Sodium Gluconate as a Complexiag Agent in the Volumetric Analysis of Aluminum Compounds H. L. WATTS
D. W. UTLEY
and
111.
Alcoa Research Laboratories, Aluminum Co. o f America, Fast St. Louis,
Tartrate has often been used as a complexing agent for aluminum. Several methods have employed barium in conjunction with tartrate to obtain a better end point for the titration of aluminum. In the present work, limited tests revealed certain basic knowledge concerning effects of the structure of complexing agents. It was found that sadium gluconate could satisfactorily replace both tartrate and barium in aluminum titrations. The advantages of gluconate are the lack of sulfate or carbonate interference and the simplicity of a single complexing reagent. Another original feature of the method is the titration of carbonate to bicarbonate in the presence of aluminum. Accurate and precise methods are included for the determination of aluminum oxide or aluminum, acids, hydroxide, and carbonate. The effects of impurities are discussed.
OST volumetric methods for the determination of aluminum are based upon the following reaction: Al(0H)s
+ 6KF
+
K3AlFs J-
+ 3KOH
in which the acidic solution containing the aluminum tartrate complex was neutralized n-ith barium hydroxide. Hale (8) substituted lithium for barium, so that the titration could be done in the presence of sulfate. For analysis of aluminate solutions, such as Bayer liquors, the authors (6) combined a titration of total hydroxide to the phenolphthalein end point with a stoichiometric modification of Viebock and Brecher's method for aluminum. Viebock and Brecher, Snyder, and Bushey (1) recognized that the presence of barium was necessary for accurate p H adjustment of the sample solution before the addition of fluoride. The authors found that a barium-to-aluminum mole ratio of 2 to 1 was required and that strontium could be substituted for barium. Curves for the hydroxide titration in the presence of 0.125 gram of aluminum oxide, shown in Figure 1, demonstrate the effect of tartrate alone, tartrate and lithium, and tartrate and barium. Lithium did not significantly change the slope of the curve over that with tartrate alone, but barium caused more than a threefold increase in the sharpness of the inflection point.
(1)
The quantity of aluminum is calculated from the titration of the released hydroxide. Several investigators have used tartrate salts or tartaric acid to complex the aluminum hydroxide so that it would be soluble during the initial p H adjustment just before the addition of fluoride. Viebock and Brecher (6) formed a harium-aluminumtart rate complex, neutralized the solution, added fluoride, and titrated the released hydroxide. Snyder (4)proposed a variation
Table I.
6
Reactions of Various Organic Reagents with Aluminum
Compound Sodium citrate
Formula COzNaCHzYOHCHzCOda
Sodium tartrate Sodium gluconate Mannitol Glycerol Sucrose
hzya COzNa(CH0H)zCOzNa CHzOH(CH0H)aCOzNa CHzOH(CH0H)aCHzOH CHzOHCHOHCHzOH CH20HCH(CHOH)zCCHzOH
,'
Yes Yes NO h-0 To
Pes Yes Yes h-0
N0
0
CHtOHCH(CHo&CH
I
0
Potassium succinate Sodium 2-ethyl hexoate Sodium propionate Sodium malonate
CO,K(CHY)SCO~S CHs(CH&CHCOzNa
No
No
5-0
N O
CHaCHsCOzNa CHz(C0zNa)n
NO N0
NO NO
I __ oZH6
I
I
I
I
I
1
1
!
5 MILL1 LITERS OF STD A t D
Reagent Aluminum Caused Soluble in Increase Neutral in p H Solution Yes Yes
I
I IO
(
I
I
l
/
15
Figure 1. Titration curves for total hydroxide in presence of various reagents
Sulfate and carbonate are precipitated by barium. The filtered precipitate of barium carbonate has been used for carbonate analysis (1, 6), and hydroxide and aluminum can be determined in the filtrate. However, precipitates of barium carbonate and barium sulfate are adsorptive, and high carbonate and low hydroxide values are obtained. A complexing reagent was needed that would permit sharp end points, be free of sulfate and carbonate interference, and form a fully associated complex with aluminum that can be completely decomplexed by the addition of fluoride, and reactions should be stoichiometric, INVESTIGATION OF COMPLEXING AGENTS
Ten organic reagents, selected for their structures were individually tested as follows:
1731
ANALYTICAL CHEMISTRY
1732
.4sodium aluminate solution containing 0.2 gram of aluminum oxide was titrated with hydrochloric acid to the first trace of precipitation of aluminum hydroxide. The solution was then titrated with a neutral (to henolphthalein) 0.4M solution of the organic reagent until the p k ceased to rise (with seven of the reagents there mxs no rise). Bn increase in pH indicated release of combined hydroxide. I n a second test a mixture of the aluminate solution and the organic reagent was neutralized to about p H 8.3 with hydrochloric acid to find whether the aluminum was complexed against recipitation as aluminum hydroxide. I n each test, 0.2 gram oralunlinum oxide and a t least 4 moles of organic reagent per mole of aluminum were used. The results are s h o m in Table I.
causcd by release of combined hydioxide. The curves show that barium increases the effectiveness of tartrate and citrate, which contain two and three carboxj 1 groups, respectively; but barium impaired the effectiveness of gluconate, which has only one carboxyl group. The curves in Figure 1 shox that the inflection in the presence of gluconate alone is as sharp as that in thc presence of both tartrate and barium. Sodium gluconate met the desired qualifications for a complexing agent. Procedures were then developed for complete analysis of both acidic and basic solutions of aluminum. CHE,MISTRY OF THE PROCEDURES
Ii
-
/
1
- CITRATE+ Ea
7
BOA
GLUCONATE t
Though sources differ concerning t'he exact reactions of complexing agents, some simple equations are included here for better understanding of the reactions. I n the equations sodium gluconate is represented by G1. Typical titration curves of the reactions are also shoxn in Figures 3 and 4. Total Acid. Free acid and the conibincd acid released by complexing aluminum with sodium gluconate are titrated to pH 8.3 with standard sodium hydroxide, as shown in Figure 3. The reaction may be represented as follows: iilC1,
I
I1
1
I
Figure 2. Titration of sodium aluminate with complexing agents
I
I
NOOH*
I
TOTAL HCl
. GLUCONATE
1
-
(K~IF~:~AI*+'l
2-
I
1
I
I
I
IO 20 30 MILLILITERS OF STD NoOH
40
-
I
10 20 M I L L I L I T E R S O SFT O
K F ADDED
-1:
I
I
30
40
nci
-
+ HC1 + nG1 + 4SaOH .41(OH),.G1,, + 4NaC1 + H20
(2)
Total Soda (Total Hydroxide Plus Total Carbonate). In the fiist part of the reaction, the sample containing sodium aluminate, sodium carbonate, and free sodium hydroxide is acidified !Tit11 excess hydrochloric acid. The carbon dioxide is evolved by boiling SaX102
+ Na&03 + KaOH + 8 H C 1 6 A1C13 + HC1 + COS t + 4KaC1 + 4H10
(3)
llftcr the sample is cooled to room temperature, sodium gluconate is added. Free acid and the combined acid released by coniplexing the aluminum are titrated to p H 8.3, as in the total acid determination (Figure 3 and Equation 2). The net hydrochloric acid titration from Equations 3 and 2 represents total soda. Hydroxide Plus One-Half Carbonate. The familiar titration of carbonate to bicarbonate can be performed in an alkaline solution of aluminum that is complexed with sodium gluconate. However, pH 8.1 is the inflection midpoint, whereas p H 8.3 is usually selected in the absence of aluminum. The correct end point is important because the titration is buffered. Sodium gluconate is added to the sample containing sodium aluminate, sodium carbonate, and free sodium hydroxide. Then, as shown in Figure 4, the sample is titrated to p H 8.10 v i t h standard hydrochloric acid. SaAlOp
Citrate, tartrate, gluconate, and ,mannitol complexed aluminum against precipitation. Each of these reagents contains one to six hydroxyl groups on chains consisting of four t o six carbon atoms. Sucrose and glycerol, n-hich have hydroxyl groups but longer and shorter chains, respectively, failed in this test Citrate, tartrate, and gluconate released combined hydroxide. They contain one to three carboxyl groups and one to five hydroxyl groups. Mannitol, which differs from gluconate only in the lack of a carboxyl group, complexed aluminum against precipitation but did not release combined hydroxide. Figure 2 shows curves of 0.2 gram of aluminum oxide complexed with citrate, tartrate, and gluconate, in both the presence and absence of 2.5 grams of barium chloride dihydrate. The mole ratios of reagent to aluminum were plotted against rise in pH
+
+ ?;a&O3 + NaOH + nG1 + 3HC1-+ ..ll(OH)3.GIn + NaHC03 + 3NaCI
(4)
Aluminum Oxide or Aluminum. The aluminum determination can be performed after titration of total acid, total soda, or hydroxide-bicarbonate, since, after any of the three titrations, aluminum is present as the gluconate complex and the solution has been titrated to either pH 8.3 or 8.1. Either pH 8.3 or 8.1 can serve as the starting pH of the aluminum titration, provided the same p H is used for the final end point. I n the recommended method (Figure 3) standard hydrochloric acid is added a t this point in an amount sufficient to give an excess over that required to neutralize the hydroxide present in the Al(0H)s G1, complex. Potassium fluoride is then added, the three equivalents of hydroxide per equivalent of aluminum are released, and the remaining excess hydrochloric acid is back-titrated to the starting p H of 8.3. The alternative method (Figure 4) is very similar. The only difference is that the fluoride is added a t the
1733
V O L U M E 28, N O . 11, N O V E M B E R 1 9 5 6
then add a 5- to 10-ml. excess of acid. Back-titrate with standard sodium hydroxide to the starting pH (8.3 or 8.1).
start of the aluminum titration instead of after the addition of excess hydrochloric acid. The hydroxide released by the addition of fluoride is titrated with hydrochloric acid, excess hydrochloric acid is added, and the excess hydrochloric acid is backtitr:tted with standard sodium hydroxide.
Grams of iil,Oa = (ml. of HCl
Apparatus. H meter. Magnetic or mechanical stirrer. Reagents. lodium gluconate (Pfizer, refined), 20% solution. Store in polyethylene. Adjust to henolphthalein neutrality. Standard hydrochloric acid a n 1 sodium hydroxide solutions, 0.3 to 0 . 4 N . Fot,assium fluoride solution, 50yo K F . 2HzO by weight, filtefed, and stored in polyethylene. Adjust the solution with sodium hydroxide and hydrochloric acid until 25 ml. of the potassium fluoride solution, when added to about 250 ml. of n-ater and 50 ml. of sodium gluconate solution a t about pH 8.3 or the phenolplit,halein end point, will change the p H by not more than the equivalent of 1 drop of standard hydrochloric acid or sodium hydroxide. Phenolphthalein, 0.1 % solution. Distilled water. The distilled rvater should be boiled and cooled for greatest accuracy in titrations of hydroxide, carbonate, or total acid. Procedure. INITIAL PREPARATIOB OF ALL SAWPLES. Pipet an aliquot of sample containing from 0.025 to 0.25 gram of aluminum oxide (0.013 to 0.13 gram of aluminum) into a 600-ml. beaker. .Idd several drops of phenolphthalein indicator. If the sample is colorless to phenolphthalein, use Procedures I and V. If pink t o phenolphthalein, use Procedures I1 and V on one aliquot and Procedures I11 and V on a duplicate aliquot. OF TOTAL ACID. Add 50 ml. of sodium gluconI . TITRATION ate solution to the sample, and dilute i t to about 250 ml. Titrate to pH 8.3 with standard sodium hydroxide.
.
Grams of HCl = ml. of XaOH X normality X 0.03646 Grams of H2S04 = ml. of NaOH X normality X 0.04904 OF TOTAL SODA(Total Hydroxide and Total 11. TITRATION Cubonate). Add standard hydrochloric acid (a recorded amount) to the sample until aluminum hydroxide precipitates and is redissolved by stirring. Cover the beaker with a watch g1:tss. Bring the sample to a boil, and allow it to simmer for about 10 minutes. Cool the sample to room temperature, wash don-n the watch glass and sides of the beaker, add 50 ml. of sodium gliiconate solution, and dilute the sample to about 250 ml. T i t u t e to pH 8.3 with standard sodiiini hydroxide.
Gi,:tnis of S a O H (as Na&OB) plus grams of YaCO:, = (ml. of HC1 - ml. of S n O H ) X normality X 0.053
111. TITRATION OF TOTALHYDROXIDE PLT-SOSE-HALF .4dd 50 ml. of sodium gluconate solution to the CARBONATE. Titrate n-ith standard sample, and dilute it, to about 250 nil. hydrochlcric acid to pH 8.10. Gr:ims oi 1:iOH (as Sa2C03) plus '/2 grams of Fa2C03 = ml. of HC1 X normality X 0.053 HYriHoXIDE A N D
X normality X 0.0169'3
Grams of hi = (ml. of HC1 - ml. of NaOH) X normality X 0.00899
PROCEDURES
I\-.
- ml. of XaOH)
CARBONATE CALCULATIOXS.
6r:uns of Na2C03 = 2 X (result from I1 - result from 111) Grams of S:iOH (as X32C03) = result from I1 - grams of Xn2CO3
TITRATIOX OF ALCXISCM OXIDEOR . 4 I-. the titration of the sample from eit.her I or I1 mended or alternative method. For titration of the sample from 111, use onlv the alt,ernative method. A . Recdniniended Method. .Idd 25 ml. of standard 0 . 3 5 hl-drochloric acid if the sample contains less than 0.105 gram of aluminum oxide or 50.0 ml. of hydrochloric acid if the sample contains from 0.105 to 0.21 gram of aluminum oxide. When using 0.4X acid and base, adjust the amount's proportionately. If the approximate content of aluminum oxide is known, add enough hydrochloric acid to provide a 5- to 10-ml. excess after release and neutralization of the combined hydroxide. Add 25 ml. of potassium fluoride solution. Back-tit'rate n-ith standard sodium-hydroxide to pH 8.3. B. Alternative Method. Add 25 ml. of potassium fluoride solution. Titrate with standard hydrochloric acid at the full speed of the buret until the sample iscolorless to phenolphthalein:
DISCUSSION OF PROCEDURES
Phenolphthalein end points can be used for all titrations except the hydroxide-bicarbonate (111) titration and the aluminum titration in the presence of carbonate.
13
I
I
1
1
5
IO
i
!
15
I
I
I
I
20
M I L L I L I T E R S OF STD
25
30
J
tic1
Figure 4. Titration curves for total sodium hydroxide plus one-half sodium carbonate and aluminum
In all titrations the st>irringrate should t i e slomr than that :tt n-hich air bubhles are whipped into the solution. The slow stirring rate helps to avoid pickup of atmospheric carbon diosidt.. If the aluminum oxide (aliiiiiinum) titration is the only olle desired, the initial adjustment, to approximately pII 8.3 can be simplified by the use of more concentr:tted solutions of sodilim hydroxide and hydrochloric acid. Combined acid or conihined sodium hydroxide can he calculilted from the aluminum result. A value for free acid or free sodium hydroxide can he obtained ljy siibtracting from the total acid or tot,al sodium hydroridr the combined acid or cqnihined sodium hydroxide. In the procedurvs separate w n p l e aliquots are used for thy hydroxide-bicarhonate a i d the total soda titrations; however, aluminum can I x determined on either or both samples. -kctually, all t,hree titrations can he performed on a single sample diquot, ns follows: Perform the hydiosidc-bicarbonate titrat,ion, add rxeess standard acid, boil out the carbon dioxide, neutralize with sodium hydroxide and add a small excess, boil again, cool, complete the total soda determination by titrating to pH 8.3 with hydrochloric acid, and titrate the aluminum. I n using this varistion, two point's must be emphasized. The addition of excess hydrochloric acid and boiling convert sodium gluconate to a ratller stable ?-lactone form of gluconic acid ( 7 ) . The reconversion t o sodium gluconate, without which inaccurate values are obtained for total soda, requires boiling the sample in the presence of excess sodium hydroxide ( 3 ) . Also, because the lactone conversion of the gluconic acid can result in partilt1 dissociation of t'he aluminum complex, enough acid should be present to convert all aluminum to aliiininiini chloride.
ANALYTICAL CHEMISTRY
1734 Table 11. Determination of Total Hydrochloric Acid and Aluminum Oxide in Hydrochloric Acid-Aluminum Chloride Solution
Table 111. Determination of Total Sulfuric Acid and Aluminum Oxide in Sulfuric Acid-Aluminum Sulfate Solution
Total HCl
.4v.
Added,
.4v.bias,
0.1062 0.2124 0.4249 0.5311 0,7435 0.8498 0.4780
+0.2 +0.3 -0.2 +0.9 +0.9 +0.7
+0.5 Std. deX7.a AltOa Recommended Method Range of Av. bias, duplicates, mg. mg.
Added, g. 0,0251 0.0501 0.1003 0.1253 0.1754 0.2005 Av.0.1128
a
Total HtSOa Range of duplicates,
-0.2
+0.3 +0.3 -0.5
Alternative Method Range of duplicates, mg. mg.
+0.2 -0.3 -0.2 -0.9
4;
-0.1
0.18 0.15
where d is difference between duplicates and n ia
PREPARATION OF SYNTHETIC SOLUTIONS
Solutions of hydrochloric acid, sulfuric acid, and a mixture of sodium hydroxide and sodium carbonate were prepared and carefully analyzed. Then, weighed amounts of 99:988% pure alumirium wire were dissolved in aliquots of the solutions. I n the case of the acids, the aluminum solutions ?\'ere prepared in a flask \%-itha ground joint attached to a reflux condenser. For the sodium hydroxide-sodium carbonate solution, a larger flask was used, and it was connected to a trap which was vented through an Ascarite tube to prevent absorption of carbon dioxide. Heat was applied in each case to hasten the reaction. The aluminum dissolved in a few hours in solutions of either hydro-
Table IV. Determination of Aluminum Oxide, Sodium Hydroxide, and Sodium Carbonate in Sodium Aluminate Solutions XaOH (as NatCOs), G. Added Found
Na.CO8.
G.
Found
AlzOa Recornmi Method), G. Added Found
Solution I
Av. found, g. Av. bias, mg. Std. dev.," mg.
0.2822
Av. found, g. Av. bias, mg. Std. dev.,a mg.
2844 2844 2854
0.5424 0.541E 0.5412
0.2848 +2.6 0.39
0.5417 -1.4 0.41
0.2856 0.2864 0.2846 0.2822 0.2848 0.2838
Solution I1 0.5431 0.5413 0.5405 0.5418 0,5432 0.5419 0.5424 0.5419
0.2846 +2.4 1.46
4
+0.4 -0.2 +1.0 +1.4 +0.7
+0.9 +0.7 +0.1 +0.8
+0.7
+o.o
Av. 0 . 1 1 4 8 Std. dev.
0.1 0.2 0.3 0.0 0.1 0.1
$0.5
(8)
=
Std. dev.'
0.4 0.4 0.9 1.2 0.8 0.63 0.52
Alternative Method Range of duplicates, nig.
Av. bias, mg.
+o
4
0'
-0
4
0.1
-1.5
0 4
0.13 Std. dev.0 0 . 1 1
42,
where d is difference between duplicates and
?i
is
number of duplicate pairs.
number of duplicate pairs.
Added
+O.J
0,0255 0.0510 0.1021 0.1276 0.1786 0.2042
a
Range of duplicates, mg. 0.1
Av. bias, mg. +I.?
A1208 Recommended Method Range of Av. Bias, duplicates, mg. mg.
Added, g.
0.2 0.2 0.1 0.1 0.1 0.4
+0.4 +0.4
+0.1 0.15 Std. dev.a 0 . 1 4
Std. dev. (s) =
Av.
Av. bias,
0.0 0.2 0.1 0.1 0.1 0.4
+0.4 +0.5
0.12 0.11
Added, g. 0,2458 0.4909 0.9819 1.2273 1.7182 1.9637 1.1046
-1.2
0.92
-
chloric acid or sodium hvdroxide-sodium carbonate. Wben sulfuric acid was used, hoG*ever, complete solution of the aluminum required several days, even though mercuric chloride (1.2 mg. per gram of aluminum) was used a catalyst. After the aluminum had dissolved completely, the solutions were diluted to volume a t room temperature. in volumetric flasks. After thorough mixing, each solution was transferred to a polyethylene bottle. PRECISION AND ACCURACY
Recoveries and standard deviations are shown in Tables 11,111, and IV.
Table V. Effect of Impurities on Aluminum Oxide Results Actual Reagent Tested
Equivalent Oxide Si02 Be0 FeO Fez01 >In0 ZrOt Ti02 GazOa ThOz SnO
vzos
0.1270
+o. 1
0.09
ZnO UOa
SI0
coo CezOa
BaO
0 2505
0.2500 0,2500 0.2501 0.2498 0.2499 0,2498 0.2499 -0.6 0.13
F)*,where (X - 3 ) is deviation of each a Std. dev. ( 8 ) = -1 determination from average and .V is number of determinations.
CrzO,
...
Ai4s;OS CUO CdO TVOS Ptoi BtOs 48203 CaO CrOq MgO >\1003
SrO Lit0
Approximate rimount of Oxide Impurity (G')Thitt Produced Significant Bias
