457
V O L U M E 27, NO. 3, M A R C H 1 9 5 5 Table 111. Determination of Phosphate by -4mperometric Titration with Uranyl Acetate after Cation Exchange Separation [Dowex 50, 2 5 X 2 1 cm column. Composition of original solution, uranium 180 nig ( U O z + “ ) , HClOI, 0 6 6 3 1 , H2901, 10 9.111 I’olume of Influent Flow R a t e , Phosphate, h l g . Difference, Solution, LI1. per Taken, Found, B - A B bll. Rlin. A 0 . (i 99.7 100 3.5 99,l 100.6 -0.2 100.8 3.1 300
4.7 4.4 4.6
64.8 68,5 100.0
08.8
0 .1 0.7 1 1 -0.0
09 2 101.7 99.5
4.3
100.1
300
3.7 3.5
148.1 148.1
147.8 148.0
-0.3 -0.1
500
4
100.1
99.9
-0.2
were: size of resin column, 24 X 2.1 em., and flow rate, 3 to 5 ml. per minute. The longer resin column effectively eliminated breakthrough of uranium (in strong sulfuric acid solution) as the anionic uranyl sulfate complex. Phosphate in solutions as concentrated as approximately 11M with respect to sulfuric acid was successfully determined. Perchloric acid presented no difficulty in the separation. The longer columns were also satisfactory with regard to holdup of phosphate in the column. The volume of influent was not critical. Precision of Method. Synthetic samples of uranium phosphate in volumes of 5 ml. were prepared which contained varying amounts of sulfuric and perchloric acids. Each contained 1 nil. of 0 67111 uranyl sulfate (180 mg. of UOz++) and 65 to 150 mg of phoqphate. The latter was added as a solution of potassium dihydrogen phosphate containing 92.56 mg. of phosphate per gram of solution. The portion of the standard solution of phosphate used to prepare each sample was measured by weighing it in a stoppered flask. Typical data, which were obtained under the optimum conditions, are shown in Table 111.
Khen the removal of uranium by the ion exchange resin appeared to be complete, in no case was the amount of phosphate found to differ from that taken by more than 1.1%. The average difference of 14 samples from the known values (68 to 100 mg ) was 0.5% and the coefficient of variation was 0.6% on a 95% confidence level. Seither precision nor accuracy appear to have been affected adversely with changes in flow rates ranging from 3 to 5 ml. per minute. Concentration Range of Applicability. The phosphate content varied from 65 to 150 mg.; hence, the quantities of phosphate in the aliquots finally taken for titration lay between 6.5 and 15 mg. S o difficulty should be expected in applying the method to more concentrated samples, because the phosphate content of the solution uhich is to be titrated can be brought within this range by taking a smaller aliquot either of the original sample or of the effluent from the ion exchange step. At the lower extreme, the limiting factor should be the titration itself. Kolthoff and Cohn ( 2 ) ,titrating with 0.01M uranyl acetate, were able to determine phosphate in solutions as dilute as 0.0002M with an accuracy of 1%. This dilution corresponds to 1 mg. of phosphate in a 50-ml. aliquot. At two to four times this dilution, results (using 0.00534 titrant) w r e high by 2 to 11%. Under the experimental conditions applied here, and using 0.035M uranyl acetate as the titrant, 3 mg. of phosphate can be determined within 1 %; for smaller quantitiee, the results are high. LITERATURE CITED (1) (2)
Kelley, lf. T., and Miller, H. H., A K ~ LCHEW, . 24, 1895 (1952). Kolthoff, I. M., and Cohn, G., IKD.ENG.CHEX.,ANAL.ED.,14, 412 (1942).
Redd;?,
C. J., “Analytical Chemistry of the Manhattan Project, p. 602, McGraw-Hi11 Book Co., New York, 1950. (4) Samuelson, O., “Ion Exchangers in Analytical Chemistry,” p. 146, Wiley, Kern York, 1953. (3)
RECEIVEDfor review M a y 8, 1964 Bccepted November 12, 1954 The Oak Ridge h’ational Laboratory is operated by the Carbide & Carbon Chemicals C o , a Division of Union Carbide & Carbon C o r p , for the -4tOmiC Energy Commission Work carried out under Contract S o K-7406-eng-2G
Weighing Pipet Method for Preparing Infrared Gas Standards for Ether and Alcohol FRANK PRISTERA
and
Picatinny Arsenal, Dover,
ALEXANDER CASTELLI
N. J.
A method is described for the preparation of infrared gas standards of ether and alcohol using a weighing pipet (micro). The method was found satisfactory when applied to sy-nthetics containing known amounts of ether and alcohol. It should be applicable also to any reasonably vola tile substances such as the numerous organic solvents which have widespread commercial application.
I
K T H E ilianufacture of solid propellants, ether and alcohol
are widely used as solvents for the nitrocellulose. The finished propellant is then usually placed in a solvent recovery house ivhere the concentration of ether and alcohol may reach appreciable proportions. It was in connection with the infrared analysis of the air for ether and alcohol in such a solvent house that the weighing pipet method for preparing infrared gas standards was developed. The infrared analysis of a gas ( I ) , in essence, consists of obtainingthe infrared absorbance of the sample and relating such absorb-
ance (usually a t some selected absorption band) to concentration from a previously established relationship (working curve) of absorbance versus concentration. The establishment of the working curve necessitates the preparation of standards containing known and varying amounts of the gas. Such gas standards are usually prepared by the introduction of controlled amounts of gas into an infrared gas cell, making accurate measurements of their pressure, and relating such pressure measurements to concentration. This method is lengthy, very difficult to control accurately, and in the case of ether and alcohol it would also be very difficult to apply as these substances are not normally in the gaseous state. For these reasons a more suitable method for preparing standards was considered desirable. In the field of organic quantitative microanalysis ( 2 ) , volatile liquids are handled in capillaries called weighing pipets. It therefore appeared reasonable to anticipate that a suitable technique could be developed to introduce known amounts of ether and alEohol in an infrared gas cell using similar weighing pipets. This paper describes the developed technique and presents some
ANALYTICAL CHEMISTRY
458 Table I.
Results Obtained on Synthetics of Ether and Alcohol E t h e r , 'Z .4lcohol, 70
Sample No.
Added 6.9 2.2 1.1 4 1.0 AV. 2.80 Std. deviation of differences 0.13
Found 7.0 2.4 1.0 1.0 2.85
Added 1.3 2.1
Found 1.3
0.58
E 5 3.1 1.76
3.0 1.75 0.082
data obtained in the application of this method to prepared samples containing known amounts of ether and alcohol. EXPERIMENTAL PROCEDURE
Preparation of Weighing Pipets. Thin-walled soft glass tubing 1 to 3 mm. in inside diameter was sealed a t one end &-ith a flame. .4t about 2 cm. from the sealed end the tube was heated in a flame, drawn out to a fine capillary, and cut a t about 3 cm. from the sealed end. (Weighing pipets were purposely made in different sizes ranging from 1 to 3 mm. in inside diameter, so as to be able to prepare standards of ether and alcohol ranging in concentration from about 1 to 8%. Using a 10-cm. gas cell, the weighing pipet with a I-mm. inside diameter, when filled to 1 cm. from the sealed end, produced a concentration of about l%.) Filling and Weighing the Pipet. The pipet, previously weighed empty on a microbalance or a good semimicrobalance, was placed with the open end down in a 10-ml. beaker containing about 1 ml. of the ether or the alcohol. The beaker was placed in a Fisher Filtrator in which the upper opening was stoppered. The vacuum was turned on for a few seconds and then turned off. In this
process some of the air within the pipet was withdrawn in the evacuation process and, upon shutting off the vacuum, the ether or alcohol moved into the pipet to replace the air withdrawn. The pipet was placed in a small centrifuge tube with the sealed end in the outward direction and centrifuged to move the liquid to the sealed end. The open capillary end was sealed in a flame and the sealed pipet was weighed on the same balance which &-as previously used. The gain in weight of the pipet was considered as ether or alcohol. (The pipet should be filled to no more than 1 cm. from the sealed end. T o get varying amounts of ether or alcohol, pipets of different inside diameters are used together with variation in the level of the liquid in the pipets. A pTeliminary weighing of the pipet can be made prior to sealing its tip to ensure that it contains the desired amount of liquid. To place additional liquid in a pipet, after centrifuging and prior to sealing, repeat the evacuation process described above. If too much liquid has been introduced in a pipet, some of it may be removed before centrifuging by warming the pipet slightly, by holding it betxeen two fingers or even by centrifuging xith the tip of the pipet in the outward direction.) Preparation of Standard Samples and Working Curves. The pipet containing a known amount of ether or alcohol was introduced through one of the threaded openings into a Perkin-Elmer 10-cm. gas cell having a metal body. With a thin metal rod heated to about 50" C. and inserted through the threaded opening, the pipet was fragmented inside the cell. The end of the rod was held in the cell for about a minute to allow evaporation of any ether or alcohol which may have deposited on the tip during the fracturing process. (The rod was heated to about 50" C. to facilitate this evaporation.) The rod was then withdrawn, and the threaded opening of the cell was suitably closed. (The other threaded opening was kept closed during the complete operation.) The cell was allowed to stand for about 0.5 hour to allow the ether or alcohol to evaporate completely and to disperse itself inside the cell. The percentage of ether or alcohol in the
CM.- 1 5000
2500
1500
1 PO0
1000
900
800
700
V O L U M E 2 7 , NO. 3, M A R C H 1 9 5 5
459
&
o’60
/ I
0.50
9 0.40 L .k2
DISCUSSION AND APPLICATION OF METHOD
/
W
s4
Oa30
0.90
0.10
1
9
3
4
5
6
7
8
, CONCENTRATION, VOL. %
Infrared absorbance of alcohol and ether
Figure 2.
analysis. The net absorbance (total absorbance minus rrll absorbance) of the various standards a t the two selected positions was plotted against concentration t,o obtain the working curves (Figure 2).
- - - At
8.00 microns in 10-cm. cell A t 6.90 microns in IO-cm. cell
The relationship of concentration to absorbance for both ether and alcohol a t both 6.9 and 8.0 microns appear t o follow a straight’ line and therefore the working curves have been drawn as straight lines. The various points lie very close t,o the lines, indicating that the weighing pipet method of preparing infrared gas standards for ether and alcohol is valid. T o test the applicability of the method, four known samples of ether and alcohol in air were prepared by introducing known amounts of both ether and alcohol in the gas cell as described above. The infrared absorbance of the prepared samples was measured a t 6.9 and 8.0 microns and the values were calculated to per cent ether and alcohol using the method of successive approximations (3). The results obt,ained are listed in Table I and $how good agreement between the values added and the values found. ACKNOWLEDGMENT
cell was calculated as follow (all n.ork as done a t 760 mm. of pressure and 25” C. or 298’ K ) : Per cent of ether or alcohol by volume =
n.t. X22,400_____ X 100x298 11.K.-x v 273
where
The authors wish to acknowledge the assistance of W.J. Huff, who weighed the pipets on a microbalance. hppreciation is further expressed to J. D. Armitage, Robert Frye. C. J. Bain, and A . J. Clear of Picatinny Arsenal for help rendered in the publication of this report.
= weight of ether or alcohol in pipet in yranis
Wt.
M.W. = gram molecular weight of ether or alcohol V = volume of gas cell in cubic centimeters ralculatcd from its geometry (fo,md to be 117.6 cc. for the cell used) I n this n-ay various standards of ether and alcohol were prepared. The infrared spectrograms of the gas cell without ether or alcohol, and of the prepared standard samples were obtained on papilr \vith alisorhmce markingr using a Pwicin-Elmer douhlebeam inf‘rarrbd sprctrophotometer (Figure 1). The 6.0-micron ether liand and the 8.0-micron alcohol band n-ere selected for
LITERATVRE CITED a n d Saier, E. L..J .Ippl. P h y s . , 17, 450-6 (1946). ( 2 ) S i e d e r l , J. E., a n d X i e d e r l , J., “ O r g a n i c Q u a n t i t a t i v e LIicro.” pp. 46-7, \17iley, S e w York. 1042. (3) P r i s t e r a , F., B p p l . Spectroscopy, 7, Xo. :3, 116-31 (1953).
(1) Coggeshall,
K, D.,
R E C E I V Bfor D review October 1 4 , 1 0 5 3 . Accepted October 27, 1954. Presented at the Annual Xeeting of t h e Society f u r Applied Spectroscopv. T r w Tork, S . Y . , May 2;. 1954.
Instrumental Variability of a Model 7 Coleman Photonephelometer HUBERT J. KEILY
and
L. B. ROGERS
Department o f Chemistry and Laboratory o f Nuclear Science, Massachusetts Institute o f Technology, Cambridge, Mass.
Instrumental variability is attributed to variations. in the blanks used to set the instrument sensitivit). and to a tendency for the readings to drift to lower values. By modifying the manufacturer’s operations and making frequent checks against standards, a standard deviation for individual measurements of 0.28 reading unit at the normal instrument sensitivity has been estimated.
D
URISG the course of a study on the nephelometric deter-
mination of sulfate (4), the need arose for an evaluation of the variability contributed to the measurements by the instrument alone. The Model 7 Coleman Photonephelometer used in the study measures the light scattered a t right angles to the direction of illumination by means of two barrier layer cells. T h e instrument, which is linear in its response t o scattered light, may be operated as either a direct or a null-reading device. Unknown turbidities may be compared ~1ith standaids, supplied by the manufacturer, which are suspensions of an inorganic salt in a highly viscous organic polymer (3). Each one is labeled with a particular Xephelos number which has been assigned with reference t o a master standard. Thus, t h e light-scattering piop-
erties of the standards, and the samples subsequently measured with reference to them, are enipirically related. I n this way, a correlation of readings in Sephelos units between laboratories should he possible. Essentially, t h e standards provide a means of reproducibly establiehing the instrument sensitivity, which is defined as the slope of the linear relationship between the instrument reading and the 90’-scattered light intensity. This relationship ibQ referred to below as the instrument-response curve. RESULTS
Using the procedure outlined by the manufacturer for the null method of operation ( 1 ), the instrument was adjusted using a standard 38 and a distilled water blank. [ A modification ( 6 ) of this procedure is now recommended by the manufacturer. 1 Five other standards were then run as “samples” in a random order which was determined by a chance selection of a 5 X 5 Latin square ( 6 ) . A different Latin square was used for the data in each table. Table I indicates the values taken after a single initial adjustment of t h e instrument with standard 38. The readings tor each
