1122
ANALYTICAL CHEMISTRY
terfere by preventing complete color development. Close control of the p H of the solution a t the time of color development is required if reproducible results are to be obtained. Measurementa show that when 15 ml. of aluminon-buffer composite solution is diluted to 25 ml. with distilled water, the mixture has a pH of 5.3. During the heating to develop the aluminum lake, some of the gelatin deposits on the walls of the volumetric flasks. Before these flasks are used again, they must be thoroughly cleaned with cleaning solution. EXPERIMENTAL
In order t o compare'samples of aluminon obtained from different sources, calibration curves were obtained using aluminon-buffer composite solutions prepared from various samples of alumiDon. The curves were obtained as directed in the method for the
analysis of zinc die casting and magnesium alloys. The results are recorded in Figure 1. Kational Bureau of Standards samples of manganese bronze No. 62-b, of zinc base die casting alloy Xo. 94-a, and of magnesium base alloy No. 171 were analyzed by the appropriate method described above. The results were recorded in Table I. LITERATURE CITED
Cheneru. E. AI.. Amlust. 73.501 (1948) , ~ , (2) Craft, C. H., and Makepeace, G. R., ISD. ENG.CHEM.,A N ~ L . ED.,17, 206 (1945). (3) Sherman, hl., paper presented before Division of Analytlcal SOCIETY, Chemistry at the 118th Meeting,AMERICANCEIEMICAL Chicago, Ill. (4) Smith, H., Sager, E. E., and Siewers, I. J., ANAL. CHEII.,21,
(1)
w.
1334 (1949). RECEIVED for review April 28,1951. Accepted May 13,1952.
Photometric Determination of Aluminum in lead, Antimony, and Tin and Their Alloys A n Aluminon Method C. L. LUKE, Bell Telephone Laboratories, Inc., Murray Hill, N . J . The work was undertaken because of a need for a reliable method for the determination of traces of aluminum in lead, antimony, and tin and their alloys. As a preparatory step toward the development of such a method, a thorough study of the specificity of the aluminon-thioglycolic acid and the oxine-cyanideperoxide photometric aluminum methods was made. As a result, an accurate specific method for aluminum has been developed. This method is applicable to the analysis of lead, antimony, and tin and their alloys and can also be adapted for use in the analysis of a wide variety of other ferrous and nonferrous alloys.
I
Ili THIS country the aluminon and the oxine methods are most
often used for the photometric determination of aluminum in lead, antimonv, and tin and their alloys. Because neither method is very specific it is necessary to isolate the aluminum rather completely before the photometric determination is attempted. I t is customary to rcmove lead as sulfate and antimony and tin by volatilization as bromide. Following this, a mercury cath6de separation is made, which removes a wide variety of metal ions but may not completely free the sample solutions of interference. In the first place, it is often difficult to obtain complete deposition of certain interfering metal ions, Moreover, several interfering metal ions are not capable of being removed a t the mercury cathode, and it is usually necessary to resort to more complete isolation of the aluminum or to the use of complexing agents to mask interference. Precipitation or extraction with eupferron has been used for the removal of traces of iron that may have escaped the electrolysis. On the other hand Chenery (1)has been able t o eliminate the interference of iron in the aluminon method by complexing with thioglycolic acid. Luke and Braun ( 4 )have used this acid to suppress the interference of copper and iron in the photometric determination of aluminum in manganese bronze using aluminon. Gentry and Sherrington (2) have markedly increased the specificity of the oxine method by complexing several metal ions with cyanide, and Kassner and Oaier (a) have further increased specificity by the use of hydrogen peroxide to complex several other metal ions. As a preliminary step toward the development of a suitable aluminum method, the author has undertaken a thorough investigation of the specificity of the oxine-cyanide-peroxide method
and of the aluminon-thioglycolic acid method. From this work a very specific method for the determination of aluminum in lead, antimony, and tin and their alloys has been developed. In this new method the solution from the mercury cathode separation is freed of such interfering metal ions as titanium, zirconium, and hafnium, by a cupferron-chloroform extraction. The solution is then neutralized to p H 5 and the aluminum is separated from such metal ions as beryllium and scandium by an oxine-chloroform extraction. Following this, the chloroform and oxine are removed by evaporation and the aluminum is determined by the aluminonthioglycolic acid method. APPARATUS
Photoelectric Photometer. An Evelyn photometer with a 515mp filter was used in the present investigation and the procedure was written with this instrument in mind. Glassware. Unless otherwise specified, all the glassware used in the method should be made of Pyrex glass No. 7740 or its equivalent. REAGENTS
Standard Aluminum Solution (0.01 mg. of aluminum per ml.). Dissolve 20.0 mg. of pure aluminum in 20 ml. of hydrochloric acid by heating gently, Dilute t o 2 liters in a volumetric flask. Cupferron Solution. Dissolve 1 gram of cupferron in 100 ml. of distilled water. Prepare fresh daily as needed. Chloroform. Redistill the commercial product to free it from traces of metalion impurities. m-Cresol Purple Indicator Solution. Dissolve 0.1 gram of mcresol purple in 10 ml. of distilled water containing 1 pellet of sodium hydroxide. Cool and dilute to 100 ml. with distilled water.
V O L U M E 24, NO. 7, J U L Y 1 9 5 2
A m m o n i u m Tartrate Solution. Dissolve 25 grams of ammonium tartrate in 250 ml. of distilled water. Store in a polyethylene bottle. Hydrogen Peroxide Solution. Dilute 5 ml. of hydrogen peroxide (30%) to 50 ml. with distilled water. Prepare fresh each day as required. pH 5 Buffer Solution. Dissolve 30 grams of sodium acetate trihydrate plus 4 ml. of glacial acetic acid in distilled water and dihit,e t o 500 . - ml. . ~The DH of this solution should be 5.0 to 5.1. Store in a polyethylene bbttle. Oxine-Chloroform Solution. Dissolve 10 grams of 8-quinolinol in redistilled chloroform in a 1-liter volumetric flask. Dilute to the mark with redistilled chloroform. Thioglycolic Acid Solution (4 to 96). Dilute 10 ml. of thioglycolic acid t o 250 ml. with distilled water in a volumetric flask and mix well. Keep stoppered when not in use. Prepare fresh every week as required. Aluminon-Buffer Composite Solution. Prepare as directed by 1.ulre and Braun (4). Hydrobromic Acid-Bromine Mixture. Pour 20 ml. of bromine into 180 ml. of hydrobromic acid (specific gravity 1.49). ~
PROCEDURE
Preparation of Calibration Curve. Transfer 0, 2, 4, 6, 8, and 10 ml. of st,andard aluminum solution (0.01 mg. of aluminum per ml.) to 150-ml. short-stemmed separatory funnels, Add 1 ml. of perchloric acid (707,) and dilute to 50 ml. with distilled water. .kdd 1 ml. of cupferron solution, swirl, add 15 ml. of redistilled chloroform, stopper, and shake vigorously for 1 minute. .Illow the layers to separate as completely as possible, swirl to dislodge any floating chloroform, aud drain off and discard as much of the lower layer as can be safely removed without loss of any of the aqueous solution. Add 1 drop of m-cresol purple solut>ioriand 2 ml. of ammonium tartrate solution. Neutralize carefully with ammonium hydroxide (using a similar solution for com arison) until the pink color changes to orange. Add 1 ml. of Xydrogen peroxide solution, swirl, and then add 15 ml. of pH 5 buffer solution. Add 25 nil. of oxine-chloroform solution, stopper, and shake manually (approximately 150 shakes per minute) for 5 minutes. Allom the layers to separate as completely as possible, swirl to dislodge any floating chloroform, and drain off the lower layer as completely as possible to clean, dry 100-ml. beakers. Place the beakers uncovered on an electric hot plate whose surface temperature is controlled to 150" to 160" C. by means of a Variac or similar instrument. If the hot plate temperature is too high, severe loss of aluminum will result. Allow the beakers to remain on the plate until the chloroform and the osine have been completely removed and only a little charred organic. matter remains. Toward the end of the distillation it is convenient to flame the condensed mine from the tops of th(z beakbut care should be taken not to raise the temperature of the kers appreciably or to ignite the vapors. When removal of the osine is complete, cool the samples somewhat,, add 1 ml. of perchloric acid, covc~,and heat gently to oxidize the organic matters in the beakers. (The amount of organic matter present is small and there is no danger of explosion.) Finally, heat unt,il copious white perchloric acid fumes start to emerge from under the cover glasses and the acid can be seen condensing near the top of the beakers. Cool, add 10 ml. of distilled water, and heat to boiling to expel chlorine. Cool to 30" C. and dilute to 50 ml. with distilled water. Add 2.0 ml. of 4 to 96 thioglycolic acid and 1 drop of uz-crwol purple solution and neutralize carefully with ammonium hydroxide (using a coinparison solution) until a definite lightening of the pink color r e d t s . Do not carry the neutralization t,o the orange color of the indicator for fear of approaching too closely the p i n t where partial hydrolysis of the aluminum occur.*. Transfer the solutions to 100-ml. volumetric flasks and add 15.0 ml. of aluminon-buffer composite solution. Swirl, place each flask in 300 ml. of vigorously boiling water in 400-ml. beakers, and allow to remain exactly 5 minutes. Remove to the bench for a minute or so and then place in a cold water bath. Cool t,o room temperature, dilute to the mark v i t h distilled wat,er, and mix well. Transfer about 30 ml. of the solutions to absorption cells, allow to stand 1 or 2 minutes, and read the per cent transmittancy a t approximately 525 mp, using distilled water as the reference solution. Prepare a calibration curve. ANALYSIS OF SAMPLE
Procedure for Lead Metal. Transfer 10.00 grams of the milled sample to a 125-ml. Vycor glass No. 7900 conical flask. Carry a reagent blank through the entire analysis. Add 50 ml. of 1 to 3 nitric acid to each flask, cover, and heat gently to dissolve the sample. When ~olutioriis complete, dilute to 100 ml. and cool
1123
t o room temperature. Add 5.5 ml. of 1 to 1 sulfuric acid to the
sample but not to the blank. Swirl, and filter through No. 40 \%-hatmanpaper into 250-ml. Vycor glass No. 7900 conical flasks. \$a'sh the precipitate thoroughly with distilled water and discard. Add 2 ml. of perchloric acid and a few grains of silicon carbide to the filtrates and boil down to about 10 ml. on a hot plate. Finally heat on a Meker-type flame until white perchloric acid fumes appear. Cool, wash down the walls with a little water, and boil down on a flame to 1 ml. to expel nitric acid and excess perchloric acid. Cool, add 10 ml. of distilled water, and heat to boiling t o expel chlorine. Cool and dilute to 45 ml. with distilled water. Filter the solutions into 100- or 150-ml. mercury cathode beakers containing 30 ml. of mercury. Wash the precipitatr: well with distilled water and discard. Dilute the solutions t o 50 ml, with distilled water and proceed to t'he mercury cathode separation as described subsequently. Procedure for Antimony Metal. Transfer 2.000 grams of the powdered metal to a 150-ml. crystallizing dish. Carry a reagent. blank through the entire analysis, Add 20 ml. of hydrobronlic acid-bromine mixture, cover with a cover glass, and heat gently, avoiding excessive loss of bromine, until dissolution of the sample is complete. Finally, raise the covers with glass hooks and heat in a well ventilated hood on a low temperature hot plate. 117 order to prevent possible loss of aluminum by volatilization, maintain the surface temperature of the hot plate a t 150" to, 175' C. Heat until the volume is reduced to 3 or 4 ml. Finally,. when it is safe to do so, remove the covers and allow the solutions to evaporate to dryness. Remove from the plate at once! cool somewhat, wash down the cover and walls with 10 ml. of hydrobromic acid, and repeat the distillation, using the covers during the initial boiling to prevent loss by bumping. Cool somewhat, add 2 ml. of hydrobromic acid plus 2 ml. of perchloric acid, and heat with the covers on to dissolve all soluble salts, Transfer t'he solutions to 125-ml. Vycor glass No. 7900 conical flasks wit'h the aid of a fine stream of distilled water from a wash hottle. Boil down on a flame to expel hydrobromic acid and finally reduce the volume to 1 ml. Cool, add 10 ml. of distilled water, heat to boiling to expel chlorine, cool, and dilute to 50 ml. with distilled water. Transfer the solutions to 100- or 150ml. mercury cathode beakers containing 30 ml. of mercury and proceed t o the mercury cathode separation as described subsequently. Procedure for Tin Metal, Tin-Base Alloys, Solders, and LeadBase Alloys. Transfer 2.000 grams of the milled sample to a 125-m1. Vycor glass No. 7900 conical flask. Carry a reagent hlank through the entire procedure. Add 10 ml. of hydrobromic acid-bromine mixture to each flask and cover with watch glasses. Cool in an ice bath if the initial reaction is violent, as it sometinies is when tin-base alloys are analyzed. Heat gently on a hot plate, avoiding excessive loss of bromine, until dissolution of the sample i q as complete as possible. '4dd 5 ml. of perchloric acid and then swirl on a Meker-type flame to expel tin, antimony, and arsenic. If lead is present, direct the flame a t the upper edge of the solution in order to avoid bumping. Avoid excessive loss of perchloric acid. \Vhen the lead bromide starts to decompose, adjust the heating so that rontinuous evolution of perchloric acid will keep the neck of the flask hot. Continue to heat until all hydrobromic acid and bromine have been expelled. If the solution is cloudy a t this point, showing that antimony has not been completely removed, add 5 ml. of hydrobromic acid and repeat the distillation. Repeat the distil1at)iona third time if necessary. Failure to expel most of the antimony may cause low results for aluminum. If part,icles of temporarily insoluble lead bromide remain after expulsion of the hydrobromic acid, continue to heat gently until solution takes place. Roil the solution down to 1 ml. to expel excess perchloric acid. I n the event that the sample contains appreciable amounts of iron, the latter may hydrolyze if too much perchloric acid is expelled. Cool, add 10 ml. of distilled water. and heat to boiling to expel chlorine. Cool and dilute to 50 ml. If the sample contains more than about 5y0 of lead, add 1 to I sulfuric acid dropwise while swirling until most of the lead present ha9 precipitated. Then, repeatedly allow the precipitate to settle, add a drop of the acid and swirl, until the addition of the drop of acid causes no visual precipitate of lead sulfate in the (.]ear supernatant solution. One gram of lead reacts with about 0.5 ml. of 1 to 1 sulfuric acid. Avoid too large an excess of sulfuric acid. When precipitation is complete, filter on No. 40 Whatman paper and wash well with distilled water. Discard the lead sulfate. Do not add sulfuric acid to the reagent blank solution. Transfer the samples to 100- or 150-ml. mercury cathode beakers containing 30 ml. of mercury. Ignore traces of lead sulfate or hydrolyzed tin or antimony, as these will dissolve subsequently. Arrange to electrolyze using a rotating gauze anode which is adjusted 80 that, its lower edge is about 0.5 inch above
1124
ANALYTICAL CHEMISTRY
the pool of mercury. Electrolyze for 30 minutes (or longer if necessar ) with a current of 4 or 5 amperea. When the electrolysis is com &,e, siphon off the solution to 100-mi. beakers, washing the wats and electrodes once in the process. If lead peroxide is seen in the solution, filter, wash, and discard it. Transfer the solutions to 150-ml. separatory funnels, add 1 ml. of cupferron solution, and proceed as in the preparation of calibration curve. With the aid of the calibration curve determine the weight of aluminum present in the sample and in the reagent blank. DISCUSSION
Specilicity of Photometric Oxine-Cyanide-Peroxide Method for Aluminum. In the usual course of events in an aluminum determination, a mercury cathode separation from perchloric acid is made to isolate the aluminum as much as possible before the photometric estimation is attempted. With this consideration in mind the following method w&s adopted for the systematic investigation of the specificity of the oxine-cyanide-peroxide method: Transfer a portion of a solution containing approximately 100 micrograms of the metal ion in question to a 125-ml. conical flask. Add 10 ml. of 1 to 9 perchloric acid and 2 ml. of ammonium tartrate solution, and dilute to 50 ml. Just neutralize to Congo paper with ammonium hydroxide and then add 2 ml. of 1 to 9 perchloric acid. Add 2 ml. of hydrogen peroxide solution and allow to stand for 5 minutes. Add 10 ml. of sodium cyanide solution (lo'%), swirl, add 3 ml. of sodium metabisulfite solution (lo'%), and heat to 80" C. Cool to 25" C., transfer to a 150-ml. short-stemmed separatory funnel, and add 25 ml. of oxine-chloroform solution. Stopper and shake manually (approximately 150 shakes per minute) for 5 minutes. Drain through glass wool to an absorption cell. If the solution is cloudy, dip the cell in hot water to redissolve the precipitated water. Measure photometrically a t 400 mp, using pure chloroform afi the reference solution. This procedure differs somewhat from that of Kassner and Ozier, the most important difference being that the sulfite is added after rather than before the cyanide. Experiments have shown that the vanadium complex and probably some of the other peroxide complexes are easily destroyed if the pH is too low at the time of the addition of the sulfite. That Kassner and Ozier did not experience difficulty in this respect is explained by the fact that the large amount of sulfite employed kept the pH near the neutral point. In the present investigation, where the sulfite content is kept to the minimum in order to reduce the size of the blank, it has appeared safer to reverse the order of adding the sulfite and cyanide in order to prevent interference from vanadium. If the addition of hydrogen peroxide is omitted in the above method, such metal ions as vanadium(1V) and cerium(1V) yield, respectively, amber and pink extracts which would cause interference in the aluminum determination. The vanadium(V) and cerium(II1) ions do not interfere appreciably if a t all. None of the four ions interferes when complexed with hydrogen peroxide. Potassium salts were avoided in the above method because of the presence of perchloric acid. Partial neutralization of the perchloric acid is made so as to yield a pH of about 9 at the time of the extraction with oxine. -4ddition of cyanide before the sulfite prevents complete reduction of copper to the cuprous state, but this is immaterial. A 3-minute manual shaking is adequate for complete removal of aluminum from alkaline cyanide solutions relatively free from ammonium salts and tartrate. In the presence of the latter a 5-minute shaking was found to be necessary. Solutions of most of the metal ions to be investigated were prepared by dissolving an appropriate amount of the oxide, chloride, sulfate, or acetate in 10 ml. of hydrochloric or perchloric acid, followed by dilution to 500 ml. Each solution was made up to contain 10 micrograms of metal ion per ml. Various other metal ion solutions were prepared from ammonium vanadate, uranyl acetate, ammonium molybdate, potassium chromate, sodium silicate, sodium germanate, sodium tungatate, sodium niobate, and sodium tantalate For the sake of completeness, several commonly
encountered heavy metals, which would normally be previously removed, were included in the investigation, Experiments showed that 100-microgram portions of the following metal ions, when separately carried through the procedure described above, caused little or no absorption a t 400 mp: germanium, arsenic( V), antimony(V), tin, silver, mercury(II), copper( 11), copper(I), cadmium, molybdenum( VI), thallium( 111), thallium(I), zinc, nickel, cobalt, iron(III), iron(II), chromium(VI), chromium(III), manganese, magnesium, silicon, tungsten, tantalum, niobium, thorium, hafnium, zirconium, cerium(IV), cerium(111),vanadium(V), vimadium(IV), lanthanum, samarium, neodymium, praseodymium, boron, phosphorus, barium, strontium, and calcium. Further experiments showed that 10-mg. portions of iron(III), iron(II), nickel, zinc, copper(II), and copper(1) caused little or no absorption. Table I. Effectiveness of Cupferron-Peroxideoxide Separation for Elimination of Interference in AluminonThioglycolic Acid Method for Aluminum NO. 1 2 3
Metal Ions Added 50 y AI
100 y AI
100 y T h 100 y Hf
4
inn
5
6
7
8
9
+
.,Ti
io0 5
20 y Al 100 y each of Ti, Zr, and T(V) 100 y each of Al Ti Zr and V(V) 100 y A1 500 $ eachbf Ti, Zr, and V(V)
+
.4Iuminum Found, 49
y
100 0 0 1
0
19
103 107
The metals that were found to interfere are herewith listed in decreasing order of severity of interference: beryllium, titanium, gallium, yttrium, indium, bismuth, lead, uranium(VI), and scandium. In addition, magnesium, when present in larger amounts, interferes in the manner described by Gentry and Sherrington. Perhaps this is a good place t o insert a warning. The conclusions in the present investigation are based upon extractions of 100-microgram portions of individual metal ions. The extraction of mixtures was not investigated. The interference of some of the metal ions mentioned above can be eliminated. Thus, bismuth and lead can be previously removed by a mercury cathode separation. In addition, complete removal of the color due to beryllium, scandium, yttrium, and lead can be obtained by running the chloroform extract from the cyanide solution through glass wool to a second 150-ml. separatory funnel containing 15 ml of pH 5 buffer solution and shaking for 3 minutes. Contrary to the experience of Kassner and Ozier, it was found that titanium and uranium(V1) interfere. Apparentlytheperoxide complexes are not very stable. The interference can be greatly reduced by omitting the heating to 80' C. after the addition of the sulfite, but, strangely enough, when this is done, zirconium is found to extract and cause interference. Because of the above difficulty and the fact that no satisfactory separation of gallium and indium n as found, the oxine method for aluminum was abandoned in favor of the aluminon method. Specificity of Photometric Aluminon-Thioglycolic Acid Method for Aluminum. The use of thioglycolic acid in conjunction with the aluminon method for aluminum greatly increases the specificity of the method. The interference from a number of metal ions can be completely eliminated. In other instances the interference can be greatly attenuated. This latter is of great value because, in many cases, it permits correct results to be obtained for aluminum even though the preliminary separations of the interfering metal ions may not be complete. -411 that is required is that the concentration of the interfering metal ion be reduced to the point where the interference is negligible. The cupferronchloroform separation of titanium and zirconium provides a good example of such a case. Complete extraction of these metal ions may not be obtained, but this is immaterial, because as much as 20 micrograms of one or the other can be present in an aluminum determination.
1125
V O L U M E 24, NO. 7, J U L Y 1 9 5 2 In order to establish the specificity of the aluminon-thioglycolic acid method, the following procedure was used: Transfer 10-ml. portions of the metal ion solutions used in the tests of the oxine-cyanide-peroxide method t o 100-ml. beakers. Add 1 ml. of erchloric acid followed by 40 ml. of distilled water. Add 2 ml. o r 4 t o 96 thioglycolic acid and 1 drop of m-cresol purple indicator solution, neutralize, and carry the samples through the photometric aluminon determination as in the preparation of calibration curve, Experiments showed that 100-microgram portions of the following metal ions, when separately carried through this procedure, caused little or no absorption at 515 mp: germanium, arsenic(V), antimony(V), molybdenum(VI), niercury(II), thallium(III), thallium(I), cadmium, zinc, nickel, iron(III), iron(II), manganese, magnesium, silicon, tungsten, tantalum, niobium, yttrium, indium, uranium(F'I), gallium, cerium(IV), cerium(III), lanthanum, samarium, neodymium, praseodymium, boron, phosphorus, barium, strontium, and calcium. A turbidit,y was noted at the time of the photometric measurement in tests on the heavy metals, lead, bismuth, tin, and silver. A brown color, which absorbs somewhat' at 515 mp, appeared when t.he thioglycolic acid was added t,o a solution cont,aining 100 micrograms of cobalt. The following metal ions yielded red colored lakes wit,h aluminon : beryllium, scandium, chromium(VI or 1111, vanadium(\- or IV), copper(I1 or I), titanium, zirconium, hafnium, and thorium. The magnitude of the interference in an duminum determination can be estimated from the fact that t'he color caused by 100 micrograms of beryllium corresponds t o about 75 microgramsof aluminum, that, from 100 micrograms of scandium corresponds t o about 10 mirrograms of aluminum, and that from 100 micrograms of each of the others including cobalt corresponds to about 5 micrograms of aluminum. Experiments have shown that it is pm~i1)leto eliminate the interference of all of the met,al ions mentioned. In the method for aluminum described above, lead, bismuth, silver, tin, cobalt, chromiuni(VI), chromium(III), copper(II), and copper(1) are removed by a mercury cathode separation. [At t,his point, vanadium, if present, is probably in the van:adium(IV) state.] Following this, titanium, zirconium, iron(III),some of the vanadium(V), some or all of the hafnium, possibly some of the thorium, and several ot'her noninterfering met,al ions are removed by a cupferron-chloroform extraction. The aqueous phase is then neutralized t o pH 5 and treated with hydrogen peroxide t o complex vanadium(V) arid vanadiuni(1V) and thus prevent extraction of these metal ions hy oxine. hfter this the aluminum is removed
Table 11. Determination of Aluminum in \-arious Metals and Alloys
16
Samples Trbt lead Test lead Test lead Test lead Antimony metal Antimony metal Tin metal Tin metal Tin metal Tin metal Tin metal Tin metal Tin metal Tin metal NBS tin base No. 54-c NBS tin base
17
NBS tin base
NO.
16
11 12 13 14 15
s o . 54-0
_-
Metal Ions -4dded
...
... ... Be +'Ga
19
20 21
22
.%
80 20 60 10
so
40
Zr
50 80
h!IO(VI) U(V1) Ti 4- V(V)
v(vj co
'
'
ni
..
50 .XI
M ~.
so
Aluminum Found, y 8 28 45 74 17 58 12 40 50
50 ~.
49 49
83 81
10
11
40
38
80
82
Th
10
vcvij
40 10
9 40 10
...
40
37
*..
YO
75
W n 5A.p _.l.
18
A1 uminum Added, y 10 30
~
NBS solder N o . 117 SBSsolder No. 127 NBS lead base No. 53-2 NBS lead base Yo. 53-c NBS lead base No. 58-c
by an oxine-chloroform extraction. This separates the aluminum from any beryllium or scandium as well as from a number of other metal ion8 including lead, chromium(V1). chromium(III), and yttrium. The treatment with hydrogen peroxide does not prevent uranium(VI), titanium, or zirconium (if present a t this point) from being extracted by oxine. Thus uranium(V1) will a h a y s accompany aluminum and be present at the photometric estimation. Fortunately, ho\vever, uranium(V1) does not interfere. The behavior of hafnium and thorium in the cupferron-peroxideoxine separations has not been thoroughly investigated. Suffice it to say that the interference of 100-microgram portions of each of these metal ion? is eliminated by carrying them through the separations. Miscellaneous Comments on Method. Experience has shown that there is 5 t o 10% loss when a measured amount of aluminum is extracted with oxine-chloroform solution previous to the photometric analysis. It has not been established whether this loss is due t o incomplete extraction or whether it occurs during the expulsion of the chloroform and oxine. Attempts t o improve the recovery by omitting the addition of tartrate or by repeated extraction were not successful. Because of this it is necessary to eliminate the error by including the oxine extraction in the preparation of the calibration curve. For convenience, the cupferron extraction and the treatment with hydrogen peroxide have also been included. The aluminum calibration curve obtained is similar in form t o that shown by Luke and Braun (4). In the photometric estimation of the aluminum it is important t o use distilled water rather than the blank as the reference solution. I t is probable that few analysts realize what large errors can result from the practice of using the blank as the reference solution where Beer's law does not hold. During the development of the method, attempts were made to remove some of the heavy metals by an acid sulfide separation. However, surprisingly low results for aluminum n ere obtained whenever the sulfide precipitate was large. Some loss of aluminum by occlusion occurs when large amounts of lead are removed as sulfate, as can be seen in Table 11, but this is less than 10% and can therefore be ignored. In all trace analyses it is imperative that reagent blanks be carried along with the samples. Pyrex KO.7740 glassware contains about 2y0 of aluminum oxide. Vycor S o . 7900 glassware, on the other hand, contains less than 0.5%. Because of this, the latter should be uaed in the presence of alkali and for all fuming or boiling operations. ,211 reagents should be of analytical reagent grade, in order t o keep the blanks as low as possible. If proper precautions are taken, the reagent blank for the entire analysis should not amount to much more than 3 or 4 micrograms of aluminum. If necessary, the ammonium tartrate solution and the pH 5 buffer solution can be cleansed of aluminum by a preliminary extraction with oxinechloroform solution. Whenever excess acid in a solution containing traces of aluminum i q neutralized, it is necessary t o have a complexing agent present t o prevent hydrolysis of the aluminum. I n the author's opinion the failure t o observe this precaution is one of the most important sources of trouble in aluminum determination. I n the neri method described above, tartrate and thioglvcolic acid are the complexing agents used, Because the latter forms a relativelx- weak complex with aluminum, i t is important t h a t addition of excess ammonium hydroxide be avoided during the neutralization of the qolution previous to the addition o f the aluminon solution. Difficulty has been encountered in the analysis of certain samples of antimony metal which contain appreciable amounts of iron 9 o t only does the iron cause bumping during the distillation of the antimony. but it apparently forms a compound with antimony and thus severely rctnrds the removal of the latter I ) Y (iirtill*itioiias 131 omide
ANALYTICAL CHEMISTRY
1126
The n.ethod is applicable to the analysis of a m-ide variety of ferrous and nonferrous metals and alloys, providing suitable separations are devised for removal of the bulk of the sample without loss of aluminum. EXPERIMENTAL
In order to demonstrate the effectiveness of the cupferronperoxide-oxine separations as a means of removing interference in the aluminon-thioglycolic acid method for aluminum, portions of solutions of various metal ions were transferred to 150-ml. separatory funnels, 1 ml. of perchloric acid was added, the samples were diluted to 50 ml., and the determination of aluminum was performed as in the preparation of calibration curve. The results obtained are recorded in Table I. I n experiments 7,8, and 9 , 3 ml. of cupferran solution mas added andtheextracted solution was extracted with 5 ml. of pure chloroform to free it of
lead metal, 2 grams of antimony metal, 2 grams of tin metal, 2 grams of NBS tin base alloy KO.54-c, 2 grams of NBS solder No. 127, or 2 grams of NBS lead base alloy No. 53-c were added; the mixtures were then analyzed for aluminum as directed in the method described above. A 10-gram portion of test lead or a 2-gram portion of the other samples was carried through the analysis to serve as a blank. The results obtained are recorded in Table 11. “Aluminum Found” represents the weight of alunlinum found in the mixtures minus that found in the blank. ACKNOWLEDGMENT
The author wishes to express his appreciation to Mary E Camphell for her assistance throughout the investigation. LITERATURE CITED
traces of extractable metal ions and cupferron.
(1)
I n order to test the accuracy of the new method for aluminum portions of standard aluminum solution plus, in certain instanrex, 100-microgram portions of various other metal ions were rvaporated to about 2 ml. in an appropriate flask or dish; 10 grams of
RECEIVED for review January 2 2 , 1952.
Chenery, E. AI., A n a l y s t , 73,501 (1948). (2) Gentry, C. H. R., and Sherrington, L. G., Ibid., 71, 432 (1946). (3) Kassner, J. L., and Ozier, hI. A., ANAL.CHEM.,23, 145.3 (1951). (4) Luke, C. L., and Braun, IC. C., Ibzd., 24, 1120 (1952). Accepted May 13,1952
Automatic Karl Fischer Titration Apparatus Using Dead-Stop Principle H. A. FREDIANI, Merck & Co., Znr., Rahway, iV. J . A simple and relatively foolproof automatic apparatus for Karl Fiscliertypc titrations was needed in order to obtain higher precision and greater rapidity in moisrure determinations. A novel electrical apparatus has been developed which utilizes the dead-stop principle, automatically adds the reagent, and differentiates between “true” and “false” or fleeting end points. No vacuum tubes are used. Greater precision is possible because of the automatic addition of reagent and electrical timing of the permanence of the end point. High precision is possible even with laboratorians unfamiliar with the Karl Fischer technique. Relatively complex samples may be titrated rapidly and the possible effect of side reactions is minimized.
T
HE volume of literature on the use of the Karl Fiecher reagent for moisture determinations in many types of samples
amply attests the utility of this technique. The equivalence point involved in these titrations, however, has posed a problem sufficient to prevent maximum use of this method. An experienced chemist, working with relatively clear or colorless solutions, can carry out the Fischer titratio? with a high degree of precision. Because the color change involved is usually from canary yellow to a light amber, practice is required, and the chemist who analyzes infrequent samples usually encounters difficulties. For colored samples, or suspensions of solids, it is often impossible to use the visual end point and recourse must be had to an instrumental method. Although the potentiometric method has been ufied, experience has shown that ( 2 ) the deadstop method serves best for electrically indicating the end point in Karl Fischer-type titrations, as the analysis must be carried out under anhydrous conditions and the usual reference electrodes (salt bridges, calomel, and silver chloride) cannot be used. 81though many ingenious devices have been suggested (4, 6) for automatic potentiometric titrations and a t least three surh instruments are now commercially available, none of these will function for the dead-stop type of titration. The advantages of an automatic titration apparatus, both for laboratories that routinely require many titrations per day and
for ihr laboratory that analyzes infrequent samples by the Karl Fischer technique, are many. The dead-stop technique, which really involves a “polarization” end point, may be employed n herever a sharp transition occurs from the polarization of a t least one electrode to the depolarization of both (or vice versa), this transition coinciding with the end of the chemical reaction being used. Under these conditions a polarized pair of electrodes (10) will have oxygen adsorbed on the anode and hydrogen on the cathode, the former being depolarizable by a suitable reducing agent and the latter by an oxidizing agent. In practice, a very small potential difference, of the same order ot magnitude as the back e.m.f. of polarization, is applied to two phtinum electrodes immersed in the solution t o be titrated. At tlic end point of the titration, when depolarization occurs, a sudden increase of current becomes apparent. Observation of this current provides a far more sensitive indication that the end point h:ts been reached than attempts to measure the e.m.f. involved. The e.m.f. usually is of the order of 10 t o 20 mv. and, because the rapacity of the cell is low, potentiometric measurements must be nmde slowly and carefully. Foulk and Bawden ( 2 ) have aptly summarized the advantages of the dead-stop end point. Gradually increasing excursions of the galvanometer give evidence of the approach of the end point.
