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Mercurous salts as reductimetric reagents for titrations in alkaline medium
VOL.
10 (1954)
ANALYTICA
CHIMICA ACTA
3o*
MERCUROUS SALTS AS REDUCTIMETRIC REAGENTS TlTRATIONS IN ALKALINE MEDIUM I. TITRATION OF FERRICYANIDE
I’. BURRIEL-MARTi,
B. LUCENA-CONDE
AND
S.
AI
FOR
J IMENO
Mercurous salts have a small reducing property but in the prcscncc of a large concentration of thiocyanatc ions these reducing propertics arc cnhanccd as a consequence of the formation of the [Hg(SCN),]-2 complex. ‘l’his enhancement has been applied in quantitative analysis by I~RADBURY ANI) ~SDW~~I~L)S~ to the direct titration of ferric salt. Later, I'uGH~, and I~IXCHER AKD WE& have carried Some other applications have been studied by out a study of these titrations. these authors4 and the presentG. In spite of the great interest these reagents have acquired, there are no references concerning titrations in alkaline medium with mcrcurous salts as reagents. In alkaline medium, mercurous salts yield I-160 and Hg. But in the prcsencc is not produced but of complex-forming ions with Hg +2, the HgO precipitate instead the respective complex is formed. For instance, in the presence of SCN-, CN- or I-, WC have noticed that many oxidizing substances are rcduccd in alkaline medium, and [Hg(SCN),Ie2, [Hg(CN)4]-2 and [Hg141e2, respectively, are formed. The most important redox titration in alkaline medium is that of ferricyanidc, not only on account of its own interest, but also because it is able to conduce to a great number of indirect titrations. In fact, potassium ferricyanide has been used for the titrations of arsenic, antimony, vanadium salts, sulphur compounds, sulphate and other organic compounds. hydrogen pcroside, sugars, hydrazin In spite of this, the number of direct titrations for ferricyanide is rather small: DIX I~ZESNO AND VALDBSO used vanadyl sulphate for this titration in strong alkaline solution and in the absence of air. HAHN’ used chromic salt in concentrated NaOH. DICKENS AND MAASSEN~ reduce with cobaltous salt in an ammoniaammonium chloride buffer. WILLIARD AND MANALO~ used hydrogen peroxide in a hot basic medium. No reference has been found to titration with mercurous salt in the prcsencc of enhancing substances. In the presence of cyanide the titration is not convenient since the cyanide anion is oxidised by the ferricyanide. In the presence of thiocyanate the titration Re/evcnccs
*/rr. 309
F.
302
BURRIEL-MARTi
et at?.
VOL.
10
(1954)
not convenient either, since a precipitate (mercuric oxide, mercurous and mercuric ferro- and fcrricyanidc) totally insoluble in an excess of thiocyanate is formed. When iodide is present the results obtained arc satisfactory. The fcrricyanide is reduced quantitatively to ferrocyanide, the mcrcurous salt being oxidised to and the end-point can bc detected potentiometrically (the soluble [IlgI,]-2, change of potential at the end-point being about ISO mv), or with diphcnylamine sulphonatc. In a strong alkaline medium the ferricyanide has no cffcct on iodide (Fig. T).
is
liig.
The
reaction
must
r. Curve Curve Curve Curve
1 z 3 4
-
syhtcm system system system
Hg+a/Hg2+a/Mg CN),/CNf.Fe(CN),,]-3/[l%(CN),]-” 1,/I
be as follows:
z[Fe(CN),]-3+I-Ig,+2
+81-
--> z[Fe(CN),]-4+z[Hg14]-2
but this other takes place simultaneously: Hg2+2-j- 4I- -+ [Hg14]-2 + Hg +
z[Fe(CN)J3
Hg
+ 4I- --z z[Fe(CN),]-”
-+ [Hg14]-2
In both ways the total reaction is the same and it is only necessary to have a concentration of iodide high enough, at least o.afif, to help the formation of intermediate compounds (mercuric iodide and ferrocyanide). References
$.
309
VOL.
10
(1954)
TITRATION
OF
FERRICYANIDE
303
The results obtained are correct, the usual volumetric errors being obtained. The titration must be carried out at temperature below 30” C. Otherwise, high results can be obtained since the iodide and the ferricyanide react. The acidity of the reagent must be controlled, since with solutions containing much free mineral acidity wrong results are obtained. In this way, a solution of mercurous perchlorate prepared as described must be preferred to that of mercurous nitrate, since the hydrolytic effect is less, it can be kept at a higher value of pn. The influence of the different anions has been considered. Anions having oxido-reducing properties in alkaline medium do interfere. Chloride, nitrate and sulphate do not interfere in fairly high concentration. The great advantage of the method over those previously proposed is the noninterference of the air on the reagent and during the titration.
Xeagercts Mercurozrs $erchZorate must be preferred to the nitrate since the solution is more stable and can be obtained less acid without hydrolytic precipitation. This compound is obtained from the following substances: Red mercuric oxide, 26 g Perchloric acid (d = 1.5), 30 ml Metallic mercury, 30 g Water, 50 ml These substances are mixed thoroughly, stirring and heating till soluiton of mercuric oxide. The solution is filtcrcd, and diluted to zooo ml. It is titrated as mercurous chloride. Potnssiz~~n ferricyanidc o.IN. By solution of pure Merck salt. It is titrated by iodometry periodically. Potassium iodide, rM Sodimrt hydroxide, 4&i.
r7Lfzw?tce of pf? Pvocedwe. IO ml of ferricyanide solution in a volume of IOO ml containing 40 ml of iodide solution are titrated with mercurous perchlorare. The prr is measured with the glass electrode. The usual potentiometric technique with platinum wire and calomel saturated electrode is followed. The results are included in Table I. During the titration it is noticed that at the moment the mercurous salt drop falls on the solution a dark-black precipitate is formed, which is quickly dissolved, and the yellow colour of the ferricyanide decreases. At the end-point the reaction is rather slow and it is advisable to proceed slowly. Rcfcrcnces
p. 309
F.
304
BURRIEL-MARTf
TABLE
Et
at?.
VOL.
10 (1954)
I
Solutions.
I. potassiu m ferricyanide, o.xN (f = 0.982) 2. mcrcuro us perchlorate, o.xN (f = 1.014) solution In all cases 10 ml of ferricyanide solution, or 9.82 ml of fcrricyanide o.~oooN are titrated. ..- ._-____-______ _ ___ _ _-- --_____ --__ml of Difference cyO Conditions of p11 ml of IWWCIW H&,(ClOI), o.1N found consumed __ ___-_-. - --____--_. ----.. __. -10.48 10.64 SaF;;t;w& solution of + 8.5 0; PII = 8.5 Ruffcr solution NI-I&l-NH3; prr =
10.10
IO.24
-1-4.3
10.58
10.73
+9.2
IO.09
42.7
10.3.t
+.5.2
9.90
x0.05
+2.3
9.85
9.99
+1.7
I&Z’ NaOH solution PII = 14
9.70
9.Q
+0.2
2M NaOH solution pr-1 = > 14
9.70
9.81
+0.2
2.6114 NaOH solution PII = > 14
g.GS
9.82
3M NaOH solution PFI = > 14
9.65
9.79
9.75
9.89 --.-. .-_-__---_--
1 oh, solution of Na,COo; p11 = IM
9.0
10.8
NH,, solution PII = 11.5 NaOI-I solution PII = 12.7
9.95
o. rilf
o.gIM NaOH solution PII = 13.2 o.GgM NaOI-I P1‘ = 13.6
solution
4M NaOH solution PI-l = > 14 -----
Observations. ence of a I-# InfZuencc
10.20
0.0 -*3 -t_o.G _.--__--_
For a PH less than 14 the values obtainedare high. In the presNaOH solution the results are correct.
of the conmttration
of iodide
The experiments are carried out in xM NaOH solution. The same conditions before described are used, but the iodide concentration is variable. The results are included in Table II. Rejcrences
p.
3og
VOL.
TITRATION
10 (rg54)
OF
FERRICYANIDE
TABLE
305
II
potassium ferricyanide o.xN (f = 0.974) mercurous perchlorate O-IN (f = 1.014) In all cases 10 ml of solution equivalents to 9.74 ml o.roooN are titrated. ___ __..__ -_--Solutions.
T .
2.
Concentration of I<1 solution _.-_._ __ __-____
ml of Hgz(CIO,), consumed --_--_-.--___-_-----
0. r5Al
.
_
10.48 IO.45
z;
g.89
+L5
9:% g.60
9.63 __--_-._.
_
._
--
Oh
-t-o.5 -1-0.5 -0.1
9-79 9.79 9.73 9.76
;*z;
___ ____ _
Difference ---_
0.2M
0.25M 0.3Al <>.‘I&1 0.g.w
solution
------._-
ml of IC,[Bc(CW),] o.~oooN found
IO.33 10.30
O.IM
ferricyanidc
..-___----
._- ---_-.
.___-_____
+0.2 .._--_ _ __
Observations. The solution must be at least 0.~~11 in KI; high results being obtained for smaller concentrations. If the iodide concentration is less than o.1ll1 a yellow precipitate is formed, insoluble in this iodide concentration. InfZzrence
of the concentration
o/ /erricyanide
The concentration of fcrricyanide is variable; the concentration of iodide and NaOH being 0.4Af and TM, respectively. The results are included in Table III.
TARLE Solutions.
o. I ;V potassium N mercurous
0.1
ml o.loooN (correct) of ferricyan~tle adtlcd .J.S8
9.75 J 4.63 19.50 24.38 29.25 _
..- .._ .-. -..
III
fcrricyanidc (f 0.975) perchlorate (f = 1.020) . --. ..-.-. . .--- - -_-_ ---__--.-_._
-.
_ - __-__--.. _---.._-
ml of o.ror:V ml of o.roooN ferirmcu-;;idc 1-igr(ClO,), consumed --_-.__._ ..__..-. _-_--. _- _____ _-
4.80 9.55 x1.30 23.90 23.70
- ..--_-.
Difference _ ___.
_----
__._ .__. --.-.-
29.27 _.__
_ . o/o
____ __- __._.__ _ .._
j-o.4
4.90 9.74 X4.59 19.48 24.38
19.10
---_.
4.1
--0.2 --0.x
--
._.----
,-::: .___ .. . . ..-_-- _.
_
Obseyvcctions. It can be observed that the results are all correct for the iodide concentration proposed and the usual ferricyanide amount. Rc/rrewccs p. 309
36
I?. BURRIEL-MARTi
et
VOL.
cd.
10
(1954)
Influence of the time IO ml of o.og8zN potassium ferricyanidc were titrated with mercurous perchlorate. In every case the time was measured from the moment at which the ferricyanide and the iodide were brought together till the moment at which the titration was commenced. The results are as follows (Table IV). TARLIZ IV fcrricyanide (f = 0.982) 0.1 N mercurous pcrchlorate (f = 0.952) In all C~LSCS10 ml of solution cquivalcnts to 9.8 2 ml o. I oooiV correct fcrricynnidc solution arc titrated. _.._-- .._- ____ -- .__-_ _ _- _. _.. ___--_-__ -.. ..__ -_- _ -.-___ -._--._-_. .-. _- ..-.- . Rcduccd fcrriTime 1111 of H&&IO,), ml ;r Ti,[Fc(CN),-J cyanide in OA, (minutes) o.og52N consunlctl o.roooN found . -._- _._._ -. .- _.----.... _ _.--. _ . _ ..___ .--. - . . ____-_._.. T 9.81 0.1 10.30 cj.81 0.L 2 10.30 0. I g.81 10.30 3 10.25 0.5 9.76 1 9.48 9.95 3 1.5 9.6.5 9.19 7 5 10 8.85 9 *0 E’d 8.38 15 z: CL+8 617 27 24 110urs I .oo 91 0.95 _--_ -. . .._._. _ .__.. . . ..._____ __- - _ .- -__ . _--___. . _ ____%llltiCJnS.
0.1
hr pc~tassilln~
Obscvvations. The reaction bctwecn the ferricyanide and the iodide is very slow, but it seems that once it is initiated it conrinucs more rapidly. From the practical point of view, it is evident that it is convenient to add the iodide just at the moment the titration is to bc commenced.
The different solutions are heated before they arc mixed. During the titration with the cold mercurous salt the tempcmturc &creased. The initial and final temperatures are shown in table V. TARLE 1’ potassium fcrricyanidc (f = o.gSa) o.rN mercurous perchlorate (f = 0.952) In all cases IO ml of solution ecluivnlcnts to 9.82 nil 0.10ooN cyanide solution are titrated. -- .--- -_-.---. _ . _-_--.__ _ ____-. ..--._. ----.-. I:nitial Final ml of H~,(ClO,), ml of I&[Fe(CN),,] tcmperatu rz o.oggzN consumed tcmpcrature o.roooN founcl _. ----.-.--__-~-_-__ _--_-___ -_ ._--__.--.. _-TO0 C lz” c 10.30 g.s1 20 20 10.32 o.s2 22 10.2s 9.79 ;z 28 IO.25 Solutions.
;:
__- .----_-
12cjcYemcs
0. r N
2:
_-- .__- ____..._ ___._ ^_ f?. 309
10.10 g.so
_ _-
Cxri-
(correct)
_ -----
--
0.0
0.3 0.6 2
;*s:.
__- _- __- _.____ --. 9.33 .- --__--_.
---
Reduced ferricy. y. .___.--- --.- . 0.1
---
5
-_ ---
--
VOL.
10
TITRATION
(1954)
OF
FERRICYANIDE
307
Observations. The results are only correct for a temperature below 30~ C. Above this temperature a reaction between the iodide and the fetiicyanide cannot be excluded. Influence
o/ the ucidity
of the reagent
All the experiments described above have been made with a reagent containing very small amounts of free acid. In the following tables experimenrs made with different mercurous pcrchlorate and nitrate solutions show that when the pH the solution decreases the results incrcasc. In general, a rcagcnt of very slight acidity must be used. The results are included in Table VI.
Solutions. o.Ih’
potassium ferricynnidc (f = 0.975) mcrcurous pcrchloratc of varying acidity 0. IN mcrcurous nitrate of varying acidity. In all cases 10 ml of potasGun1 ferrlcyanlclc solution cclui\*alcnts to 9.75 nil o.rooo:V (correct) iLrc titratccl. _-____-..-_ _ .._. __ __._._ _ _ _. . . _. - - pr1 of Mf&(C104), -- Diffcrcncc “A, ml of K,[Fc(CN),] o. Iooohr found solution _ _-_. __ ._ . ._..-.. ._ ..-__ . ..---_. - .- . . _.-..- _-.-... 0.1 IV
3.0 3.0 3.0 1 .H I .x 1 .s I .o I .o I .o
i-o:3 f2.7 3-2 f3
9.98 10.08 10.14 0.9G IO.1 4
plx of Hg2(NO~~3 solution
:: -l-4 0.0 -0.2
9.75 9.73 9.70 IO.12
1.7 1.7 1.7 I.1 I.1 1.1
Infhiencc
:z
9.78 10.02
0.3 0.3
0.5 0.5 0.5 ._ . .._. -
-0.1 -kO..l +0.2
9.74 9.79 9.77 9.80 9.80
_--__
of the. different
__---_..-..
9.98 10.03 10.17 9.96 10.07 __._. --_
-0.5 d-3.8 -t-2.3 +2.8 -J-Z.” _ - _._..
. _. -
_
+3 _-_ . --_
anions
The influence of fairly large amounts of different anions has been tested. Nitrate, chloride and sulphate do not interfere at a concentration of 5%. Some Referewes
p. 309
I;. BURRIEL-MARTf et al.
308
VOL. 10 (x9.54)
oxidizing anions such as IO-, BrO- and ClO- do interfere through their oxidizing power. The reducing agents S-2, SO,-2, S20ae2, AsO,-~ and CN- interfere. Tihulion with
7evmsiOZc indicator
intcvnal
All the above experiences were carried out with potentiometric indication of the end-point. Looking for a chemical indicator, methylene blue, picric acid and ferrous dimethylglyoxime were tried, but the results were poor, a sharp end-point not being obtained. Recently, WILLARD AND MANALO~ have proposed the use of some derivatives sulphonate seems to be the most of diphenylaminc ; barium diphcnylamine suitable. In fact, the green form of this indicator obtained according to WILLARD AND YOUNGER have proved quite suitable. At the end-point a change from red to colourlcss is obtained. Table VII shows some tirrations with this indicator. TAULl< VIL Si,lutiuns. 0.1 N putashiunl fcrricyanicle (f ==0.975) 0. I N n1c:rcnxous pcrchloratc (f = I .020) - ---._-.-_-_-
-._-.._--.
ml of o.roooZV fcrricyaniclc
- .
--
..--
(correct) added
__.._ ____.__ ._-._-.__ _-_-____--_------
4.88 9.75 1.I.63
._.____ _.-_----.-
4.85 0.74 I‘{._58
I ql..lS ‘4.38 29.30
19.50 24.3s 29.25
- ._ - _- ___. . __.
._..-.
_..__ -_-_.-__-__-_-..
_ _-__--
____ _
ml of 0. roooN--_---..--;.__. fcrricyanicle found ._. ._ .___. ._-_-. Di f fercllce T;, Potcntiomct\Vlth (with indicator) rically indicator
_.-_
.__ _. _._- .
_....
__.. -_ ..-_--.-.-_
4.90 9.78
._
-I-0.4 -l-O.3
14.70 1O.S.3 2‘~.‘@ ‘9.30
.__.._..-- .___ -_.-_.
_._
-to..5 +o.r 4-0.4 -I- 0. 2
._ .
PROCEDURE 1’1~OI’OSED Taking into account the whole study and the considerations indicated in the last paragraph, the following procedure has been proposed for Ihc titration of fcrricyanidc with mercurous salt : To cold solution containing the ferricyanide arc added 25 ml of 4111 NaOH, the necessary amount of water to give a final volume of IOO ml; and 40 ml of I/\[ potassium iodide just as the titration is going to be started. Four drops of the indicator diphenylamine sulphonate alkaline oxidized form are added (or the electrode introduced if the potentiomctric method is followed) and the standard solution of the mercurous pcrchloratc is immediately added. Near the end-point this addition is made slowly. With potentiometric determination a sharp 100 mv change can be observed, and with the indicator the characteristic change from red to colourless. l?efeve~accsp. 309
VOL.
10
TITRATION
(1954)
OF
PI3RRIC\‘ANIl~@
3el
For the first time the application of mcrcurous salt for rcductimctric titrations in alkaline medium has been studied. In the presence of iodide the ferricyanidc ion can be titrated accurately. The end-point can be determined potentiometrically The influence of the concentration of or with barium dlphenylamine sulphonate. acidity of the reagent, and time are iodide, ferricyanide and alkali, tcm crature, considered. The greater ad\*antage o r”this proceclurc lies in the fact that the reagent is stable to air and the titrations can be carried out without elimination of the latter.
Pour la premiQrc fois, l’utili.sation des scls mercurcux pour Its titrations rcSductlmdtriques, en milieu alcalin, a cStd envlsag&e. En prtiscnce d’iodure, l’ion fcrricyanLc point final est ddtcrmind par potentiohydriquc peut Ctre titr& avcc prdcislon. I,‘influencc de la conmdtrie ou ;LU moyen de diph~nylaminesulfonatc de hnryunl. centration en iodurc, fcrricyanure ct hydroxydc alcalin, dc la tcmp&aturc, de l’aciditd du rCactif et du tcmps. cst titurli&. TX plus grand avantage de cette mCthodc Its titrations pcuvcnt done Ctrc effectu&es sans cst clue le rcSactif cst stable h l’air; chasser cclui-ci.
Zum erstcn Ma1 wurrlc clie Anwentlung eincs I-I~-~~-~;~lms bci der rcduktimctrischen Titration im alkalischen Mecllum untersucht. 1%~ dcr Ccgenwart von Jodid Dcr Knclpunkt kann potentiomckann das Ferricyanation genau titricrt werclcn. trisch (Jder mit I~ariumdiphcnylamindisulfonat hcstlmmt werdcn. l&r Einfluss cler Konxntration van Jodid, Fcrricynnid untl Alkalt, clcr Tempcratur, dcs %urcgrades dcr Reagenzicn untl der &it wcrden crwogen. Der grdsserc Vorteil dicscs class tlas ICcagctiz an dcr Luft best%ndig ist untl Verfalirens licgt in dcr Tatsache, die *~itrati(Jn so ohnc :\usschlu~s clcr letztcrcn clurchgcfuhrt wcrtlen kann.
Ha sldo estucliada lx)r primcra vcz la aplicaclGn tic las salts mercuriosas en lab I
and I?. LUCESA, o c. DEZ. ~il~usN;o
Zl
‘I * B 10
F. I’. H. l-1.
Alaules AND
real SCJC.espuii.
I,. V,\LDas.
.dnh?s
fis sot.
y qrrtm,
40
l'sf~O1. /&.
(B) y C/dJJt.,
(x953)
45.
27
(1929)
368;
10 (1954)
ANALYTICA
CHIMICA ACTA
3o*
MERCUROUS SALTS AS REDUCTIMETRIC REAGENTS TlTRATIONS IN ALKALINE MEDIUM I. TITRATION OF FERRICYANIDE
I’. BURRIEL-MARTi,
B. LUCENA-CONDE
AND
S.
AI
FOR
J IMENO
Mercurous salts have a small reducing property but in the prcscncc of a large concentration of thiocyanatc ions these reducing propertics arc cnhanccd as a consequence of the formation of the [Hg(SCN),]-2 complex. ‘l’his enhancement has been applied in quantitative analysis by I~RADBURY ANI) ~SDW~~I~L)S~ to the direct titration of ferric salt. Later, I'uGH~, and I~IXCHER AKD WE& have carried Some other applications have been studied by out a study of these titrations. these authors4 and the presentG. In spite of the great interest these reagents have acquired, there are no references concerning titrations in alkaline medium with mcrcurous salts as reagents. In alkaline medium, mercurous salts yield I-160 and Hg. But in the prcsencc is not produced but of complex-forming ions with Hg +2, the HgO precipitate instead the respective complex is formed. For instance, in the presence of SCN-, CN- or I-, WC have noticed that many oxidizing substances are rcduccd in alkaline medium, and [Hg(SCN),Ie2, [Hg(CN)4]-2 and [Hg141e2, respectively, are formed. The most important redox titration in alkaline medium is that of ferricyanidc, not only on account of its own interest, but also because it is able to conduce to a great number of indirect titrations. In fact, potassium ferricyanide has been used for the titrations of arsenic, antimony, vanadium salts, sulphur compounds, sulphate and other organic compounds. hydrogen pcroside, sugars, hydrazin In spite of this, the number of direct titrations for ferricyanide is rather small: DIX I~ZESNO AND VALDBSO used vanadyl sulphate for this titration in strong alkaline solution and in the absence of air. HAHN’ used chromic salt in concentrated NaOH. DICKENS AND MAASSEN~ reduce with cobaltous salt in an ammoniaammonium chloride buffer. WILLIARD AND MANALO~ used hydrogen peroxide in a hot basic medium. No reference has been found to titration with mercurous salt in the prcsencc of enhancing substances. In the presence of cyanide the titration is not convenient since the cyanide anion is oxidised by the ferricyanide. In the presence of thiocyanate the titration Re/evcnccs
*/rr. 309
F.
302
BURRIEL-MARTi
et at?.
VOL.
10
(1954)
not convenient either, since a precipitate (mercuric oxide, mercurous and mercuric ferro- and fcrricyanidc) totally insoluble in an excess of thiocyanate is formed. When iodide is present the results obtained arc satisfactory. The fcrricyanide is reduced quantitatively to ferrocyanide, the mcrcurous salt being oxidised to and the end-point can bc detected potentiometrically (the soluble [IlgI,]-2, change of potential at the end-point being about ISO mv), or with diphcnylamine sulphonatc. In a strong alkaline medium the ferricyanide has no cffcct on iodide (Fig. T).
is
liig.
The
reaction
must
r. Curve Curve Curve Curve
1 z 3 4
-
syhtcm system system system
Hg+a/Hg2+a/Mg CN),/CNf.Fe(CN),,]-3/[l%(CN),]-” 1,/I
be as follows:
z[Fe(CN),]-3+I-Ig,+2
+81-
--> z[Fe(CN),]-4+z[Hg14]-2
but this other takes place simultaneously: Hg2+2-j- 4I- -+ [Hg14]-2 + Hg +
z[Fe(CN)J3
Hg
+ 4I- --z z[Fe(CN),]-”
-+ [Hg14]-2
In both ways the total reaction is the same and it is only necessary to have a concentration of iodide high enough, at least o.afif, to help the formation of intermediate compounds (mercuric iodide and ferrocyanide). References
$.
309
VOL.
10
(1954)
TITRATION
OF
FERRICYANIDE
303
The results obtained are correct, the usual volumetric errors being obtained. The titration must be carried out at temperature below 30” C. Otherwise, high results can be obtained since the iodide and the ferricyanide react. The acidity of the reagent must be controlled, since with solutions containing much free mineral acidity wrong results are obtained. In this way, a solution of mercurous perchlorate prepared as described must be preferred to that of mercurous nitrate, since the hydrolytic effect is less, it can be kept at a higher value of pn. The influence of the different anions has been considered. Anions having oxido-reducing properties in alkaline medium do interfere. Chloride, nitrate and sulphate do not interfere in fairly high concentration. The great advantage of the method over those previously proposed is the noninterference of the air on the reagent and during the titration.
Xeagercts Mercurozrs $erchZorate must be preferred to the nitrate since the solution is more stable and can be obtained less acid without hydrolytic precipitation. This compound is obtained from the following substances: Red mercuric oxide, 26 g Perchloric acid (d = 1.5), 30 ml Metallic mercury, 30 g Water, 50 ml These substances are mixed thoroughly, stirring and heating till soluiton of mercuric oxide. The solution is filtcrcd, and diluted to zooo ml. It is titrated as mercurous chloride. Potnssiz~~n ferricyanidc o.IN. By solution of pure Merck salt. It is titrated by iodometry periodically. Potassium iodide, rM Sodimrt hydroxide, 4&i.
r7Lfzw?tce of pf? Pvocedwe. IO ml of ferricyanide solution in a volume of IOO ml containing 40 ml of iodide solution are titrated with mercurous perchlorare. The prr is measured with the glass electrode. The usual potentiometric technique with platinum wire and calomel saturated electrode is followed. The results are included in Table I. During the titration it is noticed that at the moment the mercurous salt drop falls on the solution a dark-black precipitate is formed, which is quickly dissolved, and the yellow colour of the ferricyanide decreases. At the end-point the reaction is rather slow and it is advisable to proceed slowly. Rcfcrcnces
p. 309
F.
304
BURRIEL-MARTf
TABLE
Et
at?.
VOL.
10 (1954)
I
Solutions.
I. potassiu m ferricyanide, o.xN (f = 0.982) 2. mcrcuro us perchlorate, o.xN (f = 1.014) solution In all cases 10 ml of ferricyanide solution, or 9.82 ml of fcrricyanide o.~oooN are titrated. ..- ._-____-______ _ ___ _ _-- --_____ --__ml of Difference cyO Conditions of p11 ml of IWWCIW H&,(ClOI), o.1N found consumed __ ___-_-. - --____--_. ----.. __. -10.48 10.64 SaF;;t;w& solution of + 8.5 0; PII = 8.5 Ruffcr solution NI-I&l-NH3; prr =
10.10
IO.24
-1-4.3
10.58
10.73
+9.2
IO.09
42.7
10.3.t
+.5.2
9.90
x0.05
+2.3
9.85
9.99
+1.7
I&Z’ NaOH solution PII = 14
9.70
9.Q
+0.2
2M NaOH solution pr-1 = > 14
9.70
9.81
+0.2
2.6114 NaOH solution PII = > 14
g.GS
9.82
3M NaOH solution PFI = > 14
9.65
9.79
9.75
9.89 --.-. .-_-__---_--
1 oh, solution of Na,COo; p11 = IM
9.0
10.8
NH,, solution PII = 11.5 NaOI-I solution PII = 12.7
9.95
o. rilf
o.gIM NaOH solution PII = 13.2 o.GgM NaOI-I P1‘ = 13.6
solution
4M NaOH solution PI-l = > 14 -----
Observations. ence of a I-# InfZuencc
10.20
0.0 -*3 -t_o.G _.--__--_
For a PH less than 14 the values obtainedare high. In the presNaOH solution the results are correct.
of the conmttration
of iodide
The experiments are carried out in xM NaOH solution. The same conditions before described are used, but the iodide concentration is variable. The results are included in Table II. Rejcrences
p.
3og
VOL.
TITRATION
10 (rg54)
OF
FERRICYANIDE
TABLE
305
II
potassium ferricyanide o.xN (f = 0.974) mercurous perchlorate O-IN (f = 1.014) In all cases 10 ml of solution equivalents to 9.74 ml o.roooN are titrated. ___ __..__ -_--Solutions.
T .
2.
Concentration of I<1 solution _.-_._ __ __-____
ml of Hgz(CIO,), consumed --_--_-.--___-_-----
0. r5Al
.
_
10.48 IO.45
z;
g.89
+L5
9:% g.60
9.63 __--_-._.
_
._
--
Oh
-t-o.5 -1-0.5 -0.1
9-79 9.79 9.73 9.76
;*z;
___ ____ _
Difference ---_
0.2M
0.25M 0.3Al <>.‘I&1 0.g.w
solution
------._-
ml of IC,[Bc(CW),] o.~oooN found
IO.33 10.30
O.IM
ferricyanidc
..-___----
._- ---_-.
.___-_____
+0.2 .._--_ _ __
Observations. The solution must be at least 0.~~11 in KI; high results being obtained for smaller concentrations. If the iodide concentration is less than o.1ll1 a yellow precipitate is formed, insoluble in this iodide concentration. InfZzrence
of the concentration
o/ /erricyanide
The concentration of fcrricyanide is variable; the concentration of iodide and NaOH being 0.4Af and TM, respectively. The results are included in Table III.
TARLE Solutions.
o. I ;V potassium N mercurous
0.1
ml o.loooN (correct) of ferricyan~tle adtlcd .J.S8
9.75 J 4.63 19.50 24.38 29.25 _
..- .._ .-. -..
III
fcrricyanidc (f 0.975) perchlorate (f = 1.020) . --. ..-.-. . .--- - -_-_ ---__--.-_._
-.
_ - __-__--.. _---.._-
ml of o.ror:V ml of o.roooN ferirmcu-;;idc 1-igr(ClO,), consumed --_-.__._ ..__..-. _-_--. _- _____ _-
4.80 9.55 x1.30 23.90 23.70
- ..--_-.
Difference _ ___.
_----
__._ .__. --.-.-
29.27 _.__
_ . o/o
____ __- __._.__ _ .._
j-o.4
4.90 9.74 X4.59 19.48 24.38
19.10
---_.
4.1
--0.2 --0.x
--
._.----
,-::: .___ .. . . ..-_-- _.
_
Obseyvcctions. It can be observed that the results are all correct for the iodide concentration proposed and the usual ferricyanide amount. Rc/rrewccs p. 309
36
I?. BURRIEL-MARTi
et
VOL.
cd.
10
(1954)
Influence of the time IO ml of o.og8zN potassium ferricyanidc were titrated with mercurous perchlorate. In every case the time was measured from the moment at which the ferricyanide and the iodide were brought together till the moment at which the titration was commenced. The results are as follows (Table IV). TARLIZ IV fcrricyanide (f = 0.982) 0.1 N mercurous pcrchlorate (f = 0.952) In all C~LSCS10 ml of solution cquivalcnts to 9.8 2 ml o. I oooiV correct fcrricynnidc solution arc titrated. _.._-- .._- ____ -- .__-_ _ _- _. _.. ___--_-__ -.. ..__ -_- _ -.-___ -._--._-_. .-. _- ..-.- . Rcduccd fcrriTime 1111 of H&&IO,), ml ;r Ti,[Fc(CN),-J cyanide in OA, (minutes) o.og52N consunlctl o.roooN found . -._- _._._ -. .- _.----.... _ _.--. _ . _ ..___ .--. - . . ____-_._.. T 9.81 0.1 10.30 cj.81 0.L 2 10.30 0. I g.81 10.30 3 10.25 0.5 9.76 1 9.48 9.95 3 1.5 9.6.5 9.19 7 5 10 8.85 9 *0 E’d 8.38 15 z: CL+8 617 27 24 110urs I .oo 91 0.95 _--_ -. . .._._. _ .__.. . . ..._____ __- - _ .- -__ . _--___. . _ ____%llltiCJnS.
0.1
hr pc~tassilln~
Obscvvations. The reaction bctwecn the ferricyanide and the iodide is very slow, but it seems that once it is initiated it conrinucs more rapidly. From the practical point of view, it is evident that it is convenient to add the iodide just at the moment the titration is to bc commenced.
The different solutions are heated before they arc mixed. During the titration with the cold mercurous salt the tempcmturc &creased. The initial and final temperatures are shown in table V. TARLE 1’ potassium fcrricyanidc (f = o.gSa) o.rN mercurous perchlorate (f = 0.952) In all cases IO ml of solution ecluivnlcnts to 9.82 nil 0.10ooN cyanide solution are titrated. -- .--- -_-.---. _ . _-_--.__ _ ____-. ..--._. ----.-. I:nitial Final ml of H~,(ClO,), ml of I&[Fe(CN),,] tcmperatu rz o.oggzN consumed tcmpcrature o.roooN founcl _. ----.-.--__-~-_-__ _--_-___ -_ ._--__.--.. _-TO0 C lz” c 10.30 g.s1 20 20 10.32 o.s2 22 10.2s 9.79 ;z 28 IO.25 Solutions.
;:
__- .----_-
12cjcYemcs
0. r N
2:
_-- .__- ____..._ ___._ ^_ f?. 309
10.10 g.so
_ _-
Cxri-
(correct)
_ -----
--
0.0
0.3 0.6 2
;*s:.
__- _- __- _.____ --. 9.33 .- --__--_.
---
Reduced ferricy. y. .___.--- --.- . 0.1
---
5
-_ ---
--
VOL.
10
TITRATION
(1954)
OF
FERRICYANIDE
307
Observations. The results are only correct for a temperature below 30~ C. Above this temperature a reaction between the iodide and the fetiicyanide cannot be excluded. Influence
o/ the ucidity
of the reagent
All the experiments described above have been made with a reagent containing very small amounts of free acid. In the following tables experimenrs made with different mercurous pcrchlorate and nitrate solutions show that when the pH the solution decreases the results incrcasc. In general, a rcagcnt of very slight acidity must be used. The results are included in Table VI.
Solutions. o.Ih’
potassium ferricynnidc (f = 0.975) mcrcurous pcrchloratc of varying acidity 0. IN mcrcurous nitrate of varying acidity. In all cases 10 ml of potasGun1 ferrlcyanlclc solution cclui\*alcnts to 9.75 nil o.rooo:V (correct) iLrc titratccl. _-____-..-_ _ .._. __ __._._ _ _ _. . . _. - - pr1 of Mf&(C104), -- Diffcrcncc “A, ml of K,[Fc(CN),] o. Iooohr found solution _ _-_. __ ._ . ._..-.. ._ ..-__ . ..---_. - .- . . _.-..- _-.-... 0.1 IV
3.0 3.0 3.0 1 .H I .x 1 .s I .o I .o I .o
i-o:3 f2.7 3-2 f3
9.98 10.08 10.14 0.9G IO.1 4
plx of Hg2(NO~~3 solution
:: -l-4 0.0 -0.2
9.75 9.73 9.70 IO.12
1.7 1.7 1.7 I.1 I.1 1.1
Infhiencc
:z
9.78 10.02
0.3 0.3
0.5 0.5 0.5 ._ . .._. -
-0.1 -kO..l +0.2
9.74 9.79 9.77 9.80 9.80
_--__
of the. different
__---_..-..
9.98 10.03 10.17 9.96 10.07 __._. --_
-0.5 d-3.8 -t-2.3 +2.8 -J-Z.” _ - _._..
. _. -
_
+3 _-_ . --_
anions
The influence of fairly large amounts of different anions has been tested. Nitrate, chloride and sulphate do not interfere at a concentration of 5%. Some Referewes
p. 309
I;. BURRIEL-MARTf et al.
308
VOL. 10 (x9.54)
oxidizing anions such as IO-, BrO- and ClO- do interfere through their oxidizing power. The reducing agents S-2, SO,-2, S20ae2, AsO,-~ and CN- interfere. Tihulion with
7evmsiOZc indicator
intcvnal
All the above experiences were carried out with potentiometric indication of the end-point. Looking for a chemical indicator, methylene blue, picric acid and ferrous dimethylglyoxime were tried, but the results were poor, a sharp end-point not being obtained. Recently, WILLARD AND MANALO~ have proposed the use of some derivatives sulphonate seems to be the most of diphenylaminc ; barium diphcnylamine suitable. In fact, the green form of this indicator obtained according to WILLARD AND YOUNGER have proved quite suitable. At the end-point a change from red to colourlcss is obtained. Table VII shows some tirrations with this indicator. TAULl< VIL Si,lutiuns. 0.1 N putashiunl fcrricyanicle (f ==0.975) 0. I N n1c:rcnxous pcrchloratc (f = I .020) - ---._-.-_-_-
-._-.._--.
ml of o.roooZV fcrricyaniclc
- .
--
..--
(correct) added
__.._ ____.__ ._-._-.__ _-_-____--_------
4.88 9.75 1.I.63
._.____ _.-_----.-
4.85 0.74 I‘{._58
I ql..lS ‘4.38 29.30
19.50 24.3s 29.25
- ._ - _- ___. . __.
._..-.
_..__ -_-_.-__-__-_-..
_ _-__--
____ _
ml of 0. roooN--_---..--;.__. fcrricyanicle found ._. ._ .___. ._-_-. Di f fercllce T;, Potcntiomct\Vlth (with indicator) rically indicator
_.-_
.__ _. _._- .
_....
__.. -_ ..-_--.-.-_
4.90 9.78
._
-I-0.4 -l-O.3
14.70 1O.S.3 2‘~.‘@ ‘9.30
.__.._..-- .___ -_.-_.
_._
-to..5 +o.r 4-0.4 -I- 0. 2
._ .
PROCEDURE 1’1~OI’OSED Taking into account the whole study and the considerations indicated in the last paragraph, the following procedure has been proposed for Ihc titration of fcrricyanidc with mercurous salt : To cold solution containing the ferricyanide arc added 25 ml of 4111 NaOH, the necessary amount of water to give a final volume of IOO ml; and 40 ml of I/\[ potassium iodide just as the titration is going to be started. Four drops of the indicator diphenylamine sulphonate alkaline oxidized form are added (or the electrode introduced if the potentiomctric method is followed) and the standard solution of the mercurous pcrchloratc is immediately added. Near the end-point this addition is made slowly. With potentiometric determination a sharp 100 mv change can be observed, and with the indicator the characteristic change from red to colourless. l?efeve~accsp. 309
VOL.
10
TITRATION
(1954)
OF
PI3RRIC\‘ANIl~@
3el
For the first time the application of mcrcurous salt for rcductimctric titrations in alkaline medium has been studied. In the presence of iodide the ferricyanidc ion can be titrated accurately. The end-point can be determined potentiometrically The influence of the concentration of or with barium dlphenylamine sulphonate. acidity of the reagent, and time are iodide, ferricyanide and alkali, tcm crature, considered. The greater ad\*antage o r”this proceclurc lies in the fact that the reagent is stable to air and the titrations can be carried out without elimination of the latter.
Pour la premiQrc fois, l’utili.sation des scls mercurcux pour Its titrations rcSductlmdtriques, en milieu alcalin, a cStd envlsag&e. En prtiscnce d’iodure, l’ion fcrricyanLc point final est ddtcrmind par potentiohydriquc peut Ctre titr& avcc prdcislon. I,‘influencc de la conmdtrie ou ;LU moyen de diph~nylaminesulfonatc de hnryunl. centration en iodurc, fcrricyanure ct hydroxydc alcalin, dc la tcmp&aturc, de l’aciditd du rCactif et du tcmps. cst titurli&. TX plus grand avantage de cette mCthodc Its titrations pcuvcnt done Ctrc effectu&es sans cst clue le rcSactif cst stable h l’air; chasser cclui-ci.
Zum erstcn Ma1 wurrlc clie Anwentlung eincs I-I~-~~-~;~lms bci der rcduktimctrischen Titration im alkalischen Mecllum untersucht. 1%~ dcr Ccgenwart von Jodid Dcr Knclpunkt kann potentiomckann das Ferricyanation genau titricrt werclcn. trisch (Jder mit I~ariumdiphcnylamindisulfonat hcstlmmt werdcn. l&r Einfluss cler Konxntration van Jodid, Fcrricynnid untl Alkalt, clcr Tempcratur, dcs %urcgrades dcr Reagenzicn untl der &it wcrden crwogen. Der grdsserc Vorteil dicscs class tlas ICcagctiz an dcr Luft best%ndig ist untl Verfalirens licgt in dcr Tatsache, die *~itrati(Jn so ohnc :\usschlu~s clcr letztcrcn clurchgcfuhrt wcrtlen kann.
Ha sldo estucliada lx)r primcra vcz la aplicaclGn tic las salts mercuriosas en lab I
and I?. LUCESA, o c. DEZ. ~il~usN;o
Zl
‘I * B 10
F. I’. H. l-1.
Alaules AND
real SCJC.espuii.
I,. V,\LDas.
.dnh?s
fis sot.
y qrrtm,
40
l'sf~O1. /&.
(B) y C/dJJt.,
(x953)
45.
27
(1929)
368;