Table 1.
KO
of
Date
Trials
2/13/56 11/16/56
3 3 6 1
Av .
2/29/56 3/ 2/56 3/ 6/56 3) 7/56 3/10/56
Av .
2/18/56 11/16/56
Av. 2/23/56
1 1 2 1 6 3 3 6 3
Calibration Data for an Integrating Motor
Resistance, Ohms
Current Level, Ma.
10,000 10 ,000
2.170 1.795
4,000 4,000 4,000 4,000 4,000
5.066 5.086 5.032 5.035 5.049
2,500 2,500
8.090 8.304
2 ,000
Exptl. Factor, Meq./ Count
x
10.67
108 0.1380 0.1382 0.1381 0.3421 0.3412 0.3416 0.3411 0.3413 0.3414 0.5447 0.5443 0.5445 0.6794
Av. Devn. between
Trials, P.P.T.
*l.O 0.7 0.9
0.1381
...
...
-0.06
0.90 &O 0.5 0.3 0.8
0.3412
0.9054
+o. 04 ...
1.357
-0.07
1.695 2.711
+ O . 06
4
1,500
13.82
0.9054
0.3
1,000 1,000 1,000
20.73 20.88 21.62
800 800 800
27.22 27.70 27.63
500
40.22
1.354 1.357 1.356 1.356 1.691 1.696 1.696 1.694 2.711
1.3 0.7 0.7 0.9 1.1 1.0 0.3 0.8 0.6
Av.
7%
0.5
3 5 3 11 3 3 3 9 4
11/16/56
103
...
2/18/56
Av.
x
... ...
2/12/56 3/14/56 11/16/56 . . 2/14/56 3/10/56 3/14/56
Theoretical Factor, Diff. in Me%/ Count Factor,
0.5443 0.6797
+ O . 04
...
Average deviation of six results shown.
At least 100 motor counts or 600 seconds were used in each calibration run. The leads to the electrolysis cell were either shorted or the current was discharged through a 1000-ohm resistance. No difference in result was observed. RESULTS
Table I shows the theoretical and actual motor factors obtained. The theoretical factors were calculated from the relationship :
(meq. per count)
-*:"- + 2.7 x 10-6
where R is the ohmic value of the parallel resistance. This relationship is b, where x is of the type, y = mx taken as 1/R, 1.354 is the slope, m, determined by the ratio of two points; and 0.0027 is a constant. The constancy of a given motor factor calibration with time is very good to excellent up to a t least 9 months. Variations are largely due to inconstancy
+
of line frequency and voltage, neither of which was controlled during these measurements. The effect of temperature variation is unknown, as all measurements were done a t room temperature in order to simplify the apparatus as much as possible. DISCUSSION
An empirical relationship of this type greatly extends the usefulness of this integrating motor, as lengthy calibration a t many current levels is unnecessary. Calibration a t two current levels employing two different resistors should be sufficient to establish the motor constants. By a simple calculation the motor factor may be accurately determined for any combination of resistance setting and current level. The only additional criterion is that the voltage input to the motor be within the linear velocity region. If the necessary motor calibrations are obtained with apparatus that is independent of current or frequency fluctuations, the motor integrator should vield titration results indeDendent of these common sources of error. LITERATURE CITED
Amick, R. M., senior thesis, Princeton University, May 1955. Bett, N., Nock, W., Morris, C., Analyst 79, 607 (1954).
Parsons, J. S., Seaman, W., Amick, R. M., ANAL.CHEW27,1754 (1955). Reilley, C. N., Adams, R. N., Furman, N. H., Ibid., 24, 1044 (1952). Reilley, C. N., Cooke, W. D., Furman, N. H., Ibid., 23,1030 (1951). RECEIVEDfor review August 6, 1956. Accepted January 23, 1957.
Colorimetric Determination of 1,2-PropanedioI and Related Compounds LAWRENCE R. JONES and JOHN A. RlDDlCK Commercial Solvents Corp., Terre Haute, Ind. Propylene glycol dehydrates and rearranges in concentrated sulfuric acid to a mixture of allyl alcohol and the enolic form of propionaldehyde. The mixture forms a violet-colored complex with ninhydrin (triketohydrindene hydrate) in a strong sulfuric acid solution. The color is suitable for quantitative measurement of propylene glycol at a wave length of 595 mp. It follows Beer's law in the range of 5 to 50 y and has an expected
1214
ANALYTICAL CHEMISTRY
accuracy within +2% and a precision within k l % . This colored complex is specific for propylene glycol and its polymers in mixtures of glycols.
M
published methods on the determination of propylene glycol (1,%propanediol) have been modifications of the Malaprade (19) reaction, which involves oxidation of the vicinal glycols with periodic acid to aldehydes, OST
followed by various means of detecting the aldehydes. Warshowsky and Elving (16) and Cannon and Jackson (2) used the polarograph, Braumel(1) ultraviolet absorbance, and Reinke and Luce ( I S ) and Hoepe and Treadwell (8) volumetric procedures to measure the acetaldehyde. Dal Nogare, Norris, and Mitchell (5) and Jordan and Hatch (10) determined acetaldehyde as iodoform, while Desnuelle and Naudet (6) used the colorimetric procedures of Schryver (1.4)
and Fromageot and Heita (7) to distinguish acetaldehyde. Recently, Dal Nogare and Mitchell (4) extended the Komarowsky (11) reaction for monohydric compounds to the determination of glycols. p-Hydroxybenzaldehyde was condensed with glycols in concentrated sulfuric acid to yield a colored complex suitable for quantitative measurement. Propylene glycol can be determined in the presence of ethylene glycol, although some interference occurs. This paper presents a colorimetric procedure for the determination of propylene glycol which is similar to the Komarowsky reaction. This test is, however, more specific for propylene glycol, as other glycols or monohydric alcohols do not interfere, and therefore has a greater sensitivity than other published techniques. According to Curme and Johnston ( S ) , propylene glycol is dehydrated by concentrated sulfuric acid to a mixture of allyl alcohol (2-propen-1-01) and the enolic form of propionaldehyde: -H10 CH3--CHOH--CH*OH _ CHI= CH-CHeOH CHa-CH=CHOH
+
The mixture of allyl alcohol and propionaldehyde can be determined by a modification of the Jones and Riddick (9) procedure for propionaldehyde. Allyl alcohol and propionaldehyde both form a violet-colored complex with ninhydrin (triketohydrindene hydrate) in the presence of concentrated sulfuric acid. The complex is suitable for quantitative measurement a t 595 mp and follows Beer's law in the range of 5 to 50 y of propylene glycol. The proposed test is specific for propylene glycol or its polymers in mixtures of other glycols. It is extremely sensitive and has been adapted to the determination of as little as 1 part of propylene glycol in 1000 parts of ethylene glycol. PROCEDURE
Apparatus. Spectrophotometer, Beckman Model DU. Tubes, Lewis-Renedict, graduated a t 12.5 and 25.0 ml., Corning No. 7860. Water bath, 70" C. Cooling bath, 25" C. Reagents. Propylene glycol standard, purified by fractional distillation. Analyzed 99.5% by the method of Siggia (15). Sulfuric acid, specific gravity 1.84, hlallinckrodt, low nitrogen. Ninhydrin reagent, 3y0 solution (Eastman Organic Chemicals No. 2495), 1,2,3-triketohydrindene crystals in a 5% aqueous sodium bisulfite solution. This solution is stable a t room temDerature (9). Preparation of Calibration Curve. Prepare a solution of propylene glycol standard in water t o contain 1.00 mg. per ml. Transfer 0.0, 0.50,
Table I.
Determination of Propylene Glycol in Glycol Mixtures
Composition of Mixtures, Wt. yo Ethylene Diethylene Triethylene Propylene glycol glycol glycol glycol 40.50 67.32 78.60 95.00 99.00 99.50 99.99
19,so 17.33 8.25
...
... ...
...
19.50
...
3.15
... ... ... ...
1.00, 3.00, and 5.00 ml. of the standard solution to 100-ml. volumetric flasks and dilute each to volume with water. Transfer 1.00 ml. of each diluted standard to Lewis-Benedict tubes, carefully add 5.00 ml. of concentrated sulfuric acid, and mix. Stopper the tubes and immerse in the 70" water bath for 10 minutes. Remove and cool to 25" C. Add 0.20 ml. of ninhydrin, mix, and allow to stand for 1 hour at 25' C. Dilute the contents to 12.5 ml. with concentrated sulfuric acid, mix, and allow to stand for an additional 5 minutes to develop full color intensity. (The intensity will increase rapidly upon the addition of the acid, reaching a maximum after 5 minutes. The color is stable for a t least 30 minutes.) Transfer to a 1-cm. Corex cell and read the absorbance a t 595 mp, using the zero prepared standard for setting the spectrophotometer. When concentrations are plotted against absorbance on linear graph paper, the above standards equal 0, 5,. 10,, 30,. and 50 Y of propylene glycol. Determination of ProDvlene Glvcol in Glycol Mixtures. @&gh 1 t'o 5 grams of the glycol mixture, the amount depending upon the expected propylene glycol content, into a 100ml. volumetric flask and dilute to volume with mater. Transfer a 1.00ml. aliquot containing 50 y or less of propylene glycol to a Lewis-Benedict tube and treat in the same manner as the calibration standards. Prepare a blank, similarly treated, from 1.00 ml. of water. Calculate the percentage of propylene glycol in the glycol mixture from the standard calibration curve. Replicate results on the determination of propylene glycol, in known mixtures of other glycols, are presented in Table I. The glycols used were purified by fractional distillation. The samples were prepared so that a 1.00-ml. aliquot contained 50 y of propylene glycol. EXPERIMENTAL
The effects of several variables on the degradation of propylene glycol and color formation were investigated. These variables included the time and temperature of degradation, the absorption curve, stability and intensity of color, time and temperature of color formation, concentration of acid and
20.20 15.35 10.00 5.00 1.00 0.50 0.010
Propylene glycol Found, Wt. yo 20.25 15.35 10.05 5.051.00 0.50 0.010
20.20 15.34 10.00 4.99 0.99 0.49 0,010
ninhydrin, conformity to Beer's law, and interference from other compounds. Time and Temperature of Degradation. The completeness of the degradation of propylene glycol was measured by the intensity of the color formed with the ninhydrin. Propylene glycol dehydrates readily in strong sulfuric acid a t elevated temperatures. The heat of dilution between the 1.00-ml. aliquot of sample and the 5.00 ml. of concentrated sulfuric acid was used in the preliminary testing, but consistent results were not obtained. Various temperatures were tested and a Jeaction time of 10 minutes a t 70" mas found optimum. Maximum degradation was reached a t 100" in less time, but the reproducibility of the reaction was impaired by the volatility of the dehydration products. Absorption Spectrum. The violet complex resulting from the reaction of propionaldehyde (9) and ninhydrin yielded a maximum absorption a t 595 mp. As allyl alcohol is also formed in the reaction, a spectral-absorbance curve was prepared from purified allyl alcohol as well as propylene glycol and propylene oxide. The four compounds gave a colored complex with identical absorbance curves. It is believed that the compounds dehydrate and rearrange to the same type of olefinic structure. The sulfuric acid dehydrates the hydroxyl group on the second carbon atom to form allyl alcohol and the enol form of propionaldehyde as the primary reaction. However, in the prolonged treatment with concentrated acid, it is believed that the remaining hydroxyl is also dehydrated t o yield a propadiene compound that couples with the ninhydrin. This observation is further substantiated by the fact that neither propane nor propene reacts. These observations are in agreement with the suggestions of Dal Nogare and Mitchell ( 4 ) , mho postulated the formation of olefins a s intermediates in the reaction of hydroxy compounds and aromatic aldehydes. Concentration of Acid. The optimum acid content for color development was approximately 83 volume %. However, as the differences in the intensity of color, developed in different acid concentrations, were slight, a more convenient ratio of water to acid was VOL. 29, NO. 8, AUGUST 1957
1215
used. One milliliter of aqueous sample, 5.00 nil. of acid, and 0.20 ml. of reagent, corresponding to a n acid content of 80.6 volume %, were chosen. Time and Temperature of Color Development. The complex between ninhydrin and the dehydration products of propyltne glycol is heatsensitive. Attempts t o speed color development a t elevated temperatures resulted in loss of color intensity; therefore the ninhydrin reagent must be added after the reaction mixture is cooled at 25’. Addition of 0.20 ml. of reagent to the cooled mixture does not raise the temperature sufficiently to affect color stability. Total color develops in ap: proximately 50 minutes at 26” and remains constant for 30 minutes more. The color development isunusual, in that after the samples stand for 50 to 60 minutes and are diluted t o 12.5 ml. with concentrated acid, the color rapidly gains in intensity for 3 to 5 minutes. A 1-hour period for color development at 25O, followed by dilution with concentrated acid and a n additional 5minute waiting period, was chosen as optimum. Reagent Concentration. The concentration and stability of the ninhydrin reagent have been discussed for propionaldehyde (9). T h e same conditions prevailed for the mixture of propionaldehyde and allyl alcohol. INTERFERENCE A N D APPLICATION T O OTHER COMPOUNDS
Additional glycols were tested to determine the specificity of the ninhydrin reaction toward propylene glycol; ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, dipropylene glycol, 1,3-propylene g1: col, 1,2,3-propylene glycol (glycerol), 1,3, butylene glycol, 1,4-butylene glycol, 2,3butylene glycol, and Z-ethyl-l,3-hexandiol. Only 1,Bpropylene glycol and dipropylene glycol gave a positive test,
indicating that the test is specific for propylene glycol and its polymers in a mixture of glycols. Because the Komaroa-sky (4, 11) test reacts n ith aldehydes, ketones, and monohydric compounds a s well as glycols, these types of compounds were tested. Previous work (9)showed that propionaldehyde was the only aldehyde that gave a color reaction, while ketones gave no color. The monohydric compounds tested included methanol, ethanol, 1-propanol, 2-propanol, 2methyl-1 -propanol, 1-butanol, 2-butanol. 3-methyl-1-butanol, 3-methyl-1-butanol, 32-methyl-2-butano1, I-pentanol, pentanol, and allyl alcohol (2-propen-l01). Only the latter gave the violet complex with ninhydrin. This method has been used for propionaldehyde (9) and could be adapted to the determination of allyl alcohol itself or any compounds that would yield either allyl alcohol or propionaldehyde. Compounds tested included polymers and derivatives of propylene glycols and condensation products of ethylene oxide or propylene oxide and alcohols such as Cellosolves, Carbitols, and Dowanols: propylene oxide, dipropylene glycol, polypropylene glycol P-750, polypropylene glycol P1200, propylene glycol monobutyl etber, Pluronic L-60 (a propylene oxide-ethylene oxide polyether), ethylene oxide, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutpl ether, ethylene gl!-c.ol diethyl ether, diethylene-glycol diethyl ether, and diethylene glycol monobutyl ether. Compounds containing the propylene molecule reacted quantit7tively. The others did not react. Apparently the polymers and derivatiL es of propylene oxide are dehydrated as readily as the simple glycol in strong sulfuric acid. They gave the coloi intensity expected when compared to propylene glycol-example, 50 y of dipropylene glycol yielded a color in-
tensity equivalent to 56.6 y of propylene glycol, compared to 56.7 y calculated. Several miscellaneous compounds were investigated: propane, propene, 3chloro- 1,2-propanediol, 3-chloro- 1,2epoxypropane, 1,2-dichloropropane, 1chloro-2-nitropropane, 2-chloro-1-nitropropane, I-chloro-3-nitropropane, 1amino - 2 - propanol, and 2 - amino - 1propanol. Only 2-amino-1-propanol yielded a positive reaction. LITERATURE CITED
(1 ) Braumel, I. &I., Ax.4~.CHEII. 26. 930 (1954). (2) Cannon, IT. -4.,Jackson, I,. C., Ibid.. 24. 1053 (1952). (3) Curme, G. O., Johnston, F., “Gl~‘cols,’’ p. 3, Reinhold, Sew York, 1952. (4) Dal Sogare, S., Mitchell, J., Jr., ASAL. CHEJI.2 5 , 1376 (1953). (5) I h l Nogare, S., Norris, T. D., l‘litchell, J., Jr., Ibid., 2 3 , 1473 (1951). (6) Desnuelle, P., Saudet, M., BiiII. SOC. chim. France 12,871 (1945). ( 7 ) Fromageot, C., Heitz, P., Mikrochim. Acta 3, 52 (1928). (8) Hoepe, G., Treadwell, IT. D., Helv.Chim. Acta 2 5 , 353 (1912). (9) Jones, 1,. R., Riddick, J. A , , AXAL. CIIEM.26, 1035 (1954). 10) Jordan, C. B., Hatch, V. 0 . : I b i d . 25,636 (1953). 11) Komarowsky, A , , Chem.-Zfg. 27, 807 (1903). 12) RIalaprade, L. A., Bull. ~ O C . chim. France 43, 683 (1928). 13) Reinke, R. C., Luce, E. S., IND. ESG.CHEX.,ASAL. ED. 18, 244 (1946). 14) Schryver, S. B., Proc. Roy. SOC. (London) 82B, 226 (1909).
Riggia, S., “Quantitative Organic Analysis via Functional Groups,” p. 16, Wiley, New York, 1954. 16) Warshowsky, B., Elving, 1.’ J., IND. ESG. CHEX.,ASAL. ED. 18, 15)
253 (1946). RECEIVEDfor reviex Xovember 1956. hccepted January 30, 1957.
29,
Removal of Interferences in the Scott-Sanchis Fluoride Determination R. E. SHOUP Ohio Department o f Health, Division o f laboratories, Columbus
b
Aluminum, carbonates, organic colorproducing material, hydroxides, iron, manganese, phosphates, and SUIfates-interfering substances commonly found in public water supplies-are removed b y precipitation with cadmium and mercury in the presence of sufficient boric acid-sodium hydroxide buffer to maintain a pH of 8.0.
1216
ANALYTICAL CHEMISTRY
A
IO, Ohio
of ions cause error in fluoride determination. Although distillation ( I ) is efficient, it is time-consuming, and must be manipulated with precision to avoid contamination of the distillate. By precipitating the interfering substances with cadmium and mercury, excellent recovery of fluorides has been obtained by the Scott-Sanchis SUMBER
( 2 ) method. Sufficient boric acidsodium hydroxide buffer is used to maintain a pH of 8.0, or over, after addition of about 0.25 gram of cadmium nitrate per 150 ml. of sample. The precipitate thus formed removes aluminum, color from organic matter, iron, manganese, and phosphates, and reduces the carbonates and hydroxide
