also assure complete and reproducible wetting of the plummet wire. The procedure employed in using these materials is to add a s m d l drop of a dilute solution (approximately 1 per cent) of sorliuin lauryl sulfate to the surface of the aqueous solution in the measuring cylinder, after immersion of the plummet of the 1)alance. I n this way, diffusion of the addition agent into the aqueous solution is minimized and its effect on the specific gravity of the solution will be negligible. -4s an example of the lieneficial effects of this procedure, the following measurements on the same distilled water sample as that previously cited are given. Ohserved Specific Gravity at 23.0' C. 0.9996 0.9995
Specific GraTjty a t 203/20 1 0001 1.0000
The d u e s obtained are independent of the direction of niotion of the plummet and are reproducible to the limits of
519
sensitivity of the balance. T h e behavior of the balance is very different after addition of the surface tension depressant. The short fast oscillations previously obtained with water are replaced by the long slow swings characteristic of a slightly damped analytical balance. The readings can be taken by balancing to zero in the usual manner, or can be obtained Ly the method of swings. As further proof that balancing difficulties are due to surface tension and wettability effects on the plummet nire, it is only necessary to examine the water surface by oblique illumination. With pure water, the meniscus a t the wire can be observed to move in the same direction as the plummet, as it is raised or lowered. After addition of the surface tension depressant, the plummet can be raised or lowered a t mill without visible alteration of the meniqcuj at the wire, thereby indicating its complete wettability. The operating procedure given above has been adopted as standard in the author's laboratory and is recommended R S being of material assistance in measurements of specific gravities of liquids of medium to high surface tension. RECEIVED J u l y 7 , 1938.
Determination of Ortho-, Pyro-, and Metaphosphoric Acids By Colorimetric pH Titrations ARTHUR B. GERBER
T
AND
F R i N C I S T . 3IILES, Rlonsanto Chemical Company, ;iriniston, Ala.
HE recent offering in conmercial quantities of anhydrous
phosphoric acids has occasioned interest in the coinpositions and hydration characteristics of such conipounds ( 7 ) . These acids, made by the partial hydration of phosphoric a n hydride, contain from 73 to 88 per cent of phosphorus pentoxide. They range from sirupy to thick viscous liquids; impurities are negligible. This paper describes a method for the determination of their compositions in terms of ortho-, pj-i-0-, and metaphosphoric acid by the use of colorimetric pH titrations. For the determination of the three phosphoric acids, Aoyama ( 2 ) neutralized with sodium hydroxide solution, added a measured excess of silver nitrate and enough alcohol to make the content 50 per cent, and determined the excess silver after removal of the precipitated silver phosphates by filtration. Dmorzak and Reich-Rohrwig ( 4 ) revised the procedure by introducing a two-step neutralization. The solution was first neutralized to phenolphthalein indicator, and then, after treating with silver nitrate and alcohol folloTved by filtration, it was again neutralized using 0.1 i\; sodium hydroxide until the newly formed silver precipitate showed a distinct gray tint due to the precipitation of silver oxide which is formed when all the phosphate is precipitated. After filtration, the excess silver was determined as by Aoyama. The precipitated silver phosphates were treated Ti-ith hydrogen sulfide (2) or hydrochloric acid (4) t o form the phosphoric acids which, after separation by filtration, were titrated with sodium hydroxide to both the methyl orange and phenolphthalein end points. From these two titrations and the silver determination the amounts of ortho-, pyro-, and metaphosphoric acids were calculated. These methods of determination may be greatly simplified,
especially for anhydrous acids as inentioned above, if the silver determination is replaced by measurement of the sodium hydroxide required to neutralize in the presence of an excess of silver, while the accuracy and precision of the acidimetric titrations may be greatly inrreased by making allowance for the differences between the equivalence points of the ortho- and pyrophosphates.
Outline of Proposed Method For titration purposes, meta-, pyro-, and orthophosphoric acids are considered, respectively, as mono-, cii-, and tribasic acids which may he quantitatively neutralized in three steps. For the determination a portion of the anhydrous acid, carefully diluted with ice n a t e r to avoid hydration, is titrated under stated conditions with standard sodium hydroxide solution to pH 4.4 (a provisional value) a t which point the reactions are : HPO, + SaOH = Sap03 HzO H4Pz07 2SaOH = Sa2HzPz07 2Hz0 H3POI XaOH = XaH?P04 HzO
++
+ +
By continuing the titration to pH 8.8 (a provisional value) in the presence of a suitable quantity of added sodium nitrate to reduce hydrolysis, additional hydrogens of ortho- and pyrophosphoric acids are neutralized, giving:
+
+
SazHzP20, 2XaOH = NaaPzO;. 2H20 SaHZPO4+ NaOH = Iia:HP04 + 1120
If a t this point an excess of silver nitrate is added, normal silver phosphates are subsequently precipitated, and the remaining hydrogen of the orthophosphoric acid can be titrated with sodium hydroxide, using methyl red as indicator
