V O L U M E 2 3 , NO. 9, S E P T E M B E R 1 9 5 1 after the latter has been laced on the steam bath. A hypodermic syringe is ideal for t%is purpose] and can be fitted with a fine glass tip drawn out from an adapter (available from Becton Dickinson & Co.,Rutherford, N. J.). A current of air from the jet is blown a t the solution a t such a rate that the surface of the solution is barely disturbed. If an aqueous solution is to he analyzed, it will require 2 to 3 minutes for a 1-ml. aliquot to be brought to dryness a t 100’ C. Higher or lower temperatures can be reached as desired by placing liquids of suitable boiling temperature in the bath. During the time required for the evaporation of the first aliquot, others can he started on the remaining three holes of the bath. As soon as the solution comes to dryness, the liquid clinging to the bottum of the shell is touched off with cotton gauze, and the shell is placed in its proper position on the wire frame. When all the shells have been so treated, the wire frame with its shells is placed in a large glass test tube fitted with a rubber collar near its upper end. The tube and its contents are supported in a steam bath by means of this collar, and the open end is closed by a one-hole rubber stopper with a glass t u h and rubber hose connection to a high vacuum oil pump. 811 the samples and the tare are dried together a t looo, 0.2 mm., for the required time, usually about 5 minutes. The shells are then weighed on a semimicro balance to &0.01 ing. I n weighing, the tare alnays nearly counterbalances the shell containing the residue. The Seederer-Kohlbusch balance used in this laboratory utilizes riders for weights up to 100 mg., and hundredths of a niilligram are measured by the degree of deflection. As the shells differ from the tare by no more than 100 mg., the entire series of shells can be weighed by use of the riders alone. The balance is magnetically dampened] and each neighing requires no longer than 1.5 minutes. DISCUSS103
Thiii hell^ have heen fouiici prefcra1)le to heavier ones. Because they provide less resistance to the interc1i:irige of h w t , evaporation of wlutions is more rapid, and in the stwm bath the final drying temperature is reached more quickly. Even more iiiiportnnt is the rapid adjustment to atmospheric temperature. This permits reliable weighing immediately after removal from the ste:tni bath. Finally, there is the added prccision of weighing
1327 very sinall residuw iii cont~aiiirrso f the smallest possible weight. The method of evaporatioii presents several advant2tgcs. The stream of air not Oll~ycarries away solvent vapors but also prevents particles of dust from falling into the shells. The shc~lls are suficieritly large that the level of the liquid k well below the rim of the hole in the steam plate, even with a %nil. aliquot. Thus, any solution tending to creep up the wall evaporates befcire it travels beyond the h(1atr.d surface. It is advisable to evaporate sufficient solution so that :i residue of 0.5 to 5 nig, is oljtained. With samples of t,his size tliero is no loss by spattering, r v ( ~ with ~ i cryptalline residues. Usually a thin layer of the residue covers the bottom of each shell, and c.ontlit,ions for final drying are optimal. The fragile shells can be washed out easily withiiut reliiovhig them from the wire frame by flooding them two tCJ three times with an appropriate ~ o l v e n t . Solvent remaining in the shells is aspirated conveniently kiy a i,uhber tube with :i syringe nerdle attached lor a nozzle. The empty shells return to their original weight altcr n.:rsliing. I t is t,herefore necessary trJ recalibrate their weights only at infrequent intervals, or when there is reason to suspect an insoluble residue. Thus, the clean, wet shells can be plared directly on the evaporator for the next serier; of determinations. As the bottom o f each shell is of clear glass, a residue of 0.10 mg. can be wen with the nakcd eye. The above procedure Itas been in use in this laboratory for the past 2 years, and has been applied successfully to the measurement of partition ratios and to the analysis of countercurrent distributions of amino acids, polypeptides, bile acids, and lipides. The volatility of some fatty acids required that water be replaced by ethyl alcohol i ] ~the evaporator, and some proteins :ind lipides could be freed of water only by final drying in the stcant hath for 15 to 20 minutes. RECEIVED 12ehruary 20, l O > l
High Frequency Titrations of Calcium and Magnesium Ions in Aqueous Solution F. W.JENSEN, G. M. WATSON,
4ND
L.
C,.
iigriculti~raland Mechanical College of Tezbs, College Station, T P X .
IIE purpose of this work was to irivcstigate the use of high rfrcquency titration in the deterinination of calcium and magnesium ions in water, and to improve on the standard soap method ( I , 8 ) of estimating total hardness. The apparatus used was developed by Jensen and Parrack (6, 6). An alternating current model was used in this determinntion. The wiring diagram is given in Figurv 1. SOLUTIONS
The soap solutions were prepared according to the standard procedure ( 1 ) and diluted x i t h ethyl alcohol to approximately 0.2 N . The sLmdard calcium chloride solutions were prep‘ved by dissolving a weighed amount of pure calcium carhonate \yith dilute hydrochloric acid (1). The standard magnesium sulfate solutions were prepared from the best reagent grade magnesium sulfate heptahydrate and standardized gravimetrically by precipitation of niagnesiuni :as magnesium ammonium phosphate. The hydrochloric acid was prepared by dilution from constant boiling mixture ( 7 ) . The sodium hydroxide solution w:is prepared carbonate-free by decantation from saturated sodium hydroside solution ( 7 ) and was stored and standardized with the usual precautions. .___
Present address, Galveston Laboratories, Galveston, Ter
Figure 1.
Electrical Circuit for Alternating Ciirrent Titrimeter
80 1.4. IO-mh. H.F. chocb VKiR .MI. 0 to 10 ma. 6CJ M2. 0 to 50 pa. 6. 8-mfd. elec,trulytic R I . 100,OOO ohms C,. 0.01-mfd. mica R : . 35,000ohma Cj. 10 to50mmfd. trimmer Rz. 10,OOO ohms Ci. Ct. 100-nimfd. variable R I . 50,000ohmb Ce. 0.003-mfd. mica HI. 1OOOohms LI. 15-hcnry choke Re. 500,000ohms I.,, L?. Identical coils of N o . 14 copper wire, each 8 turns. 1.5 inch in diametcr and evenly spaced o v e r &inch length
A.
B.
c.
ANALYTICAL CHEMISTRY
1328 Table I.
Titration Data for Curves I, 11, 111, and IV
Curve NO.
I
I1 I11 I\'
Titrant
Solution Titrated
Soap solution Hydrochloric acid (0.00870iV) Soap solution Calcium chloride (0.116N ) (0.00400N ) Soap solution Magnesium sulfate (0.116N ) (0.00200'3') Magnesium sulfate Sodium hydroxide (0.020N) in alcohol (0.00189X)
E n d Point, M I . Experimental Calculated
2.46
..
1.78
1.72
0.98
0.86
4.78
4.72
Typical titration curves for the standardization of soap solut,ion, and the determination of calcium with soap, of magnesium with soap, and of magnesium with sodium hydroxide in alcohol are shown by curves I, 11,111,and IV, respectively. The relative loading of the oscillator in microamperes is plot,ted as ordinate against the volume of titrating agent added. Typical t,itration data arc given in Table I. Similar precision was obtained when the titrated solutions varied in concentration range by &50% of these values. DISCUSSION
The standardization of the soap solution could be reproduced within 2%, which is believed t,o be an improvement over the conPROCEDURE ventional method (f). However, the soap solutions used in this invedgation were more concentrated than those ordinarily used I n all titrations a definite volume of the solution to be titrated was placed in a test tube, which in turn was introduced in the and t,he precision with more dilut,e solutions was not investigated. core of the plate tank coil of the titration apparatus. This soluCalcium determinat,ions with soap could be reproduced within tion was kept in a carbon dioxide-free atmosphere by the con3% of the theoretical value. This precision compares favorably tinuous introduction of dry carbon dioxide-free natural gas on with the standard method ( I ). top of the solution. 4 stirrer kept the solutions homogeneous. The instrument was turned on and allowed to reach thermal Magnesium determinations directly with soap solutions using equilibrium and a convenient setting was chosen. The titrant the titrator were not satisfactory. I t is believed that this was was added from a microburet and readings of the microammeter due t o t'he relatively large solubility of magnesium oleate (8). were taken a t suitable intervals. I n all caws the titrant used The unprecipitat,ed magnesium oleate more than offset the reTvas at least ten timee as concentrated as the solution being titrated, to minimize dilution effects. placement, of magnesium with slower moving sodium ions and the loading of the oscillator increased. The end point was indicated merely by a change in slope instead of a definite IV) break in the titration curve. LEGEND Except in the titration of magnesium 0 -CURVE I sulfate with soap solution the precipiA - C U R V E II tated subst,ances were sufficiently insolu20 --CURVEIII blr, to give sharp breaks in the curves at X -CURVE IV t,hc end point. The effect of rrlative solubility has been pointed out by Jciisen and Parrack ( 6 , 6). 10 Mixtures of calcium and magnesium with soap were not determined satisfactorily and it is suspected that the reasons given above were Rsponsible for the masking of the end point. Magnesium was determined satisfaco~ torily as the hydroxide. However only where alcohol was used as diluent, for the sodium hydroxide were the titrations lo! successful. When water was used as diluent,, the shapes of titration curves were functions of time and did not re!O veal an end point. This was probably W A due to supersaturation and slow precipitation of magnesium hydroxide. Ethyl alcohol was added t o decrease the solu-30 bility of magnesium hydroxide and thereby increase its rate of precipitation. Once ethyl alcohol was added, no further difficulties were encountered and stable -40 readings could be obtained rapidly. For future extensions of this preliminary investigation, work is under way with temperature control of the reacting mixture and it is planned t o use other -50 reagents ( 2 , 4 ) in the determination of magnesium. Because of the slow precipitation of magnesium it is thought that thermostatic control is essential (4)if stable readings are t o be obtained in the titrator. Figure 2. Titrations Continued study of the applications of I. Hydrochloric acid with soap solution 11. Calcium chloride with soap solution the use of high frequency titration in 111. Magnesium sulfate with soap solution water analysis is contemplated in this IV. Sodium hydroxide in alcohol with magnesium sulfate
V O L U M E 2 3 , NO. 9, S E P T E M B E R 1 9 5 1 laboratory. A study of the application of this method for the determination of salts of weak acids is also under way.
1329 (5)
Jensen, F. W., and Parrack, d.L., IND.ENG.CHEX.,A N ~ LED., . 1 8 , 5 9 5 (1946).
(6) Jensen. F. W., and Parrack. A. L., Texas 9. and M. College. - Ena. Expt. Sta., Bull. 92 ( 1 9 4 6 ) . 17) Rieman. W.. Keus. J. D.. and Naiman. B.. “Quantitative Analv~, ” sis,” 2nd ed., Xew York. McGraw-Hill Boo-k Co.. 1942. Seidell, Atherton. “Solubilities of Inorganic and Metal Organic (8) Compounds,” 3rd ed., X e w York, D. \-an Kostrand Co., 1940. I
LITERATURE CITED
American Public Health Association, “Standard Methods for the Examination of Water and Sewage,” 9th ed., 1946. ( 2 ) Betz, J. D., and Xoll, C. A , , J . A m . Water W o r k s Assoc., 42, 49 (1)
(1950).
( 3 ) Clark, T., Chem. Gal., 5, 1 0 0 ( 1 8 4 7 ) . ( 4 ) Corwin, J. F., Dresel, A. P.. and Osuch, G. E., As%L.CHEM..22, 653 (1950).
RECEIVLD Kovember 13, 1950. Presented a t the Fifth Southwest Regional N SOCIETY, Oklahoma City, Okla., December Meeting, . ~ V E R I C ~ CHLVICAL 10, 1949.
Determination of Rotenone by the Use of Mercuric Acetate IRWIN HORNSTEIN Cnited States Department of .Igriculture, Bureau of Entomology and Plant Quarantine, Beltsville, Md.
I
T HrlS been found possible to determine rotenone quantitatively by adding an excess of mercuric acetate in methanol to a solution of rotenone or its carbon tetrachloride solvate in ethylene dichloride. Whitmore (6) states that mercuric salts in methanol solution add HgX and OCH, to a double bond and in the process release 1 mole of HX. Analytical methods based on the addition to ethylenic double bonds of mercuric acetate in methanol have recently been described by hlarquardt and Luce ( 3 , 4 )and Martin (6). The reaction between rotenone and mercuric acetate may be formulated as follows: CHaO
\
0
C Hi
Sodium chloride, aqueous solution containing 35 grams per 100 ml. Ethylene dichloride, neutral to phenolphthalein. Phenolphthalein indicator, 1% in ethyl alcohol. PROCEDURE
Extract the root, powder sample, resin, or dust and crystallize the solvate from carbon tetrachloride ( 1 ) . Dissolve the precipitated carbon tetrachloride-rotenone solvate without drying and weigh into a 500-ml. Erlenmeyer flask, using approximately 50 ml. of ethylene dichloride. Pipet in 50 ml. of the mercuric acetate solution and let stand at room temperature for 25 minutes. Add 100 ml. of the sodium chloride solution and approximately 1 ml. of the 1% phenolphthalein solution. Titrate to the first pink end point with standard 0.1 N sodium hydroxide. Shake the flask vigorously during titration to remove acetic acid from the ethylene dichloride layer. Run a blank containing all reagents with each determination, duplicating all conditions. Each milliliter of 0.1 N sodium hydroxide after subtraction of the blank is equivalent t o 39.4 mg. of rotenone. To determine the effects of temperature and time on the rate of the reaction between mercuric acetate and rotenone, samples of pure rotenone and pure rotenone-carbon tetrachloride solvate were analyzed by the volumetric method described. The analyses were run at 25” and 0 ” C. and the time interval between the addition of the mercuric acetate reagent t o the rotenone sample and the titration was varied from 15 to 75 minutes. The results are shown in Table I. A time of 25 minutes a t room temperature was adopted.
Table I.
Effect of Temperature and Time on Reaction % Rotenone Found
Temp.,
The isopropenyl double bond in the rotenone molecule reacts quantitatively with the mercuric acetate-methanol reagent. For each mole of rotenone 1 mole of acetic acid is formed. Titration with 0.1 N sodium hydroxide gives a direct measure of the ro tenone present. In the procedure developed, sodium chloride is used to convert excess mercuric acetate to the chloride and thus permit direct titration of the acid Fith standard alkali. An excess of phenolphthalein is used to increase the sharpness of the end point. REAGENTS
hlercuric acetate, c . P . , reagent grade. Dissolve 50 grams in 500 ml. of methanol, add 0.20 ml. of glacial acetic acid, and dilute t o i50 nil. with methanol; the solution should be acid t o phenolphthalein; if necessary, filter. Sodium hydroxide, 0.1 iz’ and carbonate-free.
0
0
’ C.
Time, Minutes
15 20 45 60
I n pure rotenone
I n pure rotenoneCClr solvate“
91.3 94.0 99.7 99.8
65.8 68.3 71.7 71.9
Actual rotenone in pure rotenone-CClr solvate is 71.94%.
Table 11. Comparison of Gravimetric and Volumetric Methods of Determining Rotenone in Samples Sample Pure rotenone Pure rotenone-CClr solvate Tephroaia viryiniana root Derris root Cube resin Prepared 5% rotenone dust
Gravimetric Method, % 99.9 71.9 2.4 1.8 34.2 4 9
Volumetric Method, % 100 0 72.0 2.5 1 8 34 4 5.0
