Vol. 2 , s o . 1
A S A L YTICAL EDI T I O S
126
the liquid from the beaker to the spot plate. This should be emphasized because the operator is likely to obtain a false end point unless this precaution is observed. The indicator is very sensitive to molybdenum. The spot plate should be well washed after each determination. It should have its depressions filled with the indicator and should be allowed to stand about a minute before commencing operations to be sure that no color will develop from contaminations. Large quantities of ammonium acetate appear to prevent coloration of the indicator by sodium molybdate. Small quantities of iron and copper do not interfere. Of course, if rather a large quantity of iron were present and qhould become oxidized on the spot plate, the red color of ferric thiocyanate would appear. Precipitated lead molybdate is dissolved by mineral acids and also by alkalies unless they are very dilute. For this reason the titration must be made in a solution nearly neutral. The lead solution during titration should be kept near the boiling point by frequent heating. Metals which under the conditions of the determination form insoluble molybdates or form colored compounds with the indicator would interfere with the accuracy of the determination. I t would seem that the converse of this method might be used for the determination of molybdenum. Work is being done along this line in this laboratory. The lead molybdate
formed near the boiling p3int readily setkles, so that the liquid transferred to the spot plate need contain very little of this compound. B large drop of liquid should be used near the end of the reaction for transfer to the spot plate. A brown color which appears on the spot plate after some time should be disregarded. It is due to the dissolving lead molybdate. When the end point is apparent'ly reached reheat the contents of the beaker and test again in about 20 seconds. The reaction between the lead nitrate and ammonium molybdate may not have been completed. The precipitate is not readily dissolved by the indicator solution. If other metals which form insoluble molybdates are absent a neutral solution of lead nitrate may be titrated directly against the molybdat,e. It should be emphasized that for accurate work the solution before titration should be nearly neutral. The appearance of a permanent precipitate upon addition of ammonium hydroxide shows that the lead solut,ion has the proper hydrogen-ion concentration. Literature Cited (1) Epperson, Chemist-Analysl, 25, 9 (1918). (2) Kedesy, Mitt.k g l . .I.laterialprufungsaml Berlin-Lichleufelelde W e s t , 31, 173. (3) hlaag and hlcCollam, IND. ENG.CHEY.,17, 524 (1925). ( 4 ) Weiser, J. Phrs. C h e m . , 20, 640 (1916).
Perchloric Acid as Oxidizing Agent in the Determination of Chromium' James J. Lichtin VEROXA CHEMICALCo., NEWARE,.'IP
iX ACCURATE and rapid method for the determi-
A
nation of chromium in chrome-alum liquors and crystals has long been needed. The usual procedure of precipitating the chromium from its acid solution with ammonia as Cr(OH)3 and igniting to Cr203 necessitates a separate determination of iron and aluminum and gives high results due to the formation of some alkali chromate (1). The alkali oxidation methods are rather long, especially \\-hen the contents of impurities are required. -4new method has been found Tv-hich is an acid oxidation method, being based on the observed fact that perchloric acid oxidizes quantitatively chromic salts to chromates. I t also offers a rapid and accurate method for the separation of iron and aluminum from chromium. Perch!oric acid (60 per cent c. P.) is a very stable acid, does not liberate free iodine from its salts, and is not easily decomposed by concentrated hydrochloric and sulfuric acids. These properties make it a very suitable reagent for the purpose. The reactions involved are probably:
+ 3HCI04 =+2Cr03++Clz 3HC103 + 202 + + H 2 0 + C1, + 20' + H20
Cr203 and
or and
3HC103 = HCIOl H20 Cr203 2HC10r = 2CrO3 K20 f 2HC104 = 2KC104
It was found that an excess of perchloric acid was necessary to obtain very accurate and concordant results, and the following procedure was adopted. Method
Weigh out 1 gram of chrome-alum crystals or its equivalent of chrome-alum liquor into a 100-cc. Erlenmeyer flask. Add 5 cc. of water and 5 cc. of perchloric acid (60 per cent 1
Received November 15, 1929.
J.
c. P.) and heat on a hot plate under the hood. Keep the flask covered with a small funnel, the stem of which has been cut off, in order to prevent loss by spattering. The reaction occurs when the solution has been evaporated to about half its volume. After the reaction is over, which is shown by the change of the chromic green to a deep orange chromate color, heat the flask for an additional 5 minutes on the hot plate. Remove from the hot plate and allow to cool to room temperature. Add 40 to 50 cc. of water, heat to boiling, and boil for 2 minutes to drive off any free chlorine that may still be present. The absence of free chlorine is shown when starch-iodide paper held in the neck of the flask, during boiling. is not turned blue. Transfer and wash the chromate liquor from the Erlenmeyer into 150-cc. beaker, add ammonia (1:l) solution to a slight excess. and bring to a boil. The iron and aluminum present are completely precipitated, filtered, washed, and determined in the usual way. Acidify the filtrate with hydrochloric acid ( I : 1) and determine the chromium iodometrically. Wash the acidified chromate solution into a 500-cc. glass-stoppered bottle, containing 25 cc. of 15 per cent potassium iodide solution and 5 cc. of concentrated hydrochloric acid. Add 60 cc. of watcr and let stand for 1 minute. Titrate the liberated iodine with 0.1 N thiosulfate solution. Add starch solution towards the end and continue titration until the green color appears. 1 cc. of 0.1 11- thiosulfate is equivalent to 0.002533 gram of chromic oxide. Results
This method has been used in the writer's laboratory for the last six years and has always given concordant results, as shown in Table I.
I.VDUSTRIAL i l S D ENGINEERISG C H E X I S T R Y
January 15, 1930 RUN
T a b l e I-Experimental Data CriOa Rus
Per cent 1 2
3 4
6.20 6.20 8.60 8.60 8.36 8.36 7.68 7.70
5
6
127
tested against a n alkali oxidation method, which it checked very closely, as is seen from Table 11.
CrzOa Per cenl 15.05 15.05 8.77
T a b l e 11-Comparison
of R e s u l t s b y Alkali O x i d a t i o n and P e r c h l o r i c Acid M e t h o d s CHROMIC OXIDEIIY ALKALIOXIDATION CHROMKOXIDEBY METHOD PERCHLCIRIC METHOD Per cenl P e r cenl 15.12 15.05 l5,08 15.10 1 5 , 10 15,lO 1.510 I5 05
8.75
15.10 15.12
RCN
1 2 3 4
Comoarative Accuracv of Method The accuracy of this method was tested b y comparing it with c. P. potassium dichromate which had been dried a t 110' C. This salt is used as a primary standard in iodomet r y . for the standardization of thiosulfate solution. A 0.2-gram sample of the dichromate v a s reduced by adding to its aqueous solution 5 cc. of alcohol and 2 cc. of concentrated hydrochloric acid. The solution was evaporated to dryness o n the water bath. It was taken u p with a little Jvater and oxidized by the addition of 5 cc. of perchloric acid (60 per cent c. P.) as described above. The oxidized solution required the same amount of thiosulfate solution for its titration a. the original dichromate solution. It was also
The perchloric acid method is more accurate than the ~1~~ advantages of this method over gravimetric the alkali oxidation method are its rapidity, the comparatively easy separation of iron and alumirlum from chromium. and the purity alld stability of the oxidizing reagent. Acknowledgment This investigation chief ,,llen1ist of the indebted to hinl for
J
at the suggestion of J. Ehrlich. chemical ~ c0., ~ alld~the FT.riter ~ is~ instructive suggestions in this Tvork, -
Literature Cited (1) Treadwell and Hall, "Analytical Chemistry." 4th ed., Vol 11, p. 102.
A New Application of the Abb6 Refractometer in the Analysis of Lacquer Thinners' J. D. Jenkins PITTSBURGH PLATE GLASSCOMPANY, MILWAUKEE, WIS.
T
HE use of the dispersion scale on an Abbe refractonie-
ter has been found to be a rapid and comparatively accurate method of determining the toluene or benzene content of lacquer thinners. While taking the dispersion on a number of common solvents and diluents it was noticed t h a t they were sharply divided into two classes by this determination-the dispersion scale reading of the aliphatic compounds, alcohols, esters, ketones, etc., areraging very close t o 41.4, while the aromatic hydrocarbons fell very near 36.6. The determinations on a number of these materials are shown in the accompanying table. A study of this table will show the extent of variation of SOLVENT Ethyl alcohol, tech. Methyl alcohol, synthetic Isopropyl alcohol, tech. Isopropyl alcohol, c. P. Pl-ormal butyl alcohol, tech. Sec-butyl alcohol, tech. Tertiary butyl alcohol, tech. Amyl alcohol, tech. Isoamyl alcohol, c. P. Acetone, tech. Methyl ethyl ketone, tech. Ethyl acetate, tech. Isopropyl acetate, tech. Isopropyl acetate, tech. Normal butyl acetate, tech. Sec-butyl acetate, tech. Pentaacetate, tech. Amyl acetate, c . P. Butyl propionate, tech.
REFRACTIVE DISPERSION ISDEX SCALE 25' C. READING
the dispersion reading from the mean value for aliphatic X more marked variation is evident compounds-41.4. among the aromatic compounds. The large majority of lacauer thinners in Dresent use. however. contain toluene as 1 Received September 16, 1929. Presented before the Division of Paint and Varnish Chemistry a t the 78th Meeting of the American Chemical Society, Minneapolis, Minn , September 9 t o 13, 1929.
the only aromatic hydrocarbon. When homologs are used they are generally present in relatively small amounts. The presence of large proportions of higher homologs of toluene will usually be evident b y the odor or evaporation characteristics, and in this case the results of the refractometer test should be checked b y a determination of the per cent insoluble in 75 per cent (by weight) sulfuric acid. The dispersion scale reading of rnixtures \\as found to be a straight-line function of the volume composition within the limits of accuracy of the determinations. This fact permits a yery rapid and reasonably accurate determination of the content of toluene. SOLVENT Cellosolve, tech. Methyl Cellosolve, tech. Butyl Cellosolve. tech. Cellosolve acetate, tech. Ethyl lactate, synthetic Methyl oxybutyrate, tech.
DISPERSION REFRACTIVE IKDEX SCALE 250 READING 1,4073 41.4 41.5 1,4035 41.3 1,4183 41.3 1,4048 41.3 1.4116 41.4 1.4099
c.
Petroleum naphthas: Oleum spirits V. hl. & P. naphtha Toluene substitute Toluene substitute
1,4364 1.4322 1,4059 1.4133
40.7 40.8 41.1 41.4
Turpentine, gum Benzene, tech. Toluene, tech. Xylene, c. P . Xylene, tech. Solvent naphtha, tech.
1,4533 1,4955 1,4930 1,4940 1.4940 1,4846
40.0 35.4 35.6 35.9 36.0 36.5
On a mixed lacquer thinner where the hydrocarbon is toluene, it has been found that the average accuracy is about f1 per cent of toluene. This is sufficiently accurate on most work. such as the analysis of comDetitive products, because the rariation from b a t l h to batch-will be of the same order Of ma@itude' This method does not indicate the presence of petroleum
