to blue-green. Calculate the per cent carboxylate. An analysis can be completed in 90 minutes. In general, clean the equipment with much distilled water, then alcohol, ether, and last by suction; if necessary, use 5% aqueous hydrofluoric acid a t first. Analysis of silver benzoate for benzoate requires a dry condenser jacket to avoid crystallization of benzoic acid; add 1 gram of water each time in five distillations instead of the usual four; otherwise the procedure is the same as that used with the liquid carboxylic acids. RESULTS AND DISCUSSION
Fable I clearly shows a reliable 90minute analysis for acetate, propionate, butyrate, dduoroacetate, and penta-
fluoropropionate through reaction of phosphoric acid with the metal or organometal carboxylate. HaPo'
acid boils a t 187'. An altered method without water in the condenser jacket allows analysis for benzoate, but not for cinnamate. These distillations have a resemblance to the Duclaux procedure, which is of little use today.
+ AgOCOCsHs = AgHZPOh + CzHsCOOH
Analyses for formate run 1 to 6% low because of decomposition. Fluoroacids give satisfactory analyses, in keeping with the high bond energy (1) of 107 kcal./mole for C-F, but chloroacids furnish some hydrochloric acid. Complete transfer of the liberated acid into the receiver requires an adequate volatility of the acid with s t e m and an adequate solubility of the acid in water a t 20" C.; analysis of n-valerate should be possible since the
LITERATURE CITED
(1) Anderson, H. H., J . Am. Chem. SOC.
79, 326 (1957). (2) Dahmen, E. A. M. F., Chim. Anal. (Paris)40,430 (1958). (3) Fritz, J. S., Yamamura, S. S., ANAL. CHEM.29, 1079 (1957). (4) Sorensen, P., Ibid., 28, 1318 (1956). ( 5 ) Veibel, S., Chim. Anal. (Paris) 41, 12 (1959). HERBERT H. ANDEMON Chemistry Department Drexel Institute of Technology Philadelphia 4, Pa.
Paper Chromatography of Iron(II) and Iron(III) SIR: Separation of different valencies of the same element is important from the theoretical as well as the practical point of view. Iron is widely used in analytical chemistry in the ferrous and ferric states. For the separation of Fe+* and Fe+* Stevens (1) used a mixture of 1-butanol-ethanol-acetic acid-water (40:25:25:35) after an overnight equilibrium. The salts were adjusted to p H 4 prior to chromatography and spotted on pre-acid-washed Whatman No. 1 filter paper. The method is rather tedious. It needs a close adherence to pH, an acid-washed paper, and an overnight equilibrium. An effort waa therefore made to develop a method of paper chromatographic separation which does not suffer from these limitations. A number of solvent systems were studied. Those that give best results are : (4M) Ha-1-butanol-acetic arid-acetone
S & S 2043a chromatography paper gives good separations with this method.
butanol. If the proportion of butanol is increased, separation between Fe+2 and Fe+s improves ( M I = 0.52) but the Fe+S spot begins to spread. The addition of acetic acid and acetone to the 4 M HC1-1-butanol system not only improves the separation but also keeps the spots compact. It therefore appears that the presence of acetic acid and acetone in solvent system 1 is helpful but not necessary.
Comparison of the use of ammonia gas, oxine, and ferroferricyanides as detectors showed that for moderate concentrations NHa (gas) is the most convenient, while the cyanides are the most sensitive. NH, (gas) alone gives a green spot with Fe+* and a brown
A 0.1M solution of ferrous sulfate was brought to the required pH by the addition or either dilute sodium hydroxide solution or dilute hydrochloric acid. Similarly the ferric chloride solution was adjusted to the required pH. The NaC1-HC1 buffer was added. The buffered sample was then applied to Whatman No. 1 paper for experiments summarized in Table I. The paper was developed with the ascending technique. The detector was ammonia gas. All chemicals used were reagent grade.
Table 1. RI Values of Fe+* and Fe+' at Different pH Values with 4M HCI1-Butanol-Acetic Acid-Acetone
(1:lr 1:l) pH of Sample 2.25 2.48 2.93 3.95
Time of Develop., Hr. 1
1 1 1
Rf Fe+* 0.31 0.31 0.22 0.14
Fe+* 0.74 0.74 0.60
0.74
( 1 : l : l : l ) (1)
I-Butanol-acetic acid4M HCl ( 1 : l : l ) (2)
Table It.
4M HCI-ethanol-acetic acid (1 :1: 1) (3) In none of these systems was overnight equilibration necessary. The results for system 1 are summarized in Table I. Experiments were carried out to see if acetic acid and acetone are necessary for good separations in solvent system 1. From Table I1 it follows that it is possible to obtain a satisfactory separation with the 4M HC1-1-butanol system. Best results are obtained ( M I = 0.24) if the two solvents are mixed in the ratio 1 to 3. No separation could be obtained with a smaller proportion of
System No.
2
Separation of Unbuffered Samples at pH 4 with Different Solvent Systems
Rf
Composition of System FeC2 4M HC1-1-butanol0.22 acetic acid-water
Fe 0.71
(1:l:l)
4a
4b 4c 5
4M HC1-1-butanol (1:l and 1:2) 4M HC1-1-butanol (1:3) 4M HC1-1-butanol (1:4 and 1:5) 4M HC1-1-butanolacetone (1:1: 1)
Remarks Spot of Fe+' lees compact than with solvent 1
N o sep.
No sep.
0.19
0.43
Both spots compact
0.06
0.58
Fe+a spot lese compact than with system 4b
0.21
Spreads with solvent front
VOL 34, NO. 10, SEPTEMBER 1962
1341
spot with Fe+3. Separation is not impaired by 100yo impurities of Al+3 and Cr+3. Good separations can be obtained with this solvent system a t any pH from 1 to 5 without the use of buffers (Table 111). As can be seen from Table 111, even unbuffered samples separate well. Best separations are obtained a t pH 1 and 4. A close control of pH is not essential for good separations. Further work is in progress and results will be published later.
ACKNOWLEDGMENT
Table 111. Separation of Unbuffered Samples at Different pH Values with Solvent System 1
The authors are grateful to A. R. Kidwai, head of the Department of Chemistry, for research facilities.
Rf
LITERATURE CITED
PH
Fe + 2
Fe +a
1 2 3 4 5
0.17 0.16 0.17 0.15 0.24
0.80 0.62 0.68 0.78 0.69
(1) Stevens, H. M., Anal. Chim. Acta 15,
538-42 (1956).
MOHSIN QURESHI ICJBAL AKHTAR Chemical Laboratories Aligarh Muslim University Aligarh, India
Rapid Potentiometric Determination of Ascorbic Acid SIR: Visual titration with 2,6dichlorophenol-indophenol is the official method for determining ascorbic acid (4), but it cannot be used in colored solutions nor can it be readily automated. Birch, Harris, and Ray ( I ) suggested potentiometric titration with a bright platinum electrode but encountered the following difficulties : high variable initial potential, slow equilibrium, drifting of potential, and indefinite end point. Harris and Olliver (2) improved the end point by using a mercury-platinum electrode. However, the electrode had to be prepared fresh each day and the response was sluggish past the end point. Kontio and Casagrande (3) raised the p H from the usual pH of 2 to 3-4 and obtained a 40-50 mv./ml. change a t the equivalence point. Carbon dioxide was used to prevent oxidation by air, and each type of solution required a different optimum pH. Using an untreated platinum electrode (or one aged in water for one week) eliminates the above difficulties, and ascorbic acid can be titrated rapidly a t pH 2. The method has been applied to an automatic titrator and is recommended even for colorless solutions.
mately 70% of the dye required to reach the end point was added. One-ml. aliquots of dye were then added and the potential was measured about 30 seconds after each addition until the end point was passed. The baseline potential was the lowest mv. reading obtained during the titration. The instrument was set to 35 mv. above the baseline potential. The standard and test solutions were titrated to %IO mv. of this arbitrary end point. RESULTS AND DISCUSSION
Figure 1 is an example of a complete titration curve. The readings before the equivalence point are stable after about 30 seconds of stirring, while past the equivalence point the readings are stable immediately. The slope between 2 5 4 5 mv. above the baseline is 230 % 50 mv./ml. (average of 27 plots by four different workers). Therefore, a t the arbitrary end point of 35 mv. above the baseline, an error of % l o mv. has an effect of only %0.6% for a 15-ml. titer. The baseline potential of a multiple vitamin solution containing ascorbic acid was measured daily. Each day the baseline potential decreased, asymptotically becoming stable after three successive days. This effect occurred
EXPERIMENTAL
Apparatus. Manual potentiometric
measurements using standard calomel and platinum electrodes were made with a Beckman Model G p H meter and automatic titrations were performed on a Fisher Model 36 Automatic Titrimeter. The platinum electrode was used as received from the manufacturer and kept immersed in water between use. Acid-cleaned platinum electrodes become usable only after 1 or 2 weeks of aging in water. Procedure. The U.S.P. method (4) was followed except for the end point. First the approximate baseline potential was determined. Approxi1342
ANALYTICAL CHEMISTRY
290 270
I70
i 1,
even when an electrode was used after a period of storage in water. During a given day, a different type of solution (in this particular case orange juice as compared to a vitamin mixture) exhibited a significantly different baseline potential, and therefore, it is recommended that the baseline be redetermined for each new type of solution. Amounts of thiamine, riboflavin-5phosphate, or nicotinamide equivalent to the ascorbic acid in solution did not interfere with the assay; nor did a mixture of these vitamins with sodium pantothenate and pyridoxine interfere, even when decomposed (120’ C. for 1 hour). Also, no difficulty was encountered in determining the ascorbic acid content of orange juice. The precision of the method calculated from 35 manual titrations on standard solutions, vitamin mixtures, and orange juice was +o.5y0(average deviation) for 15- to 16-ml. titer. The precision of the automatic titration using the Fisher Titrimeter was within *O.ly0. Recovery experiments on vitamin mixtures, both fresh and decomposed, indicated a maximum error of %l.Oa/,. Assays of vitamin mixtures containing known amounts of ascorbic acid also indicated a maximum error of f1.0%. LITERATURE CITED
,
2
,
,
4
,
, , ,
6
,
,
,
,
,
, , ,
8 I O 1 2 1 4 M L OF DYE
,
,
]
1618
Figure 1. Titration curve of ascorbic acid with 2,6-dichlorophenolindophenol
(1) Birch, T. W., Harris, L. J., Ray, S. N., Biochem. J . 27, 590 (1933). (2) Harris, L. J., Olliver, M., Zbid., 36, 155 (1942). (3) Kontio, V. P., Casagrande, V. M., Suomn Kemistilehti 18B. 9 (1945). (4) “Pharmacopoeia of the United States of America,” 16th Rev., p. 66 (1960).
EDMUND E. SPAETH~ VICTORH. BAPTIST MARTINROBERTS
Don Baxter, Inc. Glendale 1, Calif. 1 Present address, California Institute of Technology, Pasadena, Calif.
