Determination of Organic Soda in Aluminate Solutions by an Ion Exchange Method HENRI SHEHYN Aluminium Laboratories Limited, Arvida, Que., Canada
A fast and accurate method was needed for the determination of the organic soda content of aluminate solutions. Fluorides and phosphates are removed with alumina by a carbon dioxide precipitation. Subsequent treatment with a cation exchanger removes the remaining cations, molybdate, and most of the vanadate. Organic acids plus the sulfuric and hydrochloric acids are titrated w-ith standard sodium hydroxide in the effluent. After sulfate and chloride have been determined in the titrated solution, the sodium hydroxide equivalents are calculated and subtracted from the titration which represents the organic soda content of the sample. Five determinations may be completed in one day. Increased information on the various forms of alkali present in aluminate solutions is made available, which is of interest for the control of the Bayer process for obtaining alumina.
W
Table I.
Recovery of Sodium Oxalate, Acetate, and Formate as Organic NaK03 Taken
Salt IianCgO4
CHaCOONa HCOONa
Equiv. organic soda, mg.
52.7 78.9 131.1 131,l
Found, Organic Soda,
RI g. 52.4 78.6,78.8, 78.4 130.9 130.9
change resin ( I ) , it is removed a t that point and does not appear in the effluent. Vanadate. The case of this compound, which is always present in Bayer solutions, is slightly more complicated for small amounts find their way into the column effluent. I n an Amberlite IR-120 column, 60 to SOToof the vanadate has been found to be reduced and retained. T h e soda equivalent of the amount remaining in the effluent is very small; tests carried out on synthetic samples have indicated that this error is compensated by a small loss of organic acids by adsorption on the precipitated alumina. Chloride and Sulfate. These follolv the organic acids throughout the procedure and the resulting hydrochloric and sulfuric acids are titrated a t the same time as the organic acids. These two constituents must be determined in the titrated solution in order to make the proper corrections. Sulfate is easily determined hy the usual barium sulfate precipitation method. T h e chloride determination is generally complicated by the presence of traces of iodide and measurable amounts of thiocyanate. No appreciable error is introduced by counting the former as chloride, but the latter must be eliminated before determining the chloride. For this purpose, the destruction of thiocyanate by boiling with hydrogen peroxide in alkaline solution ( 2 ) has been found most satisfactory.
H E S bal.ixite is submitted to the Bayer process for ex-
tracting alumina, part of the small amount of humic matter present is degraded and oxidizes to acids which remain in t'he aluminate solution in the form of sodium salts. The fate of starch, sometimes used as a filter aid, is similar, so that this addition also contributes t o the organic content of aluminate solutions. T h e amount of sodium thus bound to organic acids and expressed as sodium carbonate is called organic soda. Because this form of alkali is not directly accessible to the usual acidimetric determination of total soda, a method of detcrmination is desirable. Until recently the organic soda content of aluminate solutions was obtained by determining the total sodium content of the sample by the classical zinc uranyl acetate method and expressing the result as sodium carbonate. T h e total sodium carbonate b y titration and the sodium carbonate equivalents of the sodium sulfate and sodium chloride present were subtracted and the difference was called organic soda. Corrections for silicate, vanadate, phosphate, and fluoride were neglected. This rather unsatisfactory system was used because a thorough literature search had failed to reveal any method for determining organically bound soda in aluminate solutions. 11 more direct and accurate method has now been worked out, using an ion exchange. The principle of this method is as follows: If the alumina is removed from a suit,able aliquot' of the sample by means of a carbon dioxide precipitation and the filtrate from this separation submitted to exchange in a cation exchanging column, the effluent contains all the acidic constituents in the free state and thus they are accessible to alkalimetric titration after boiling out the carbon dioxide. This titration, corrected for the soda equivalents of the sodium sulfate and chloride present, is a direct measurement of the organic soda. In practice, the following minor constituents of Bayer solutions have to be considered. Fluoride, Phosphate, and Silicate. Fluoride and phosphate could be expected to be quantitatively recovered by ion exchange and the silicate more or less completely. T h e presence of these acids in the effluent would require correction. Fortunately, they have been found to precipitate with the alumina during the carbon dioxide precipitation and thus have been eliminated from the system. Molybdates. T h e molybdate content of aluminat,e solutions i p generally low. Because molybdate is reduced by the ion ex-
EXPERIMENTAL EQUIPMENT AND REAGENTS
Ion Exchange Resin. Three cation exchanging resins have been tried with synthetic solutions containing organic sodium salts known or suspected to be present in actual Bayer solutions. A carboxylic type was entirelv unsuitable, whereas Amberlite IR120 was the best of the two sulfonated hydrocarbon types tried. Typical results obtained with sodium oxalate, acetate, and formate are summarized in Table I. Tests parried out with sodium salts of other organic acids gave similar recoveries of 100 + 0.2'%. The same applies to other organic compounds actually extracted from Bayer aluminate solutions. Ion Exchange Column. This simply consists of a 22-mm -inside diameter glaps tube, 550 mm. long, closed a t the bottom hy a one-hole rubber stopper through n hich passes a 5-mm.-inside diameter piece of glass tubing. T o this tubing, is attached a piece of rubber tubing provided with a screx clamp vi hich permits starting, stopping, and the regulating of the flow. The resin is supported by a wad of glass wool which effectively prevents any resin particles from passing into the effluent. This column is filled with Amberlite IR-120 cation exchange resin to a height of 330 mm. Before using, water should be passed through the column until a 25-ml. portion does not require more than 1 drop of 0.1N sodium hydroxide to give a strong alkaline reaction to phenolphthalein. After using, the column is easily regenerated b y passing through slowly 100 ml. of 4JV hydrochloric acid and washing until acid-free with distilled water. PROCEDURE
Determination of Total Acids. Pipet a 50-ml. sample of the aluminate solution into a 500-ml. volumetric flask, and make up
51
ANALYTICAL CHEMISTRY
62
to the mark with water a t 20' C. Mix thoroughly and pipet a 25-m1. aliquot into a 250-ml. Phillips beaker. Add 50-ml. of hot water, heat to 80" C. on a water bath, and pass in a brisk current of carbon dioxide for 20 minutes. Set a n 11-cm. Whatman No. 52 paper in a n ordinary funnel placed on a Fisher Filtrator. Completely fill the paper with 2 N sodium hydroxide, alloiv to soak for about 1 minute, and wash thoroughly with hot water using light suction. Discard washings and replace receiver by a 250-ml. alkali-resistant beaker. This prewash removes alkali-extractable compounds from the paper and ensures constant corrections. Filter the precipitated alumina under light suction and wash 4 to 6 times with hot water. Police the precipitation flask but do not attempt to recover all strongly adhering alumina. Concentrate the filtrate to 30 to 40 ml. and cool to room temperature. Reserve as filtrate I . Open the paper containing the precipitate on a flat surface (glass or plastic) and bj- using a spatula, transfer the precipitate back into the precipitation flask. Add 5 ml. of 1 S sodium hydroxide and a little hot water; digest to dissolve the precipitate. Dilute to 75 ml. with hot \mter and reprecipitate, filter, and wash as above. Concentrate filtrate to 30 to 40 ml. and reserve as filtrate 11. Cautiously introduce filtrate I into the ion exchange column which has been washed u i t h water to the point where a 25-nil. portion of water run through will not require more than 1 drop of 0.1N sodium hydroxide to react strongly alkaline to phenolphthalein. A brisk evolution of carbon dioxide takes place; when this slows down, open the screw clamp allowing the sample to slowly displace the water in the column. Close the screx clamp and allow to stand for 5 minutes. Replace receiver by a 300-ml. Erlenmeyer flask provided with a ground joint. During the standing period, large carbon dioxide bubbles form and create gas pockets in the column. The escape of this gas will be greatly helped by tapping or vibrating the column. After the 5 minutes have elapsed, add 25 ml. of water to the column and regulate the screw clamp, so as to obtain a flow of 25 ml. per minute (a slower rate does no harm). As soon as the liquid level reaches the top of the resin layer, add another 25-ml. portion of water and repeat this until eight 25-ml. portions of wash water have been used. Close the screw clamp and remove the receiver. Add 2 to 3 glass beads, and attach the flask to a reflux condenser by a ground joint. Bring to boiling and boil briskly for 5 minutes. Remove the heat, detach the condenser, and immediately cover the mouth of the flask with a small watch glass. Cool in running Lvater. When sufficiently cool to handle comfortably, add 8 to 10 drops of 0.1 % phenolphthalein indicator solution and titrate with 0.1N carbonate-free sodium hydroxide solution. Deduct 0.2 ml. and record titration I. Reserve titrated solution for determining chloride and sulfate. After regenerating the column (or using another ready one) proceed in the same manner with filtrate 11. Titrate, deduct 0.2 ml., and record titration 11. Discard titrated solution which is sterile in chloride and sulfate. Determination of Sodium Sulfate. Introduce reserved solution I into a 250-ml. volumetric flask and adjust to the mark with water a t 20' C. Mix well, and pipet a 100-ml. aliquot into a 250-ml. beaker. Add 2 drops of methyl orange indicator solution and acidify with 12147 hydrochloric acid, adding 0.5 ml. in excess. Bring to boiling, and from a pipet introduce 3 ml. of 1K barium chloride solution while stirring. Boil for 5 to 10 minutes, cover, and digest a t 80" to 90' C. for 2.5 to 3 hours. Remove from heat, add 1 drop of S & S ash-free anticreep reagent without stirring. Filter on a 9-cm. (S & S no. 589) blue band paper. Decant all the clear supernatant liquid before transferring precipitate to the paper. Wash 5 times with hot water containing 1 ml. of anticreep fluid per 500 ml., once with alcohol or acetone, and 5 additional times with hot water containing anticreep fluid. Place the paper in a tared 15-ml. platinum crucible, dry a t 120' to 130' C. in oven for 0.5 hour; ignite a t 1000" C. for 0.5 hour. Cool in a desiccator and weigh. Determination of Sodium Chloride. Pipet a second 100-ml. aliquot of the titrated solution into a 250-ml. Phillips beaker, and add 12.5 ml. of 2147 sodium hydroxide followed by 10 ml. of Superoxol (30% hydrogen peroxide). Bring to boiling and boil for 15 minutes. Cool, add another 5 ml. of Superoxol, and boil again for 10 minutes. Cool and transfer to a 250-ml. beaker. Acidify with 2 ml. of concentrated nitric acid and stir in 6 to 8 ml. of 0.LV silver nitrate. Heat to boiling, stirring occasionally, to coagulate the precipitated silver chloride. Cover beaker and place in the dark. When the precipitate has settled. filter on a tared fritted filtering crucible and wash 3 to 4 times with cold 0.1N nitric acid. Dry a t 125' to 130' C. for 2 hours and weigh. If so desired, the chloride may also be titrated after the peroxide treatment. Calculations. The sum of titrations I and I1 multiplied by
2.12 represents the sodium carbonate equivalent oi all acids titrated, expressed in grams per liter. With the aliquoting syetcm uqrd above, 100/250 of the titrated solution represents 1.0 ml. of the original aluminate solution, so that each milligram of barium sulfate and silver chloride actually represents 1 gram per litrr. The sodium sulfate and chloride values and their sodium carbonate equivalents are obtained as follows: (Mg. BaS04) X 0.6086
g./l. S a 3 3 4 and (mg. BaS04I X 0.1511 g./l. ?;a,COa (Mg. hgC1) X 0.4078 = g./l. XaCl and (mg. A%g(l) x 0.3;O = g./l. SaCOl and finally: = =
(Sa2C03equivalent of total acids) - (Na,COy equivalent of Na2SOcplus XaC1) = organic Na&O3 content in g.11,
If other sample or aliquot sizes are taken, the calculations should be changed accordingly. This procedure applies to polutions containing approximately 20 to 80 grams per liter of alumina and 230 to 270 grams per liter of total soda. For other aluminate solutions the sampling and aliquoting may be changed t o provide suitable titrations. For practical reasons, however, the amount of sodium carbonate introduced into the exchange column should not exceed 0.6 to 0.7 gram. For weak solutions or wash solutions containing phosphate and/or fluoride, it may be advisable to raise the alumina content by adding a measured amount of pure sodium aluminate (reagent grade) solution; this is to ensure complete removal of fluoride and phosphate. Excessive frothing in the column may be controlled bv adding one drop of ether or a very thin spray of Corning antifoam; the use of the former is preferred because the latter may in time coat the resin with very hard-toremove silicone compound. illuminate solutions should not be introduced into the exchange column without first removing the bulk of the alumina by a carbon dioxide precipitation; if this is not done aluminium hydroxide v d l precipitate on the resin in the column and cause loss in efficiency and heavy adsorption losses. Such precipitated aluminum hydroxide is also difficult to remove entirely. Adding some resin to the sample before pouring into the exchange column was not found satisfactory. The operation was messy and led to uncertainties regarding the quantitative washing out of the liberated acids. ANALYTICAL RESULTS
Table I1 shows the results obtained by applying the method t o a number of Bayer aluminate solutions.
Table 11. Reproducibility of Organic Soda Determinations in Bayer Solutions (Expressed as grams per liter of KaPCOa)
Sample No. 1 32.6 32.7 32.4 32.5 32.6 32.3
.. ..
Av. 32.5 Std. dev.0.1
2
3
4
5
6
41.1 43.7 43.7 43.8 43.0 44 2 43.4 43.8 43.8
15.7 15.5 l5,7 15.6 16.0
21.9 21.7 22 0 20.8 21.8
6.8 6.3 6.5 6.2
15.7
21.0
6.4
0.3
0.2
..
. . . . . .
7
8
9
1
0
1
1
9.2 3.6 9.3 3.8 9.2 3.8 9.0 .. 9.1 .. 9 2 . .
4 . 8 2 5 , l 33.5 46.5 4 . 3 24.6 33.5 46.9 4.6 .. 33.1 .. 4.2 . . . . . . 4.6 . . . . . .
9.2
4.5
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7
24.8
0 . 2 0 . 2 0 . 1 0 . 1 0 . 2
33.3
46.7
0.2
The reproducibility is satisfactory and the accuracy is estimated as being of the same order with a slight negative bias owing to unavoidable small losses of organic acids by adsorption on the precipitated alumina hydrate According to the experience acquired in this work an analyst can easily operate five columns a t the same time and from one to five sample. may be analyzed in one 8-hour day. LITERATURE CITED
(1) Samuelson, O., "Ion Exchangers in Analytical Chetnistry," pp. 74, 1 3 6 , New York, John Wiley 8: Sons, 1953. (2) Schulek, E., 2. anal. Chem., 112, 415 (1938).
RECEIVED for review June 7, 1954. Accepted October 11, 1954
