Volumetric Analysis of Sodium Aluminate Solutions Determination of Carbonate, Hydroxide, and Alumina H. L. WATTS AND D. W. UTLEY Aluminum Research Laboratories, East S t . Louis, 111. Control of the Bayer process for extraction of alumina from bauxite requires the determination of carbonate, hydroxide, and alumina in sodium aluminate solutions. -4 procedure has been developed which permits these determinations by rapid volumetric analysis of a single sample using phenolphthalein as indicator. Carbonate is determined acidimetrically after separation as barium carbonate. Total hydroxide is determined on the filtrate by titration in the presence of barium chloride and sodium tartrate. Acid slightly in excess of the amount required to convert aluminum hydroxide to aluminum chloride is added. Alumina is determined by precipitating fluoaluminate and titrating the remaining acid with standard base. Using an alternative method, total hydroxide can be determined in the presence of carbonate. Accuracy and precision are demonstrated by the analysis of standard aluminate solutions. Effects of impurities are shown. The method should be useful to manufacturers of sodium aluminate and to aluminum fabricators who require control of caustic cleaning solutions.
I
acid against aluminum solutions of known concentration. The present authors found the alumina factor to be a function of aluminate concentration and speed of titration. By the following recommended procedure, carbonate, total hydroxide, and aluminate can be determined by titration to phenolphthalein end points. The hydrochloric acid need be standardized only against standard base for all titrations, including the alumina titration. Carbonate is separated by precipitation with barium chloride and determined acidimetrically. The filtrate containing barium chloride, free hydroxide, and aluminate is titrated with standard acid to the first visible aluminum hydroxide precipitate. Sodium tartrate is then added, complexing the aluminum and releasing one mole of hydroxide for each mole of aluminate present as represented by the equation: -1102-
+ 2Hz0 + nT
AI(OH),.T,
+ (OH)-
(1)
where T represents the tartrate radical, n the number of radicals per atom of aluminum, and AI(OH),.T, a tartrate complex. The titration is continued to the phenolphthalein end point, which coincides with a very sharp potentiometric break as a result of depletion of hydroxyl ions. The total milliliter titration to this end point represents titration of free and combined, or total, hydroxide. The milliliter difference between the turbidity and phenolphthalein end points indicates the amount of acid necessary to convert A1O2- to equivalent .41(OH), and, in the absence of a precipitate, serves as an estimate of aluminum. If this milliliter difference is multiplied by 3, the acid necessary to convert Sl(0H)s to AlC13 can be calculated. After addition of this amount and a small excess, potassium fluoride is added to form fluoaluminate. The total reaction is:
N THE Bayer process alumina is extracted from bauxite as
sodium aluminate by digestion with sodium hydroxide solution containing sodium carbonate (3). In order to control concentration of carbonate, hydroxide, and aluminate, a rapid method of analysis is necessary. The procedure used has been based on a method recommended by Bushey for aluminum (1). Bushey combined a potentiometric titration of free hydroxide with a fluoaluminate precipitation method proposed by Craig (b) and improved by Graham ( 5 ) . On a sample containing 0.25 to 0.50 gram of alumina, carbonate is precipitated with barium chloride. After filtration, the barium carbonate precipitate is determined acidimetrically. Free hydroxide in the filtrate is determined by titration with 1 N hydrochloric acid to a potentiometric end point where A ~ per H A ml. is a maximum. Acid slightly in excess of that necessary to convert aluminate to aluminum chloride is added. Aluminum is then precipitated as potassium fluoaluminate by addition of potassium fluoride. The small excess of acid is back-titrated to the phenolphthalein end point with 1 -4-sodium hydroxide. In this method the hydrochloric acid must be a t least 1 S to obtain a satisfactory potentiometric break at the free hydroxide end point. -4s the inflection is not sharp, this end point may be epread over a range of 0.3 ml. ,4s pH changes per 0.10 ml. of acid must be recorded over a range of about 2 ml., considerable time Is consumed. Viebock and Brecher ( 8 ) neutralized to the phenolphthalein end point an aluminum chloride solution, using sodium hydroxide In the presence of barium chloride and sodium potassium tartrate. After addition of potassium fluoride, aluminum mas determined by titrating the hydroxide released by formation of the fluoaluminate complex. Snyder ( 7 ) used barium hydroxide instead of sodium hydroxide and demonstrated the accuracy of the method in the presence of certain impurities. Viebock, Brecher, and Snyder recognized that the presence of the barium ion was necessary for accurate adjustment of the solution before the addition of potassium fluoride. In both procedures the titration was not stoichiometric and required the standardization of the
Al(OH),.Ta
+ 3HCI + 6KF --+ K3.41Fs 4 + 3KC1 + nT + 3H10
(2)
The small excess of acid is back-titrated to the phenolphthalein end point with standard sodium hydroxide. The titration curves for hydroxide and alumina are presented in Figure 1. RECOMMENDED PROCEDURE
Apparatus. Fisher Filtrator or 500-ml. suction flask. Mechanical stirrer, Reagents. Phenolphthalein indicator, 1% in alcohol. Barium chloride solution, 10% BaCI2.2Hz0. Standard hydrochloric acid and carbonatefree sodium hydroxide solutions, 0.3 N t o 0.4 N . P o h s i u m fluoride solution, 50% KF.2H20 by weight, filtered, and neutral to phenolphthalein. Stored in a polyethylene container (see note 6). Sodium tartrate solution, 2 5 7 SazCaH40e2Hz0 and neutral to phenolphthalein. Procedure. Pipet an aliquot of the sample containing from 0.10 to 0.30 gram of alumina into a 400-ml. beaker, and dilute t o 100 ml. with carbon dioxide-free distilled water. While stirring, add 25 ml. of barium chloride solution t o precipitate barium carbonate. Add paper pulp; then, with suction, filter immediately through an S. & S. White Ribbon paper supported by a S o . 4, 9 c m . Whatman paper and a platinum cone. Wash the precipitate thoroughly with carbon dioxide-free water containing 10 ml. of barium chloride solution per liter. Adjust final volume of filtrate to 200 to 300 ml., and titrate immediately. Determination of Total Hydroxide. Add standard hydrochloric acid to the filtrate to the first trace of permanent turbidity, and record number of milliliters required. Add 25 ml. of sodium tartrate solution and phenolphthalein indicator, and complete the titration to the last trace of pink. 864
865 judge. Good reagent should contain less than 0.10% carbon dioxide as determined by the evolution method. 6. The potassium fluoride reagent should be adjusted as follows:
.t 5
-
I-
I
10
20
30
I
I
I
40 50 60 Net MI. 0 . 4 N H C I A d d e d
I
70
I 80
Figure 1. Titration Curl-esfor Hydroxide and Alumina in Sodium Aluminate
x
normalitj- of HCI X 0.053 = total hydroxide (as grams of Na2C03) Determination of Alumina. Add to the solution three times the amount of standard acid added between the turbidity and total hydroxide end points and about 4 to 6 ml. in excess. Then, while stirring, add 40 ml. of potassium fluoride solution. Allow a few seconds for the fluoaluminate to precipitate; then backtitrate to the first permanent, pink with standard sodium hydroside. (111. of HC1 - ml. of SaOH) X normality X 0.017 = grams of .&03 Determination of Carbonate. Return the barium carbonate precipitate and paper t o the beaker and pulp with about 200 ml. of hot water. Add a measured tviofold excess of standard acid, and heat on a hot plate until the barium carbonate is decomposed and the carbon dioxide expelled. Add phenolphthalein indicator, and titrate the excess acid with standard sodium hydroxide to the first permanent pink. Total ml.
(111.of HC1
- ml. of
XaOH) X normality X 0.053 = grams of Ka2C03
NOTES Oh- PROCEDURE
J 90
Prepare a blank by adding to 300 ml. of carbon dioxidefree distilled water 25 ml. of barium chloride reagent and 25 ml. of sodium tartrate reagent. Adjust with standard base or acid t o the phenolphthalein end point; then add 40 ml. of potassium fluoride reagent. If more than 0.05 ml. of standard acid or base is required to bring the solution back to the phenolphthalein end point, adjust the stock solution by adding the required amount of acid or base.
PRECISION AYD ACCURACY
A standard sodium aluminate solution was prepared by dissolv-
ing aluminum of high purity (99.99% AI) in standardized C.P. carbonate-free sodium hydroxide. To exclude atmospheric carbon dioxide, the dissolution was made in a borosilicate glass volumetric flask and the evolved hydrogen was vented through an air condenser protected by a drying tube containing Ascarite. After the solution had been diluted to volume, aliquots containing 0.135 and 0.27 gram of alumina were analyzed in triplicate for total hydroxide and alumina. Determinations were made without filtration and without any attempt to exclude atmospheric carbon dioxide. Evidence of carbon dioxide contamination would have been noted as a result of formation of insoluble barium carbonate. contamination was not great enough to be significant. Results, averages, and average differences for total hydroxide and alumina are given in Table I. In all cases the individual differences for total hydroxide are insignificant. One difference for alumina is as great as 0.13 ml., but the average difference a t both concentrations is 0.07 ml. of 0.4 1%- hydrochloric acid.
1. If free hydroxide is desired, it may be calculated as follows:
Grains of total hydroxide - (grams of A1203 X 1.0398) = grams of free hydoxide (as hTa2C03) 2 . I t is important to filter t,he sample very soon after precipitation to minimize carbonate formation. The filtration should be rapid 2nd as free from air entrainment as possible. The filtrate should show only a surface trace of barium carbonate precipitate. 3. Considerable latitude can be allowed in the estimation of the turbidity end point; but if taken too far, the total hydroxide end point is indist,inct. Turbidity can easily be judged x-ith proper back-lighting. 4. Enough excess standard acid should be added in the alumina titration to give a 3- to 5-ml. back-titration. If too much excess acid has been added before the fluoride addition, alumina results tend to be high. If the solution increases in pinkness a t the end of the back-titration, it is an indication that not enough acid has been added to ensure complete precipitation, or not enough time has been allowed for complete precipitation. If this occurs, acid in 2- to 3-ml. excess is again added andneutralized with standard hydroxide, and second buret readings are used. 5. Potassium fluoride reagent may contain fluosilicate or carbonate. The presence of fluosilicate is indicated by a fading end point a t the sodium hydroxide back-titration. Carbonate in the reagent is indicated by a buffered end point that is difficult to
Table I. Determination of Total Hydroxide and Alumina in Carbonate-Free Sodium Aluminate Solutions 25 nn. Total XaOHa Ah08 0 1350 Added, glatn 0.4060 0 1337 Found, gram 0.4059 0 1353 0.4065 0 1353 0.4059 A 7.. 0.4061 0 1355 hv.difference SO.0001 + O 0005 a Expressed as equivalent KalCOa.
50 111. Total NaOH Alios 0 8120 0.2700 0 8120 0.2709 0 8118 0.2700 0 8118 0.2706 0 8110 0 2705 - 0 0001 +o 0005
However. because Bayer solutions contain carbonate, separation of carbonate is necessary and is performed in the ordinary laboratory atmosphere. Because of absorption of atmospheric carbon dioxide during filtration, positive differences for carbonate and negative differences for hydroxide are obtained. If the separation is poorly done, the filtrate will be turbid as a result of precipitated barium carbonate, and this Rill interfere with the turbidity end point. If the separation technique is good, only a trace of precipitate will be found in the filtrate. To determine accuracy and precision under the usual conditions of analysis, a master synthetic aluminate solution containing added carbonate was prepared from standardized sodium car-
A N A L Y T I C A L CHEMISTRY
866 Table 11. Determination of Carbonate, Hydroxide, a n d A l u m i n a in S o d i u m A l u m i n a t e Solutions
Sample No. 1 2 3" 4 5 6
7
NazCO1, Gram By COP Evolution By BaCOa Precipitation Av. of Av. of duplicates Range duplicates Range 0.0002 0.0993 0.0020 0.0975 0.1279 0.0014 0,1292 0.0002 0.1550 0.0022 0 1574 0.0008 0.0010 0.1891 0.0006 0.1912 0.0006 0.0010 0.2189 0,2224 0.2515 0.0010 0.0023 0,2548 0.0017 0.2830 0.0024 0.2843 Total NaOH, as Grams of NazCOa Calcd. (total alkali NaZCOs), av. B y titration 0.1997 0.1973 0.0022 0.2708 0.2638 0.0007 0.3434 0.3363 0.0007 0.4089 0.4017 0.0015 0.4788 0.4702 0.0015 0.5457 0.5376 0,0003 0.5457 0.5376 0,0003 0.0024 0.6139 0.6099 AlzOa, Gram By titration Added 0 0929 0.0001 0.0926 0 1236 0 0001 0.1235 0.1544 0 1537 0 0002 0 1848 0 0007 0.1852 0 2156 0 0002 0.2161 0 2461 0 0001 0.2470 0 2773 0 0002 0.2778
Did. -0.0018 +O. 0013 +0.0024 +0.0021 +0.0035 +0.0033 $0.0013
-
1 2 30 4 6 6 6 7
1 2 '3 4 5 6
7
Standard deviationb Av. difference 0 Master solution.
-
b standard dedation
n
-0.0024 -0.0070 -0.0021 -0.0072 -0.0086 -0.0081 -0.0081 -0.0040
$0.0003 +o. 0001 -0.0007 -0.0004 -0.0005 -0.0009 -0.0005
NazCOI, Gram Total NaOH AIPOI, CO, evol. BaCOl pptn. (as gram NazCOa) gram 0.0011 0.0009 0.0011 0.0002 $0.0017 -0,0063 -0.0004
.. . .
I
where R = range of duplicates and
number of duplicates ( 8 ) .
bonate and sodium hydroxide solutions and high purity aluminum. From this six other solutions were prepared to give solutions with alumina concentrations that ranged from 0.10 to 0.28 gram of alumina per aliquot. Each was analyzed in duplicate for carbonate by the evolution method (6). The average figure obtained was subtracted from the determined total alkalinity of the original hydroxide-carbonate solution to obtain a calculated total hydroxide. All solutions were analyzed in duplicate in a random manner to obtain carbonate, hydroxide, and alumina. Averages of duplicates, ranges of duplicates, standard deviations, and differences from calculated averages are given in Table 11. Agreement between carbonate results by evolution and barium carbonate precipitation is acceptable, the greatest difference being equivalent to f0.17 ml. of 0.4 N hydrochloric acid and the average difference being equivalent to +0.08 ml. The positive carbonate differences are principally due to carbonation of hydroxide during filtration. Agreement between hydroxide calculated and hydroside found is not good. The greatest difference is equivalent to -0.41 ml. of 0.4 N hydrochloric acid, and the average difference is equivalent to -0.30 ml. These differences represent hydroxide determined as carbonate plus that which has carbonated during filtration. These will vary, depending on the filtration technique of the analyst; but the results listed are in accord with experience. As elaborate precautions are necessary to exclude carbon dioxide in the filtration, the differences are accepted and allowances made. The alumina differences are small, the maximum being equivalent to -0.13 ml. of 0.4 N hydrochloric acid and the average to -0.06 ml. Standard deviations for carbonate by evolution and by barium carbonate precipitation, and for hydroxide are comparable, being about 1.0 mg. The standard deviation for alumina is 0.2 mg. While the large difference for total hydroxide is accepted when the procedure is used for control purposes, total hydroxide can be
accurately determined without separation of carbonate by an alternative procedure. A separate aliquot containing from 0.10 to 0.30 gram of alumina is diluted to 250 ml. with distilled water, and 25 ml. of barium chloride reagent is added to precipitate carbonate. Then 25 ml. of sodium tartrate reagent is added, and the total hydroxide is titrated to pH 8.3 using a pH meter. In Table I11 are results obtained on a standard sodium aluminate solution to which carbonate has been added. An average of these results a t the minimum alumina concentration differs from that added by -0.0018 gram or -0.08 ml. of 0.4 N hydrochloric acid. At the maximum alumina concentration the total hydroxide difference is insignificant. EFFECT O F IMPURITIES
Impurities usually extracted from bauxite by a Bayer digest are chloride, sulfate, silicate, fluoride, and phosphate. Chloride and sulfate in the low concentrations found do not interfere. Effects produced by silicate, fluoride, and phosphate were determined by adding them to aliquots of aluminate solution, as sodium salts in solutions neutralized to phenolphthalein. The silicate and phosphate were analyzed for carbonate, and corrections were made. Averages of results in triplicate for each concentration of impurity were compared with average values for ten determinations on the aluminate solution to which no impurity had been added. Differences found are given in Table IV. Application of Student's t test ( 4 ) indicated that differences for carbonate and hydroxide were significant when greater than 2.0 mg. and for alumina when greater than 0.5 mg. Silica a t 5 mg. significantly affected the alumina and a t 10 mg. the hydroxide. The abnormally high difference for carbonate a t 15 mg. of silica is irregular and was not verified when checked later on another solution. Sodium fluoride a t 10 mg. affected the hydroxide, and a t 50
Table 111. Determination of Total Hydroxide in S o d i u m A l u m i n a t e Solutions i n Presence of Carbonate Aliquot Size XarCOr added gram Total NaOH, &am SazCOs AIIOI. eram Total E a O H found, gram KazCOa AV. Av. difference, gram Av. difference, mi. 0.4 N HC1
Table IV.
25 RI1. 0.1997 0 3269 0 1287
50 MI. 0.2034 0 6538 0 2574
0.3248 0.3253 0.3251 0.3251 -0,0018
0.6531 0 6535 0.6538 0.6535 -0.0003
-0.08
-0.01
Effect of I m p u r i t i e s on Analy-sis of Sodium A l u m i n a t e Solutions
Average values of ten determinations without impurities. NatCOa = 0.2125 gram Total NaOH (a5 grams of XarCOa) = 0.4104 A l l 0 1 = 0.1284 gram Averagc Differences Found, M g . Amount, Total S a O H Impurity Mg. SazCOa (as NazCOd A1201 +o. 1 -0.2 Si08 1.0 +O. ?! -1.0 -2.4 J-0 7 5.0 10.0 i o 3 -2.9 -2.9 -4 1 (+I.@ -7.5 -5 1 15.0 -8.4 -9 3 25.0 +i.4 It0 0 10.0 -1.7 +2.8 KaF - 0.1 f2.8 -0.8 20.0 -0.1 -0.6 4-6.2 30.0 +o 0 -3.9 +7.1 50.0 +8.8 -0 3 -3.8 70.0 io0 f13.7 -3.9 100.0 -2.2 -0 3 PZOC 5.0 +2.6 -6.9 -0 2 10.0 4-2.4 15.0 +4.4 -10.1 +o 1 -17.6 r O 0 25.0 +4.3 Differences of 2.0 mg. for carbonate and hydroxide and 0.5 mg. for alumina are significant.
,
V O L U M E 25, NO. 6, J U N E 1 9 5 3 mg. began to affect the carbonate. Surprisingly, even a t 100 mg. it did not affect the alumina. Since fluoride is added in the aluminum determination to form AIFs--- and OH-, the increase in hydroxide would be espected to cause a decrease in alumina of about one sixth the magnitude of hydroxide increase. However, when the experiment was tried with an aluminate solution containing 0.25 gram of alumina per aliquot and 50 to 100 mg. of sodium fluoride, the effect on the alumina was evident. Phosphate had a significant effect on both carbonate and hydroxide even a t 5 mg., but there was no significant effect on the alumina even a t 25 mg. It is suspected that the silica affects the alumina by precipitating it as a barium aluminum silicate, which is separated with the barium carbonate. The magnitude of the differences for carbon:tte and hydroxide is significantly affected by rate of filtration. An impurity that increases filtration time should give correspondingly increased positive errors for carbonate and negative errors for hydroxide because of carbonation from the air. The reverse is also true. In the samples to which sodium fluoride were added, granular, easily filterable barium carbonate precipitates were formed. Based on data present in Table I1 for total hydroxide, the results in the presence of fluoride are nearer truth than those containing no impurities. CONCLUSION
The recommended procedure has the same error for hydroxide caused by carbonation during filtration as the procedure it replaces, but precision is better. Its superiority consists in making the same determinations more rapidly by titrating hydroside and
a61 alumina to phenolphthalein end points that are sharp and reproducible. Impurities in the amounts normally present in Bayer aluminate solutions do not significantly interfere. When estreme accuracy for hydroxide is required, it may be determined on a separate aliquot without separating carbonate by using a p H meter to determine the end point. ACKNOWLEDGMENT
Credit for helpful suggestions and verification work should be given H. G. Monteith, L. R. Fortner, L. J. Linder, R. L. Sommerlot, and the analysts of the Aluminum Research Laboratories a t East St. Louis, 111. LITERATURE CITED
Bushey, A H.,ANAL.CHEX.,20,169 (1948). Craig, T. J. I.,J. SOC.Chem. Ind.,30, 185 (1911). Edwards, J. D., Frary, F. C., and Jeffries, Zay, “The Aluminum Industry. Aluminum and Its Production,” pp. 124-31, New York, MoGraw-Hill Book Co., 1930. (4) Fisher, R. A., “Statistical Methods for Research Workers,” Chap. V, London, Oliver and Boyd, 1948. ( 5 ) Graham, R. P., IND. ENG.,CHEY.,ANAL.ED., 18, 472 (1946). (6) Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorgania Analysis,” p. 623, Xew York, John Wiley & Sons, 1929. (7) Snyder, L. J., IND. ENG.CHEW,ANAL.ED.. 17,37 (1945). (8) Viebock, F., and Brecher, C., Arch. Pharm., 270,114 (1932). (9) Youden, JT7. J., “Statistical Methods for Chemists,” p. 16. h’ew York, John Wiley & Sons, 1951.
(1) (3) (3)
RECEIVED for review August 22, 1951. Accepted hiarch 2, 1953. Presented a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 7, 1951.
Spectrochemical Determination of Impurities in Cobalt Oxide JOHN H. M C C L U R E AND R. E. KITSON Polychemicals Department, E. I . d u Pont de h’emours and Co., Znc., Vilmington, Del. The determination of several metallic impurities in cobaltic oxide (Co304), a problem of considerable importance in this laboratory, is difficult and timeconsuming by chemical or colorimetric methods. A direct spectrographic method was developed for the determination of calcium, barium, copper, silica, iron, manganese, chromium, aluminum, and nickel in cobaltic oxide. The procedure is rapid, as compared with chemical and instrumental methods. In general, 0.001 to 0.1% of the various impurities can be determined with an accuracy and precision of about =t209” relative. The line pairs and their usable ranges are giren, with a solution method for preparation of standard samples.
T
H E determin tion of several metallic impurities in otherwise pure cobaltic oxide (Coa04)is a problem of considerable interest in this laboratory. Chemical or colorimetric methods are available for certain specific impurities. Silica can be determined by conventional gravimetric techniques, nickel by the method of Lingane and Kerlinger (Z), manganese and iron by more or less conventional colorimetric methods, and barium by the flame photometric technique. Such determinations, however, are time-consuming, tedious, and frequently lack the desired precision owing to the complex separations involved. Consequently, the authors sought a more rapid method based on emission spectroscopy for making these and other determinations in cobalt oxide. Since no such method could be found in the literature, the necessary development work was carried out. EXPERIMENTAL DETAILS
Apparatus. A 2.5-meter grating spectrograph (Jarrell-Ash Go.) was used throughout this work. The electrodes were held
in an arc stand (Jaco). KOauxiliary lenses were employed othe than the 450-mm. lens used as a slit cover on the spectrograph. All exposures were made using a rotating step sector with a sector factor of 1.585 (loglo step factor = 0.200). Excitation power was obtained from the d.c. arc section of a Spec-power unit (National Spectrographic Laboratories) equipped with an initiator spark. Preformed, high-purity graphite electrodes (United Carbon Products) were used. The upper electrode was the No. lOOU type; the lower was the same as the No. 102 type but with a a/&nch instead of the standard 3/1e-inch deep cup. Densitometer measurements were made with a Jarrell-Ash Co. Model 200 comparator microphotometer. Eastman Spectrum Analysis KO. 1 or Eastman I-N plates were used throughout. They were developed in D-19 at 20’ C. for 5 and 3 minutes, respectively, using an Applied Research Laboratories developing machine. Preparation of Standards. Two samples of high-purity cobalt oxide were used in the preparation of known samples. One was Johnson-Matthey Specpure cobalt oxide obtained through the Jarrell-Ash Co.; the other was special laboratory purified material. Both sources were necessary since the concentration of certain impurities in one was less than in the other.
