A S A L Y T I (2.4L E DZ TZ0.V
124
volatile all of the phenol. Then connect the flasks to the condensers and place the graduates in position to receive the distillates. Apply heat and catch 240 ml. of distillate in each case. Kom put aside these distillates while the flasks and condensers are cleaiied and rinsed with distilled water. Return the portions of distillate to their respective flasks. To No. 1 add 10 ml. of the chromic acid solution mentioned and to KO. 2 add 10 ml. of sulfuric acid. Connect the flasks to the condensers and apply such heat as mill bring the contents of each to the boiling point in from 40 to 45 minutes. As soon as the boiling point is reached, remove the flames. Let the contents of the flasks stand hot for 30 minutes. During this time, if sufficient care has been taken in bringing the solutions just to the boiling point, no more than a few drops of distillate will have passed over, most of the steam formed having condensed in the bulbs of the steam traps. Following the period of standing, apply the full flame and collect 225-1111. portions of distillate. S o . 2 graduate now contains all of the phenolic bodies simulating phenol plus the phenol. S o . 1 graduate contains no phenol but all the other similar bodies. The difference between the contents of the two graduates in terms of a standard phenol solution which has received the same treatment as portion S o . 2 of the sample will be the phenol content of the sample. T o develop the color in the two portions by the method of measurement used in these experiments, treat 50 ml. of each with 4 ml. of the sulfanilic acid solution and then with 2 ml. of the sodium nitrite solution. This can be done in Xessler tubes or any suitable vessel. After a thorough mixing, add 5 nil. of sodium hydroxide and mix again. The full color is developed in about 3 minutes. Xaking the comparisons in a colorimeter greatly adds to the sensitivity of the determination, When a number of determinations are to be made it is advisable to distil a portion of a standard phenol solution having relatively the same phenol content as the unknowns and to match the color produced in the distillate with a platinum-cobalt color solution. The latter affords a permanent standard 15-ith which all comparisons can be made in a colorimeter. The accuracy of the method has been tested in the presence of a variety of phenolic substances over a range of from 0.010 to 10.00 mg. of phenol per liter. The procedure followed was to start with a portion of a standard phenol solution and to add to this varying amounts of the cresols, resorcinol, and quinol. This standard solution containing
VOI. 2, Eo. 1
these substances was then analyzed for its phenol content. The results obtained are given in Table I. Table I PHENOL I'RhSENT
3 1 g per Izicr 0 010
OTHER SVB5TAXCES
PHENOL
FQUXD
PRCSEST o-, m - . 0-Cresol-"
M g , per !7ler 0 017 0.012 0,004 0.016 0.018 0.015
ERROR Mg. per lrlcr
+0.007 p-Creso!, resorcinol +0.001 Kothing -0.006 0 015 o-Cresol +o. 001 o-Cresol, resorcinol t0.003 o-Cresol, quinol 0.000 0,100 e-, ?-Cresol 0.09s -0.002 Nothing 0.105 +0.005 Resorcinol 0,104 to.004 0.jOO 0 - , m-Cresol 0.497 -0,003 p-Cresol 0.511 +O.Oll p-Cresol 0.502 +0.002 1,000 Sothing 0.99 -0.01 m-Cresol 0.99 -0.01 e-, $-Cresol, quind 1.00 0.00 2,500 Quinol, resorcinol 2.51 +0.01 @-Cresol 2.4i -0.03 ?-othini[ 2.50 0.00 5.07 +O.OT LOO p-, m-,o-Cresol Sothiqg 5.08 +0.08 Resorcinol 4.91 -0.09 10.00 Sothing 10.02 +o. 02 @-Cresol,resorcinol 10.01 +0.01 Quinol, resorcinol 9.96 -0 04 a T h e amount of these other phenolic substances added was approximately 2 mg. per liter in each case.
Analytical Results
It may be seen that the presence of the cresols or other phenolic substances in no way interferes with the determination of t,he phenol by this method. When no other substances were added to the phenol solutions the deviations from the theoretical values were of approximately the same magnitude as when t,hese substances were present. I t might be Tvell to state again that this method of separation can be used with any volumetric, colorimetric, or other scheme for measuring the phenol. The amount of chromic acid mixture required for concentrations of phenol higher than those used here has not as yet been determined. It is, however, reasonable to suppose that higher phenol concentrations will require more than 10 ml. of this reagent to bring about complete oxidation. Xore chromic acid would also probably be required when large amounts of oxidizable organic matter other than phenol were present in the unknown. These phases of the method are now under investigation in the writers' laboratory. Literature Cited (1) Fox and Gauge, J . Soc. C h e m . I n d . , 39, 260
(1920).
Rapid Volumetric Method for the Determination of Lead' R. C. Wiley UXIVERSITYOF MARYLAND, COLLEGE PARK,MD.
HE basis for the folloTving method for the determination
T
of lead is Alexander's molybdate method. I n the past when lead salts have been titrated against amnioniuin molybdate, tannic acid has been largely used as an indicator to determine when an excess of the molybdate w s present. In some cases the end point was determined by noting the point at which no further precipitation of lead took place upon addition of ammonium molybdate. In the method here described a mixture of stannous chloride and potassium thiocyanate dissolved in water is used. 1 Received October 18, 1929.
Epperson ( 1 ), in the determination of molybdenum, titrated the molybdenum solution against a standard lead solution, using tannic acid as a n indicator. He found the results satisfactory when extreme accuracy was not required. Kedesy ( 2 ) claims that an excess of stannous chloride in the stannous chloride thiocyanate reagent will prevent color from forming when molybdate is added. Reiser ( i ) notes that lead molybdate is best precipitated from a solution slightly acid with nitric acid. T o complete the precipitation the solution is made alkaline with ammonium hydroxide and then just acid with acetic acid.
January 15, 1930
I S D C X T R I d L =IAD E S G I S E E R I S G CHEJIISTRY
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N a a g and AIcCollain (3 successfully determined molybdenum colorimetrically in steel by using the thiocyanate, stannous chloride indicator. The following procedure was used on alloys containing lead, tin, and antimony.
with hot water, cool. Make filtrate arid T\ashings u p to 500 cc. Pipet out a 50-cc. aliquot and add ammonium hydroxide until a permanent precipitate appears, but do not add an excess. Boil about 30 ieconds. Disregard any precipitate and titrate while hot with tlie standard sodium molybdate solution. Tlie solution must not be unduly Reagents acid or alkaline. The reagents used were prepared as follows: For a preliminary indicator a drop of the lead solution on a IKDICATOR-TO make the indicator for determining the stirring rod held under the tip of the buret will serve. The end point of the reaction mix 10 grams of stannous chloride clouding of the drop of lead solution d i e n coming in contact with 10 grams of potassium tliiocyanate and add 30 cc. of with the sodium molylidate from the buret is easily seen. nater. Tlie mixture will not be clear, of course, and the Toward the end of tlie reaction, however, drops of the precipitate forms an excellent Tyhite background. The lieaker should be transferred to tlie spot plitte, which has indicator is used on a spot ulate. X red coloration denotes previously been prepared, and stirred with the indicator. that all the lead is precipi- ’ The reaction is complete tated and that the molybw h e n a d r o p so t r a n s An accurate and rapid method has been described date is in excess. f e r r e d causes a brownish for the determination of lead in alloys which contain d ~ n r o s ~ v \~\IOLTRDATE r or reddish color to appear i n addition antimony and tin. The indicator is exSOLUTIOX-Weig11 out 7 . 2 on the indicator in the spot tremely sensitive to small quantities of molybdenum. grams of molybdic acid and plate. From tlie lead value With care in manipulation it is believed that this of the ammonium molybplace it in a beaker. Add method is capable of extreme accuracy. This method date solution used the perjust enough dilute sodium should prove useful for the determination of lead in ores. c e n t a g e of l e a d can be hydroxide bolution to disreadily calculated. During solve it. Then add a few the titration the solution drops of plienolplithalein a n d make the solution just acid with dilute acetic acid. should be kept near the boiling point by reheating if necessary. Dilute to 1 liter with distilled water. Results Obtained STASDARDLEADSoLuTIos-This solution may be made from pure lead salts, or as follows: Melt some c. P. lead Table I gives a description of the samples a d the results and pour ofi into a dish. Separate the bright lead globule obtained by the Bureau of Standards and Table I1 gives from the dross and allon to cool. Scrape off the outside of the results obtained by the method here described. the globule and carefully weigh out about 10 grams and drop into a liter flask. Add small quantities of dilute nitric acid Table I-Results by Bureau of Standards and boil. Coiitinue this process until tlie lead is dissolved SAMPLE So. Sn Sb 45 Cu Fe Pb and heat until all the nitric acid is driven off. Cool and make c c, io %a %a x is; u p to 1 liter. One cubic centimeter of this solution will be 1 5.40 10.70 83.60 2 9.50 18.40 olio 0:iz olio 71 60 equal to approximately 1 cc. of the sodium molybdate solution. Table 11-Results by Method Here Described STAKDARDIZATIOS O F SODIU\f h1OLYRDATE SOLUTIOXSAMPLE 1 SAMPLE 2 Carefully pipet 25 cc. of the lead nitrate into a beaker. Boil Pb Pb Per cenl Per cent the solution and titrate against the sodium molybdate solu71.68 83.08 tions as in the procedure following, without the addition 71.48 83.60 8 3 . 5 4 71.56 of ammonium hydroxide. The lead d u e of the molybdate 11 80 83.66 solution is thus calculated. 11.66 83.50 Procedure
The procedure in brief is as follows. The sample is dissolved in sulfuric acid and the lead sulfate separated. The lead sulfate is then conrerted into basic lead carbonate, which in turn is converted into lead nitrate. The lead nitrate solution is then made exactly neutral with ammonium hydroxide and the lead titrated with a standard sodium molybdate solution, using tlie stannous chloridepotassium thiocyanate solution on a spot plate as an indicator. This procedure is carried out as follows: Weigh out 3 grams of the alloy and brush i t into a 250-cc. beaker. Add 10 cc. of sulfuric acid and heat t o boiling. Boil until sample is dissolved. Then add 50 cc. of water and boil for 2 or 3 minutes longer. Examine the contents of the beaker to be sure that all of the metal has dissolved. Filter off the lead sulfate and wash with dilute sulfuric acid. Transfer filter and lead sulfate to a beaker, add about 10 grams of sodium carbonate, and boil until the sulfate is converted into the basic lead carbonate. Filter and wash with dilute sodium carbonate solution. Transfer filter and contents to a beaker and make acid with nitric acid. Boil until all the carbon dioxide is evolved. Filter, wash
Av.
83,72 83.62 83.74 83.52 83.48 83.61
71.60 11.48 (1.66 71.82 71.54 71.63
Of course the results given in Table I1 were obtained after considerable preliminary work had been done on the method. Comment and Discussion
The end point should appear nithin 10 seconds after the drop from the beaker has come in contact with the indicator. The color will slo~vlyfade. If the end point appears too soon, accidental contamination of the indicator or excess acidity or alkalinity of the solution being titrated is indicated. A red coloration of the indicator, which may appear and then quickly disappear when a drop of the solution being titrated is added to the indicator, is due to iron. Thls coloration disappears as soon as the iron is reduced by the stannous chloride. It should not be mistaken for the true end point. When lead molybdate has been precipitated on a stirring rod with an excess of ammonium molybdate the rod should not only be washed but well scoured before using it to transfer
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.
