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Titrimetric determination of aluminium in steel
VOL.
13 (1955)
ANALYTICA
TITRIMETRIC
CHIMICA
DETERMINATION
OF
ACTA
309
ALUMINXUM
IN
STEE.L
1YY
IXTRODUCTION
In a recent paper’ WC described a gravimetric procedure for the determination of aluminium in plain steels. This method involved the separation of iron from the sample using dichlorodiethylethcr and was so successful in this respect that it was decided to extend the method by using a titrimetric or calorimetric finish. The titrimetric finish which was examined was that based on the complexing of aluminium ion fluoride293. The method was based on an acid-base titration of Al4 3 first in the absence of fluoride and then in the presence of fluoride. Early methods were based on the equations:AlCl,
+
Al(OH),
3 NaOH
-> Al(OH)3
+ G NeF-+
Na,fAlF,]
-j- 3 NaCl
. . . . , . . . . . . . . . . . . . (I)
t_ 3 NaOH
. . . . , . . . . . . . . . . . (2)
but different workers were unable to agree on the choice of indicator and the conditions of titration. Low results were frequently obtained and these were considered to be caused by the formation of basis salts such as Al(OH)&X In a more recent paye+ UECK, AND SZABO overcame this difficulty in the followinli: manner. The solution was made slightly caustic and equal aliquots were then taken. The first aliquot was titrated with hydrochloric acid. To the second aliquot excess sodium fluoride was added and the soiution titrated to the same end-point as before. The equations on which this was based are as follows:Na,A103+~HC1-rA1(OH)3+3NaCl Na,AlO,
-+ 6 NaF +- 3 H,O -> Na,[AlFe?
. . . . . . . . . . . . . . . . e (3)
+ G NaOH
. . . , . . . , . , (4)
The addition of fluoride means that three extra molecules of hydrochloric acid are required in the titration for each molecule of aluminium present. They recommended that a five times excess of fluoride should be used and that neutral red should be used as indicator. This paper appeared to show promise and work was carried out to apply the method to steel analysis. Refcvences
p. 3x2.
.
C.
3x0 Invesligalio*L
curried
ELLIOTT,
J.
W.
ROUINSON
VOL.
13 (r955)
out
There were several points in BECK AND %x30’s paper which were not clear. For _ example in Table II of their papc+ calculation of the amounts of aluminium found irom their titration results do not agree with the figures they have published and in fact cleviatc significantly from the theoretical figure quoted. They also recommend taking aliquots of the original solution. This is time-consuming and can introduce errors when used as a routine method; steps were therefore taken to overcome this. The interferences they noted from carbonate and silica were confirmed and their recommendation to use a five times cxccss of fluoride was adhered to.
Titrations were carried out on a single solutions as follows. The solution was made slightly alkaline with a caustic soda. Standard hydrochloric acid was then run in until the solution had passed the end-point. The change in pa was plotted using a PH meter. Sodium fluoride was then added in excess and the solution boiled to ensure that the reaction was complete. It was then cooled to displace the equilibrium in favour of the complex formation. The solution was then titrated again with the standard hydrochloric acid, and as before the PH was plotted. The ptr curve obtained is shown in Fig. 1. 7tifafion Of 2.05mg of Aluminium
6--
g
5‘0
‘,
4.1 a #
6
t
t
72
16
*
Vo~mc%
O.i%
8
I
1
32 36 40 N HCl Iml)
It can be seen that the two curves arc not quite parallel but if the acid is standardized with a given indicator against a standard aluminium solution, reproducible results should be obtained. For this reason cresol red and thymol blue were used as indicator. Rcproduciblc results were obtained on mixture of pure iron and aluminium salts. On applying the method to steel analysis however, the other elements prcscnt caused poor results to be obtained, Their presence was eliminated by precipitating the remilining iron and aluminium with ammonia/ammonium chloride mixture, filtering and proceeding with the dissolved precipitate. It was found necessary to wash away ammonium salts from the precipitate with sodium chloride solution, otherwise ammonia was driven off when boiling before the last titration and the pH of the solution was disturbed causing poor results to be obtained. Briefly the method evolved was as follows : Referegrces p. 3x2.
13 (1955)
VOL.
TITRIMETRIC
DETERhIINATION
OF
3x1
ALUMINIUM
After extraction of the bulk of the iron, the remaining iron and aluminium were precipitated and filtered. The precipitate was washed free from ammonia with sodium chloride solution and then dissolved in dil. hydrochloric acid. Excess sodium hydroxide was then added and the solution titrated to the first end-point using cresol red/thymol blue as indicator. Sodium fluoride was added to the solution which was then boiled, cooled and titrated again with hydrochloric acid to the same end-point as before. From the volume of standard hydrochloric acid required after fluoride addition the quantity of aluminium present in the solution can be calculated.
The results obtained are listed in Table I. TA13LE --_._
.--I--
.__.
--_-
;I1
_-
-----
.-
..---.-
1,‘
-_
-
.
s:tc1
ml:
___---
._._
.__.
---.-
/I
-_ ._.
I
____--.____
_
_._
‘I’dal
I adderl
--_.
.__
Al
__-.__-_
__-___
..--
Al
.__.
nil nil nil
0.0.) 0.04 0.04
0.04 0.0.) 0.05
0 0.~
0 .)I
0.4.5
0.41
0.04
0..+1
0.4.5
0.41
0.04
0.41
0.45
0.40
0.04
4*40
4.49 4.49
4.53 4.53 4.53
4.40 4.40 4.54
27.00 27.00
27.05 27.05
2G.7 27.1
0.04 0.04
0.04 0.04 0 04 0.04
- ...-_-se-__-_-
_.__._ . -_- __.^__-_--___ RECOMMENDED
_ _-___..
.-..--..
.
.
wz _ ._-.--- --__ . ..-_ ______.._ -_ ..__-._
WT _ ._____._______ ________________nw
o.o.$
_-__.
/ouud
_____-__--___-__-_---.^
-_ -- -
PROCEDURE
Heagents
dichlor diethyl ether Hydrochloric acid Nitric acid Ammomum chloride solid solutron Sodiu’m chloridd’solution Ammonia Sodium hydroxide solution Sodium fluoride solution
/l/3’.
Standard Standard
o. I N hydrochloric acid 0.01 N hydrochloric acid
Mixture of equal parts of
0.294,
solution
Technical rcagcnt purified by steam distillation. Conccntratcd analytical rcagcnt. Concentrated analytical reagent. Analytical reagent. x0/o w/v in water. x0/o w/v in water. 50% v/v 0.880 9.6. ammonia in water. Approx. N/g NaOH carbonate fret. 4% w/v sodium fluoride in water. The solution should bc neutral to the indicator used. Reagents standardized against known quantities of aluminium.
of cresol red and
thymol
blue in water.
Pvocedwe Dissolve 5 g of steel in hydrochloric acid and oxrdizc with nitrrc acid. Bvaporatc to a paste, cool and make up to 50 ml with cont. hydrochloric acid, the solution should now be about 9N HCl. Transfer to a separating funnel washing out with the minimum of cont. hydrochloric acid and shake with IOO ml of dichlorodietbyl ether. Discard the lower (ether) layer and repeat the extraction with two further samples of dichlorodiethyl ether.
References
9. 3x2.
3*2
C. ICLLIOTT, J. W.
ROBINSON
VOL.
13 (T955)
Transfer the remaining to a 125 ml beaker and evaporate to dryness without baking. Dissolve in hydrochloric acid and dilute to IO-zo ml, filter off any silica, washing with dil. hydrochloric acid. Adjust the volume of the filtrate to go-r00 ml. Add solid ammonium chloride (z-3 6). boil and add sufficient ammonia to prccipitatc aluminium, efc. Uoil for a further five minutes, filter through a so. 41 paper and wash with ammonium chloride solution until free from Mn, Ni, efc., then wash with sodium chloride solution until free from ammonium salts. Rcdissolvc the precipitate in hytlrochloric acid and boil off any cxccss. Dilute to about zo ml add two drops of Indicator and make alkaline with carbonate-free sodium hydroxide (N/5). If sufficient iron is prcscnt to interfere with the detection of the end-point, dilute still further ant1 add a little paper pulp. l
0.8g1) mg Al
The proccdurc outlined above must be tollowcd to standardize the acid against a known aluminium solution. Prom this point of view it mi@t successfully be argued that the conversion factor for ml of acid to mg of a1uminium is empirical since the wrong answer is obtainctl when the acid is standartliacd in the normaL Way. It was also found that the factor varied slightly for different quantities of aluminiurn and it was found ntlvisablc to standardize the acid against known cluantitics of aluminium similar to that in the snnil~lc.
When large quantities of fluoride wcrc used, the end-l’oint was difficult to judge, this limited the application of the method to rather small quantities of aluminium. It was also found that the indicator changed colour at a slightly higher pn in the prcsencc of fluoride. Error caused by this is reduced when the acid is standardized in the rccommended method. ACKNOM'LEDC&blILN'lS
The anthers wish to thank I-I. GREEN, Engineer-in-Chief
Xr. 1-I. 1: I-Iourcrc;~~ for his intcrcst and help in this work and Brig. of the Gcncral Post Office for pcrmissiorr to publish this paper.
A titrimetric method has been workecl out for the &&&ination of aluminium using dichlorodicthyl cthcr to separate off the iron and titrating with hydrochloric nbscncc and then in the prcscncc of fluoride.
in plain acid first
steels in the
Unc mCthode titrimbtriquc a btb btablie pour lc dosn~c de l’aluminium dans dcs aciers. On cffcctuc unc SCpnration du fcr au moyen de 1’6thcr dic1ilorodi&liyliquc et on dose l’aluminium par titration au moycn dc l’ocidc chlorhydrique, d’aborcl cn l’abscncc, puis en prdscnce de fluor. ZUSAMIMENFASSUNG Elnc titrimctri~chc &lcthodc wurtle i~USgc!arbcitCt, urn Aluininium in Stahl zu bestimmen. Man nimmt cinc Trcnnung dcs Eiscns vor mit Milfc von Dichlorodiiithyllitl~cr und bestimmt dann das Aluminium durch Titration mit Snlzsiiurc, zucrst in hbwcsenhcit, nachhcr in Gcgenwart von Fluor.
l C. ELLIOTT AND J. W. ROIHNSON, Aqua Chiw. Acfn, 13 (1955) 235. a T. J. J. CRAIG, CJJ~UJ. ZCPJ~V., 87 (xgxG) 488. 3 0. CLIZMSET AND L. THIILBN, Z. nngcw. Clrer,r., 62 (1950) 269. ’ 4 hf. BECK AND %. G. Szmo, Aml. Cl&rr. Ada, G (1g52) 3x6 Xeccived
January
29th.
1955
13 (1955)
ANALYTICA
TITRIMETRIC
CHIMICA
DETERMINATION
OF
ACTA
309
ALUMINXUM
IN
STEE.L
1YY
IXTRODUCTION
In a recent paper’ WC described a gravimetric procedure for the determination of aluminium in plain steels. This method involved the separation of iron from the sample using dichlorodiethylethcr and was so successful in this respect that it was decided to extend the method by using a titrimetric or calorimetric finish. The titrimetric finish which was examined was that based on the complexing of aluminium ion fluoride293. The method was based on an acid-base titration of Al4 3 first in the absence of fluoride and then in the presence of fluoride. Early methods were based on the equations:AlCl,
+
Al(OH),
3 NaOH
-> Al(OH)3
+ G NeF-+
Na,fAlF,]
-j- 3 NaCl
. . . . , . . . . . . . . . . . . . (I)
t_ 3 NaOH
. . . . , . . . . . . . . . . . (2)
but different workers were unable to agree on the choice of indicator and the conditions of titration. Low results were frequently obtained and these were considered to be caused by the formation of basis salts such as Al(OH)&X In a more recent paye+ UECK, AND SZABO overcame this difficulty in the followinli: manner. The solution was made slightly caustic and equal aliquots were then taken. The first aliquot was titrated with hydrochloric acid. To the second aliquot excess sodium fluoride was added and the soiution titrated to the same end-point as before. The equations on which this was based are as follows:Na,A103+~HC1-rA1(OH)3+3NaCl Na,AlO,
-+ 6 NaF +- 3 H,O -> Na,[AlFe?
. . . . . . . . . . . . . . . . e (3)
+ G NaOH
. . . , . . . , . , (4)
The addition of fluoride means that three extra molecules of hydrochloric acid are required in the titration for each molecule of aluminium present. They recommended that a five times excess of fluoride should be used and that neutral red should be used as indicator. This paper appeared to show promise and work was carried out to apply the method to steel analysis. Refcvences
p. 3x2.
.
C.
3x0 Invesligalio*L
curried
ELLIOTT,
J.
W.
ROUINSON
VOL.
13 (r955)
out
There were several points in BECK AND %x30’s paper which were not clear. For _ example in Table II of their papc+ calculation of the amounts of aluminium found irom their titration results do not agree with the figures they have published and in fact cleviatc significantly from the theoretical figure quoted. They also recommend taking aliquots of the original solution. This is time-consuming and can introduce errors when used as a routine method; steps were therefore taken to overcome this. The interferences they noted from carbonate and silica were confirmed and their recommendation to use a five times cxccss of fluoride was adhered to.
Titrations were carried out on a single solutions as follows. The solution was made slightly alkaline with a caustic soda. Standard hydrochloric acid was then run in until the solution had passed the end-point. The change in pa was plotted using a PH meter. Sodium fluoride was then added in excess and the solution boiled to ensure that the reaction was complete. It was then cooled to displace the equilibrium in favour of the complex formation. The solution was then titrated again with the standard hydrochloric acid, and as before the PH was plotted. The ptr curve obtained is shown in Fig. 1. 7tifafion Of 2.05mg of Aluminium
6--
g
5‘0
‘,
4.1 a #
6
t
t
72
16
*
Vo~mc%
O.i%
8
I
1
32 36 40 N HCl Iml)
It can be seen that the two curves arc not quite parallel but if the acid is standardized with a given indicator against a standard aluminium solution, reproducible results should be obtained. For this reason cresol red and thymol blue were used as indicator. Rcproduciblc results were obtained on mixture of pure iron and aluminium salts. On applying the method to steel analysis however, the other elements prcscnt caused poor results to be obtained, Their presence was eliminated by precipitating the remilining iron and aluminium with ammonia/ammonium chloride mixture, filtering and proceeding with the dissolved precipitate. It was found necessary to wash away ammonium salts from the precipitate with sodium chloride solution, otherwise ammonia was driven off when boiling before the last titration and the pH of the solution was disturbed causing poor results to be obtained. Briefly the method evolved was as follows : Referegrces p. 3x2.
13 (1955)
VOL.
TITRIMETRIC
DETERhIINATION
OF
3x1
ALUMINIUM
After extraction of the bulk of the iron, the remaining iron and aluminium were precipitated and filtered. The precipitate was washed free from ammonia with sodium chloride solution and then dissolved in dil. hydrochloric acid. Excess sodium hydroxide was then added and the solution titrated to the first end-point using cresol red/thymol blue as indicator. Sodium fluoride was added to the solution which was then boiled, cooled and titrated again with hydrochloric acid to the same end-point as before. From the volume of standard hydrochloric acid required after fluoride addition the quantity of aluminium present in the solution can be calculated.
The results obtained are listed in Table I. TA13LE --_._
.--I--
.__.
--_-
;I1
_-
-----
.-
..---.-
1,‘
-_
-
.
s:tc1
ml:
___---
._._
.__.
---.-
/I
-_ ._.
I
____--.____
_
_._
‘I’dal
I adderl
--_.
.__
Al
__-.__-_
__-___
..--
Al
.__.
nil nil nil
0.0.) 0.04 0.04
0.04 0.0.) 0.05
0 0.~
0 .)I
0.4.5
0.41
0.04
0..+1
0.4.5
0.41
0.04
0.41
0.45
0.40
0.04
4*40
4.49 4.49
4.53 4.53 4.53
4.40 4.40 4.54
27.00 27.00
27.05 27.05
2G.7 27.1
0.04 0.04
0.04 0.04 0 04 0.04
- ...-_-se-__-_-
_.__._ . -_- __.^__-_--___ RECOMMENDED
_ _-___..
.-..--..
.
.
wz _ ._-.--- --__ . ..-_ ______.._ -_ ..__-._
WT _ ._____._______ ________________nw
o.o.$
_-__.
/ouud
_____-__--___-__-_---.^
-_ -- -
PROCEDURE
Heagents
dichlor diethyl ether Hydrochloric acid Nitric acid Ammomum chloride solid solutron Sodiu’m chloridd’solution Ammonia Sodium hydroxide solution Sodium fluoride solution
/l/3’.
Standard Standard
o. I N hydrochloric acid 0.01 N hydrochloric acid
Mixture of equal parts of
0.294,
solution
Technical rcagcnt purified by steam distillation. Conccntratcd analytical rcagcnt. Concentrated analytical reagent. Analytical reagent. x0/o w/v in water. x0/o w/v in water. 50% v/v 0.880 9.6. ammonia in water. Approx. N/g NaOH carbonate fret. 4% w/v sodium fluoride in water. The solution should bc neutral to the indicator used. Reagents standardized against known quantities of aluminium.
of cresol red and
thymol
blue in water.
Pvocedwe Dissolve 5 g of steel in hydrochloric acid and oxrdizc with nitrrc acid. Bvaporatc to a paste, cool and make up to 50 ml with cont. hydrochloric acid, the solution should now be about 9N HCl. Transfer to a separating funnel washing out with the minimum of cont. hydrochloric acid and shake with IOO ml of dichlorodietbyl ether. Discard the lower (ether) layer and repeat the extraction with two further samples of dichlorodiethyl ether.
References
9. 3x2.
3*2
C. ICLLIOTT, J. W.
ROBINSON
VOL.
13 (T955)
Transfer the remaining to a 125 ml beaker and evaporate to dryness without baking. Dissolve in hydrochloric acid and dilute to IO-zo ml, filter off any silica, washing with dil. hydrochloric acid. Adjust the volume of the filtrate to go-r00 ml. Add solid ammonium chloride (z-3 6). boil and add sufficient ammonia to prccipitatc aluminium, efc. Uoil for a further five minutes, filter through a so. 41 paper and wash with ammonium chloride solution until free from Mn, Ni, efc., then wash with sodium chloride solution until free from ammonium salts. Rcdissolvc the precipitate in hytlrochloric acid and boil off any cxccss. Dilute to about zo ml add two drops of Indicator and make alkaline with carbonate-free sodium hydroxide (N/5). If sufficient iron is prcscnt to interfere with the detection of the end-point, dilute still further ant1 add a little paper pulp. l
0.8g1) mg Al
The proccdurc outlined above must be tollowcd to standardize the acid against a known aluminium solution. Prom this point of view it mi@t successfully be argued that the conversion factor for ml of acid to mg of a1uminium is empirical since the wrong answer is obtainctl when the acid is standartliacd in the normaL Way. It was also found that the factor varied slightly for different quantities of aluminiurn and it was found ntlvisablc to standardize the acid against known cluantitics of aluminium similar to that in the snnil~lc.
When large quantities of fluoride wcrc used, the end-l’oint was difficult to judge, this limited the application of the method to rather small quantities of aluminium. It was also found that the indicator changed colour at a slightly higher pn in the prcsencc of fluoride. Error caused by this is reduced when the acid is standardized in the rccommended method. ACKNOM'LEDC&blILN'lS
The anthers wish to thank I-I. GREEN, Engineer-in-Chief
Xr. 1-I. 1: I-Iourcrc;~~ for his intcrcst and help in this work and Brig. of the Gcncral Post Office for pcrmissiorr to publish this paper.
A titrimetric method has been workecl out for the &&&ination of aluminium using dichlorodicthyl cthcr to separate off the iron and titrating with hydrochloric nbscncc and then in the prcscncc of fluoride.
in plain acid first
steels in the
Unc mCthode titrimbtriquc a btb btablie pour lc dosn~c de l’aluminium dans dcs aciers. On cffcctuc unc SCpnration du fcr au moyen de 1’6thcr dic1ilorodi&liyliquc et on dose l’aluminium par titration au moycn dc l’ocidc chlorhydrique, d’aborcl cn l’abscncc, puis en prdscnce de fluor. ZUSAMIMENFASSUNG Elnc titrimctri~chc &lcthodc wurtle i~USgc!arbcitCt, urn Aluininium in Stahl zu bestimmen. Man nimmt cinc Trcnnung dcs Eiscns vor mit Milfc von Dichlorodiiithyllitl~cr und bestimmt dann das Aluminium durch Titration mit Snlzsiiurc, zucrst in hbwcsenhcit, nachhcr in Gcgenwart von Fluor.
l C. ELLIOTT AND J. W. ROIHNSON, Aqua Chiw. Acfn, 13 (1955) 235. a T. J. J. CRAIG, CJJ~UJ. ZCPJ~V., 87 (xgxG) 488. 3 0. CLIZMSET AND L. THIILBN, Z. nngcw. Clrer,r., 62 (1950) 269. ’ 4 hf. BECK AND %. G. Szmo, Aml. Cl&rr. Ada, G (1g52) 3x6 Xeccived
January
29th.
1955