788
INDUSTRIAL AND ENGINEERING CHEMISTRY
Add 1 ml. of 50% aqueous potassium hydroxide solution and boil gently for 10 minutes. Cool, wash down the sides of the beaker with 30 ml. of ethyl ether, and transfer to a clean 125-ml. separatory funnel. Wash out the beaker with tn.0 25-ml. portions of distilled water, transferring each to the separatory funnel. Rotate the funnel, but do not shake. Break any emulsion with 5 ml. of alcohol. Withdraw the soap solution from the bottom. Wash the ether layer 3 times with 50-inl. portions of distilled water. If the last xvashings are not neutral to phenolphthalein, repeat until washings are neutral. Pour the ether layer into a 50-ml. beaker and evaporate on a steam or water bath until all moisture is gone. Cool and pipet 5.0 ml. of chloroform into the beaker, swirling gently to ensure solution. Transfer the solution to a clean, dry 15 X 150 mm. test tube and place in a lightproof constant-temperature cabinet a t 18" C. for 5 minutes. Proceed as directed above and determine the transmittance of each solution. From the standard
Vol. 18, No. 12
curve find the concentration of cholesterol corresponding to this transmittance. Prepare a reagent blank by placing the total solvents used for one sample in a 250-ml. beaker and evaporating as directed above. Carry through all the steps in the procedure and read the transmittance. Find the concentration of cholesterol corresponding to this transmittance from the standard curve. CALCULATIOXS. Grams of cholesterol in sample equal total grams of cholesterol found in sample minus grams of cholesterol in rclagent blank. Find per cent yolk present by consulting Table I\'. LITERATURE CITED (1) Ireland, J. T., Biochem. J . , 35, 283 (1941). (2) L a m p e r t , L. M.,IND.ENG.CHEM., ANAL.ED., 2, 159-62 (19301. (3) Reinhold, J. G., A m . J . Clin. Path., 6, 22-30 (1936).
Determination of Boric O x i d e in
Glass
MAX HOLLANDER AND WILLIAM RIEMAN 111 School of Chemistry, Rutgers University, N e w Brunswick, N. J.
A method has been developed for the determination of boron in glass which is more accurate and more rapid than the methods commonly used. Data obtained on synthetic mixtures and actual glass samples show that this method yields results with a mean error, signs disregarded, of 0.04% 8 2 0 3 with glasses containing less than 15% BzQ.
THE
essential differences among the various methods for the determination of boric oxide in glass lie in the method of separating the interfering substances, such as iron, aluminum, or silica, from the solution Containing the boric acid, \vhich is then titrated with sodium hydroxide in the presence of mannitol. Thus, Wherry and Chapin ( 5 ) dissolve the sodium carbonate fusion mixture in hydrochloric acid, and add calcium carbonate in moderate exceess in order to neutralize the free hydrochloric acid and to precipitate the sesquioxides and most of the silica. In another procedure ( 5 ), they recommend separation of the boric acid from the solution by distillation of the methyl ester. Glaze and Finn (f), on the other hand, base their method on the partition of boric acid betxeen water and ethyl ether in the presence of ethyl alcohol and a small amount of sulfuric acid. The method described in this paper is more accurate and less time-consuming than procedures which have been in use heretofore.
and repeat the same procedure, running a separate titration of the second filtrate. PROCEDURE WITH INDICATOR. Fuse and dissolve the sample as above, then add 20 drops of 0.047, bromocresol purple. Add 6 N sodium hydroxide until the first distinct color change (yellow to dirty green) can be recognized. At this point, the pH of the solution is approximately 5.5. Then proceed as in the pH meter method, Adjust the pH to 6.30 by comparing the color of the solution with that of a phosphate comparison buffer.
TITRATIOK CORRECTIOSS.The regular titration of boric acid with sodium hydroxide in the presence of mannitol, following the
Table HBOi. HzO Found, Milliequivalente 0.0 0.1 0.2 0.4
0.6 0.8
1.0
I.
Titration Corrections HBOz. Ha0
Corrections, Milliequivalent
Found, Milliequivalents
Corrections, Milliequivalent
0.000
0.005 0.008 0,013
,
0.016 0.019 0,022
EXPERIMENTAL
PROCEDURE WITH pH METER. Fuse a 500mg. sample with 3 grams of sodium carbonate for 10 minutes. Dissolve the melt in 20 ml. of 6 N hydrochloric acid and adjust the pH of the solution to 5.0 to 5.5 by means of 6 l V sodium hydroxide. Aluminum, iron, and similar elements, along mith most of the silica, are precipitated a t this point. Sweep out the carbon dioxide by bubbling air (purified by passing through concentrated sulfuric acid and Ascarite) through the solution a t 6 0 ' 1 5 " C. for 30 minutes. Filter the reaction mixture, and wash the precipitate with warm water until the volume of the filtrate is 250 ml. Cool the solution to room temperature, and adjust its pH to 6.30 by means of carbonate-free 0.05 N sodium hydroxide. Add 40 grams of mannitol to the solution. The pH is decreased markedly by the formation of mannitoboric acid. S o w titrate to a pH of 6.30 again with 0.05 N sodium hydroxide. The alkali used in this titration is a measure of the boron in the sample. I n some cases, 'a double precipitation of the residue is necessary to recover the coprecipitated boric acid. I n order to accomplish this, dissolve the precipitate in 10 ml. of 6 N hydrochloric acid,
Figure 1.
Potentiometric Titration Curves for Boric A c i d 1, Without mannitol 9 . W i t h mannitol
ANALYTICAL EDITION
December, 1946
II. Compositions of Synthetic Mixtures
Table
Mixture Number Constituent 1 2 Bz01 1.47 2.0 Si02 40.16 67.3 AhOt 2.90 2.5 Fez08 . . . Ti05 ZrOz CaO 7121 710 MgO . . . 0.17 BaO Na20 l2:06 14:O Kz0 2.00 , . . Lit0 PbO 25:50 . . ZnO .,. 7.0 MnzOa . . 0.28 . . . . . . P,Os SbrOa ... A ~ ~ . .o . ~ , , AsiOi 80, i:o F 7.10 . . C1 . . . . .
:’.
4
75 8 12.76 18.3 80.60 52.40 1.94 1.9 0.076 . . 0,027 . . 0,013 . 0.3 0:026 . .
3 3.0
4 5 6 4.13 6.15 7.22 66.80 37.04 66.90 10:0 2.54 3.70 6.38 ., 0.17 . . . 0.22 .,. , , , ... , ... 1.73 . . 7.94 4.0 2.60 0.61 , . . . , 42.04 . . . 7.40 ,.. 1.25 l2:O 1.75 2.40 . . . ,. ., ., . . ... 1.26 . . . 9.75 9.25 . .
,
.
.,. . . . .
. . . . .
7Q.5 0:5 , ,
.
. .
.
.
1.29 2.05 0:56 1:: . . . . . . . . . ... ...
.,
.
9 10 11 41.0 42.8 71.8 , .
..
.
.
.
5 :O 2 2 . 4 . , .
. . . . . . . . . . .
. . . .
. . . . . . . . . .
4.16 2.3 . 0.16 . . . 4.3 . .
:
~~-
__.
.
. . ..
,
, .
,
. ..
,
. . . . . . . . . . . . . . . . . 20.4 . . . . 0 : i i 0.1 0.085 . . , . . 0,009 . . . . .
:. 5.8
52:O 59:O
.
..
.. ,
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . .., 0,036 . . . . . . . . .
.
,
, , ,
Composition given on certificate of glass 93 of National Bureau of Standards.
of mannitoboric acid under the recommended conditions and subsequent interpretation of the graphs as explained above. The correction is to be added in all cases. The recommended conditions for the titrationi.e,, an initial and final pH of 6.30 and the addition of 40 grams of mannitol-were chosen because they give minimum corrections. The recommended initial volume of 250 ml. is the smallest that can be used without evaporation if the precipitate is to be washed thoroughly. Small amounts of carbonic acid and silica may be present in the solution during the titration without interfering because the pH is the same a t the end as at the beginning. Large amounts of these substances interfere, however, by giving small slopes to the graph in the vicinity of pH = 6.30, thus making it impossible to measure accurately the sodium hydroxide required for the titration. The recommended procedure leaves only small amounts of these substances in the solution.
Table Ill.
Determination of Boric O x i d e in Synthetic Mixtures b y p H Meter llixture No. of DeAverage Error, Mean so. terminations % Deviation, % 1 1 -0.03 ... 4-0.03 0.03 2 3 3 3 -0.03a 0.03 I 4 4-0.06 0.03 5 6 4-0.03 0.03 6 1 -0.03 ... -0.04 0.03 7 4 ... 8 1 -0.10 9 2 -0.20 0.07 10 3 -0.03 0.03 11 4 -0.36 0.10
5
Results obtained by modified method designed to remove phosphate
Table
IV. Determination of Boric O x i d e in Glasses BzOs Reported,
Glass % Bureau of Standards 128 1.52 Bureau of Standards 93 12.76 Corning, 1 0.61 Corning, 3 ca. 14.7 9.80: Armstrong 224abc Armstrong 427abc 9.75 Bureau of Standsrds, 92 0.70 Bureau of Stand. ards, 128 1.52
B208 Found,
%
Difference.
Mean Deviation,
%
%
No. of Precipitations One
1.52a
0.00
0.00
12.51b 0.59 14.71 9.91 9.66
0.25 0.02 0.0 0.11 0.09
0.01 0.01 0.0 .01 0.02
0.65
0.05
0.01
One
1.50
0.02
0.01
One
Two One Two Two . Two
Corrected for change in loss on ignition. b Dried a t 500’. By method of Glaze and Finn.
directions of Hollander and Rieman ( 2 ) . cannot be emuloved because of the interference of varying amounts of carbon dioxide and colloidal silica vihich remain in the solution. Therefore, the method of titration illustrated by the solid parts of the curves in Figure 1 is used. The first adjustment of the PH to 6.30 results in $he neutralization of a small fraction of the boric acid along curve 1, for nhich correction A B must be aoolied. Uoon the addition of mannitol, the boric acid is converted t o the stronger mannitoboric acid; consequently, the pH of the solution is depressed markedly. This is illustrated by the solid vertical line connecting the two curves. The second adjustment of the pH to 6.30 follow the path of curve 2. The pH of 6.30 is reached before the equivalence point (steepest part of curve 2 ) . Therefore, another correction corresponding to volume CD must be applied. The total corrections, A B CD, for various amounts of boric acid are given in Table I. These data were obtained by potentiometric titrations of the indicated quantities of boric acid and -
I
+
789
RESULTS
Several synthetic mixtures, similar to some glass compositions given by Morey ( 3 ) ,were analyzed by this method (Tables I1 and 111). Several glasses were also analyzed (Table 117. DISCUSSION
Table 111 shows that the boric oxide content of glasses with less than 15% B?Oa can be determined by the pH meter method with a mean error of *0.04% B203 without interference from A1,03, FeaOs, CaO, BaO, MgO, K?O, I&O, PbO, ZnO, h‘h203, SblO,, As?Ot (As2O6), TiOl, ZrO?, 803, and F. PhosDhate causes hiah - results unless the melt is dissolved in 6 A’ nitric acid and the phosphate removed by the addition of a slight excess of silver nitrate a t pH 5.5. This interference was also mentioned by Ruehle and Shock (4). It is caused by the activation of the phosphoric acid by mannitol. Method The authors’ method yielded results with a mean pH meter error of -0.04% B203on a synthetic mixture (KO.7) pHmeter identical in composition with the certified value of pH meter glass 93 of the Sational Bureau of Standards. The pHmeter p H meter authors believe, therefore, that the true boric oxide pH meter content of this glass is not 12.7670,but nearer 12.51%. Indicator The indicator method gives satisfactory results for Indicator glasses with low boric oxide contents. High-boron glasses will buffer the solution too much to yield distinct color changes by which the end point can be recognized. This method is probably applicable t o many samples other than glasses.
I
ACKNOWLEDGMENT
The authors &re indebted to the hrmstroIlg Company and the Rutgers Research Council for financial assistance in this work, and to the Armstrong Cork Company and the Corning Glass Works for their analyzed glass samples.
_
LITERATURE CITED (1)
Glaze, F. W., and Finn, 9.N., Bur. Standards J . Research, 27, 33 (1941).
(2) Hollander, M., and Rieman, W. 111, IND.EXG.CHEM.,ANAL.
ED.,17, 602 (1945). (3) Morey. G. W., “The Properties of Glass”, New York, Reinhold Publishing Corp., 1938. (4) Ruehle, A. E., and Shock, D. A,, IND.ESG. CHEY.,. ~ N A L .ED., 17, 453 (1945). (5) Wherry, E. T.. and Chapin, W. H., J . Am. Chem. SOC.,30, 1687 (1908). PRESENTED before the Division of Analytical and Micro Chemistry a t the 1Wth Meeting of the AMERICAN CHEMICAL SOCIETY, Atlantic City, N J.
