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Effect of metal ions on the determination of semicarbazide hydrochloride with potassium bromate
l-alanta,1971,WI. 18,pp. 943to 949. PuaamoaPtws. ~t~~No~~
EFFECT OF METAL IONS ON THE DETERMINATION OF SEMICARBAZIDE HYDROCHLORIDE WITH POTASSI~ BRO~TE M. 3. M. CAMPBELL, R. GRZESKOWIAK and B. PWRETT Department of Chemistry, Thames Polytechnic, London, S.E.18 (Received 23 November 1970. Accepted 15 December 1970)
Su~~~~~de can be titrated qu~titatively with potassium bromate in the presence of nickel(D), cobal@) and manganese(II) but copper causes serious interference. The effects of copper ions on the reaction between potassium bromate and semicarbaxide are investigated and the optimum conditions under which the reaction is quantitative are indicated. D~NA~ON of se~carb~ide hydroc~o~de by oxidation with potassium bromate was first suggested by Kurtenacker and Kubina.l The reaction was subsequently investigated by JanEik et aLa and Vulterin,5** and was shown to proceed according to the equation:
Tti
3NH&ONHNH,
+ 2BrO,- = 2N, + 3COa + 3NH, + 2Br + 3HeO.
Recently, the conditions under which the reaction proceeds quanti~tively have been defined by Hitchcock. 6 During our investigation of metal complexes of semicarbazide (sc),~ the Co(H), Ni(I1) and Mn(I1) complexes could be successfully analysed for ligand but the presence of Cu(I1) caused negative errors when the range of hydrochloric acid concentrations defined by Hitchcock was used, both with and without initial addition of bromide. We report in this paper the investigation of the copper interference and indicate the conditions under which the reaction is quanti~~ve. EXPERIMENTAL Reagents
All chemicals used were of analytical-reagent grade. Potassium bromate was dried at l&l”, and semicarbaxide hydrochloride over phosphorus pentoxide under vacuum, and both were stored over caltium chloride. A 0.0167&f solution of potassium bromate was used as standard. The semicarbaxide hy~~~o~de solution (0*025~), which undergoes slow auto-oxidation, was frequently restandardired. Procedures Standardization of semicarbazide hydrochloride solution. Concentrated hydrochloric acid (25 ml)
was added to 25ml of the semicarbaxide hydrochloride solution; the mixture was diluted to 100 ml with distilled water and titrated with 0*0167&fpotassium bromate. E&et of me& ions. Portions (25 ml) of standardixed semicarbaxide hy~~~o~de solution were treated with various quantities of cont. hydrochloric acid (5-70 ml) and metal salts, diluted to 100 ml with distilled water, and titrated with bromate, the addition being rapid to within 05 ml of the end-point. Analysis of nickel complexes. Approximately 1 mmole of the complex, accurately weighed, was dissolved in cont. hydrochloric acid (25 ml). After addition of potassium bromide (0.1 g) and dilution to 100 ml, the solution was titrated with bromate. Titration of semicarbazide soltifioas containing 1 :I mole ratio copper : semicarbazide~ (i) E&et ofhy&ogen ion concenfrat~n. Enough hydrochloric acid was added to 25 ml of 0@25&f semicarbaxide hydrochloride to provide the constant fInal chloride ion concentration required. The acidity was then adjusted to the required value with dilute sulphuric acid, copper solution was added (6.25 ml, O*lOM)and the solution was diluted to 100 ml and then titrated with potassium bromate. 943
944
M. J. M.
CAMPBELL,
R. Gazasrcowr~~ and B.
PERRETT
(ii) Effect of chloride ion concentration. Calculated quantities of hydrochloric acid were added to 25-ml portions of 0*025&fsemicarbazide hydrochloride, to provide final concentrations of chloride ion between @005 and SM, and the acidity adjusted to selected constant values with dilute sulphuric acid. Copper solution was then added (6.25 ml, @lOM) and the solution was diluted to IO0 ml and titrated with bromate. The haI hydrogen ion concentrations of all the solutions were determined from the quantity of acid used and tinal volume of solution, and periodically checked by acid-base titration.
RESULTS
AND DISCUSSION
Experimental results show that bivalent manganese, nickel and cobalt do not interfere with the semicarbazide-potassium bromate reaction when present in molar ratios me~l:ligand as large as 16: 1, Errors incurred in direct titrations of semicarbazide with bromate in presence of Ni(II) and Mn(H) are shown in Fig. 1 and I.4-
12-
ae b k w
08-
I
04-
!n t
o-
-0.4 -
1 iI I \ \
\ \
\
\ \ ?
-08-
-12 -
-14 -
Fm. l.--Error curves for the potentiometric titration of semicarbazide with RBrG,. AC--solution initially free from Br-; BC-Br- initially added, concentration of Brat equivalence point 0*1&f, a similar curve is obtained when Ni(II) is added instead of Br-; BD-in presence of Mn(II). are similar to those found by Hitchcock .6 The method is therefore satisfactory for titration of nickel complexes of semicarbazide (Table I). However, copper(I1) introduces a considerable error (Fig. 2) which is dependent on the rate of addition of the titrant and on other conditions (Table II). Error curves (Fig, 3 and 4) showing the effect of copper ions on the titration in hydrochloric acid medium indicate definite minima at concentrations of acid ea. 2_7M, lower acidities having a more marked effect than higher ones. The curves (Fig. 4) for the dependence of the error on the copper-semicarbazide ratio show
Determination
TABL~I.-QUANTITATWE
NH,CONHNH,
Ni(sc),Br%
Ni(sc)SO&
2HI0
Ni(sc)J&,
2+H,O
NiW%
945
found,
%
NH,CONHNH,
40.3,, 404~,40.4~ (Av. 40.42, G 0.06) 5@4*, 5@3,, 50.49 (Av. 50.41, u @OS) 33*6,, 33.3,, 33.71, 33.2s, 32*9,, 33.2,,, 32*9p, 32.94, (Av. 33.25, CT0.28) 44.01,43*91,43*9*, 43.9, (Av. 43.95, (I 0.04) 56*6,, 565B, 56.68, 56.6, (Av. 56.65, Q 0.03) 68.44, 68*4,, 68.4, (Av. 68.42, u 0.02)
HI0
Ni(sc)$04,
hydrochloride
ESTJ~WIXON OF SEMICARBAZIDE (SC)IN NICKEL CQMPLEXES
Complex
Ni(s&Cl,,
of semicarbazide
4I-W
talc., %
40.72 50.42
32.7 44.02 56.30 68.63
(I = standard deviation.
16-
A
14-
6-
4-
-a- \.
-12-
\
. \
-16-
I 02
I 0.5 Final
, 2.0
I 50
1 IO
[HCL; “!A
2.-Dependence of the error on the hydrochloric acid concentration in presence of copper at [Cul: [SC]= 1. A-in absence of Br-; D-in presence of O*lM Br-; BS and CP +mves AC and BC from Fig. 1. FIG.
M. J. M. CAMPBELL,R. GRZ~SKOWIAKand B. Pzruwrr
946
TABLEII.-EFFECT OF rrrw ADDITION RATE ON THE TITRATION OF 0.6222 mnlOle OF SEMICAlWAZIDE HYDROCHLORIDE WITH 0.01666M BROMATE IN THE PRESENCE. OF COPPER(
ti(In: SPMICARBAZIDE MOLAR LATED TITRR 2489ml
RATIO 1:1. CALCU-
Titrant used, ml
ml/set
3.OM final [HCl]. Initially bromide-free
1.0&f final [HCl]. Initially bromide-free
050 0.10 002
24.47 24.37 24.28
21.26 2052 2006
Addition rate
l*OiWiinal [HCl]. Initial addition of bromide to give 0 1M final concentration 21.73 21.46 21.30
o-2 -
4-
-6 -
2.0
3-O
40
60
60
7.0
FInal [HCLI M
FIG. 3.-Dependence of error on the hydrochloric acid concentration for semicarbazide-bromate titrations at varying ratios of Cn(II) : SC. Curve A B c D E
Cn : sc ratio 0001 o-03 0.1
curve F G H .I
Cu : sc ratio 1 2 10 100
:::5
maximum errors at mole-ratios of Cu: SCbetween O-8and 1.6. This range corresponds to the presence of ionic species [CU(SC)]~+and [CU(SC),]~+in varying proportions. These errors arise from induced side-reactions competing with the main 4-electron oxidation reaction. That induced side-reactions operate is shown by the dependence of the titration error on the rate of titrant addition (Table II), a phenomenon characteristic of induced reactions.
Determination
of semicarbazide
hydrochloride
947
o-2 -
-4 -
-69 0
-8-
‘, k w -IO-
-12-
-14 -
1
I
I
001
oool
FIG. 4.-E&&
cu (II)
:“s:. mole
ratios
I
I
I
IO
IO
Ia0
of CL@) : SCratio on the error for semicarbazide-bromate constant hydrochloric acid concentration.
titrations at
The negative errors are attributed5 to the side-reactions 2NH,NHCONH,+ 2NH,NHCONH,+
+ 2H,O = 4NH,+ + 2C0, + Na + 2e-
+ 2Hz0 = 3NH,+ + HN, + CO2 + 3H+ + 4e-
An initial addition of bromide (Fig. 2) does not appreciably reduce the error, indicating that the slow generation of oxidant via a Hinshelwood-type reaction’ HCl + HBrO, + HClO + HBrO,
in media initially free from bromide is not an important factor. Also, as observed by Hitchcock, air-induced side-reactions, if present, in media free from bromide ion can be assumed to play only a minor role. It would appear therefore, that the sideeffects leading to negative error involve the reaction between copper-semicarbazide complex species and the bromate ion. The decrease in error as the hydrochloric acid concentration is increased from 1 to 2-W (Fig. 4) would indicate that these induced reactions involve mainly the [Cu(sc)Ja+ ion, since as the hydrogen-ion concentration rises the equilibrium. [Cu(sc),]a+ + Hf e [CU(SC)]~++ scH+ is displaced to the right, and there is an associated decrease in interference. At higher acidities (greater than 3M) the increase in error is concomitant with the formation of the yellow [CUCI,]~ ion. This error continues to increase with increasing hydrochloric acid content (Fig. 3) suggesting that the induced reactions may involve ion-pairs such as [scH$+ . . . . [CuCl,ls-. Addition of hydrochloric acid introduces simultaneously into the medium two ions, H+ and Cl-, which may both inlluence the reaction. Figures 5 and 6 show the effect of hydrogen and chloride ions respectively on the semicarbazide-bromate reaction where the concentration of one of the ions was varied independently of the 7
M. J. M. CAMPBELL,R.
948
GRZESKOWLU
and B. PERRETT
-2-
-4------_----7--._ -69 * . -8 5 = w -IO-
I 01
I 001 Final
[Cl-l
I IO
IO
M
FIG. K-The
effect of chloride ion concentration on the error for semicarbazidebromate titrations at constant hydrogen ion concentration. Final [H+]: A-2.5M; B--1*OM; C-S*OM; D-6,25M; E-9*OM.
2.0
30
5.0 Final
4k+I
60
70
M
6.-Dependence of the error on hydrogen ion concentration for semicarbazidebromate titrations at constant chloride concentration. Final [Cl-]: AAIM; -.2M; C-WM; D-l*OM, E-2*OM, F-3*OM.
FIG.
80
Determination
of semicarbazide
hydrochloride
949
other and the copper-semicarbazide ratio was maintained at 1: 1. The broken line curves in Fig. 5 correspond to the condition where the reaction proceeds so slowly that excess of bromate must be added to cause the electrode system to respond. At constant chloride ion concentration (Fig. 6) the negative error decreases on increasing the hydrogen ion concentration until a minimum error plateau is reached. Two effects are apparent: (i) the greater the chloride ion level the lower the hydrogen ion concentration at which the onset of the minimum error takes place, (ii) the lower the chloride ion concentration the lower the minimum error. The optimum conditions for determination of semicarbazide with bromate in the presence of copper(I1) are high acidity and low chloride ion concentration. Thus, as seen in Fig. 5, at 9M fInal hydrogen ion concentration the minimum error is approximately 0.1% for final chloride ion contents between 0405 and 0.05M. Presumably higher acid concentrations would reduce the error further, but the handling of the solution becomes less convenient. If the hydrogen ion concentration must be maintained at about 344 then an alternative method of eliminating the error is to add an excess of copper ions, as seen in Fig. 3. Zusammenfass&emicarbazid kann mit Kaliumbromat quantitativ in Gegenwart van Nickel, Kobalt(II) und Mangan(I1) titriert werden; Kupfer(II) dagegen stiirt erheblich. Der EintW von Kupferionen auf die Reaktion zwischen Kaliumbromat und Semicarbazid wird untersucht und die optimalen Redingungen angegeben, unter denen die Reaktion quantitativ verlluft. R&urnpeut titrer quantitativement le semicarbazide avec le bromate de potassium en la presence de nickel, cobalt(I1) et manganese(II), mais le cuivre(I1) g&e serueysement. Les influences des ions cuivre sur la reaction entm le bromate de potassium et le semicarbazide sont etudiees et l’on indique les conditions optimales dans lesquelles la reaction est quantitative. REFERENCES 1. 2. 3. 4. 5. 6. 7.
A. Kurtenacker and H. Kubma, Z. Ad. Chem., 1924,64,388. F. Jan=, 0. &&ova and J. Kijrbl, Co/l. Czech. Chem. Commun., 1959,24,2695. J. Vulterin, ibid., 1963, 28, 1391. J. Vulterin and J. Zfka, T&mu, 1963, 10,891. P. H. Hitchcock, Proc. Sot. Anal. Chem., 1968,5,203. M. J. Campbell and R. Grzeskowiak, J. Inorg. Nucl. Chem., 1968,30,1865. C. Hinshelwood, J. Chem. Sot., 1947, 698.
EFFECT OF METAL IONS ON THE DETERMINATION OF SEMICARBAZIDE HYDROCHLORIDE WITH POTASSI~ BRO~TE M. 3. M. CAMPBELL, R. GRZESKOWIAK and B. PWRETT Department of Chemistry, Thames Polytechnic, London, S.E.18 (Received 23 November 1970. Accepted 15 December 1970)
Su~~~~~de can be titrated qu~titatively with potassium bromate in the presence of nickel(D), cobal@) and manganese(II) but copper causes serious interference. The effects of copper ions on the reaction between potassium bromate and semicarbaxide are investigated and the optimum conditions under which the reaction is quantitative are indicated. D~NA~ON of se~carb~ide hydroc~o~de by oxidation with potassium bromate was first suggested by Kurtenacker and Kubina.l The reaction was subsequently investigated by JanEik et aLa and Vulterin,5** and was shown to proceed according to the equation:
Tti
3NH&ONHNH,
+ 2BrO,- = 2N, + 3COa + 3NH, + 2Br + 3HeO.
Recently, the conditions under which the reaction proceeds quanti~tively have been defined by Hitchcock. 6 During our investigation of metal complexes of semicarbazide (sc),~ the Co(H), Ni(I1) and Mn(I1) complexes could be successfully analysed for ligand but the presence of Cu(I1) caused negative errors when the range of hydrochloric acid concentrations defined by Hitchcock was used, both with and without initial addition of bromide. We report in this paper the investigation of the copper interference and indicate the conditions under which the reaction is quanti~~ve. EXPERIMENTAL Reagents
All chemicals used were of analytical-reagent grade. Potassium bromate was dried at l&l”, and semicarbaxide hydrochloride over phosphorus pentoxide under vacuum, and both were stored over caltium chloride. A 0.0167&f solution of potassium bromate was used as standard. The semicarbaxide hy~~~o~de solution (0*025~), which undergoes slow auto-oxidation, was frequently restandardired. Procedures Standardization of semicarbazide hydrochloride solution. Concentrated hydrochloric acid (25 ml)
was added to 25ml of the semicarbaxide hydrochloride solution; the mixture was diluted to 100 ml with distilled water and titrated with 0*0167&fpotassium bromate. E&et of me& ions. Portions (25 ml) of standardixed semicarbaxide hy~~~o~de solution were treated with various quantities of cont. hydrochloric acid (5-70 ml) and metal salts, diluted to 100 ml with distilled water, and titrated with bromate, the addition being rapid to within 05 ml of the end-point. Analysis of nickel complexes. Approximately 1 mmole of the complex, accurately weighed, was dissolved in cont. hydrochloric acid (25 ml). After addition of potassium bromide (0.1 g) and dilution to 100 ml, the solution was titrated with bromate. Titration of semicarbazide soltifioas containing 1 :I mole ratio copper : semicarbazide~ (i) E&et ofhy&ogen ion concenfrat~n. Enough hydrochloric acid was added to 25 ml of 0@25&f semicarbaxide hydrochloride to provide the constant fInal chloride ion concentration required. The acidity was then adjusted to the required value with dilute sulphuric acid, copper solution was added (6.25 ml, O*lOM)and the solution was diluted to 100 ml and then titrated with potassium bromate. 943
944
M. J. M.
CAMPBELL,
R. Gazasrcowr~~ and B.
PERRETT
(ii) Effect of chloride ion concentration. Calculated quantities of hydrochloric acid were added to 25-ml portions of 0*025&fsemicarbazide hydrochloride, to provide final concentrations of chloride ion between @005 and SM, and the acidity adjusted to selected constant values with dilute sulphuric acid. Copper solution was then added (6.25 ml, @lOM) and the solution was diluted to IO0 ml and titrated with bromate. The haI hydrogen ion concentrations of all the solutions were determined from the quantity of acid used and tinal volume of solution, and periodically checked by acid-base titration.
RESULTS
AND DISCUSSION
Experimental results show that bivalent manganese, nickel and cobalt do not interfere with the semicarbazide-potassium bromate reaction when present in molar ratios me~l:ligand as large as 16: 1, Errors incurred in direct titrations of semicarbazide with bromate in presence of Ni(II) and Mn(H) are shown in Fig. 1 and I.4-
12-
ae b k w
08-
I
04-
!n t
o-
-0.4 -
1 iI I \ \
\ \
\
\ \ ?
-08-
-12 -
-14 -
Fm. l.--Error curves for the potentiometric titration of semicarbazide with RBrG,. AC--solution initially free from Br-; BC-Br- initially added, concentration of Brat equivalence point 0*1&f, a similar curve is obtained when Ni(II) is added instead of Br-; BD-in presence of Mn(II). are similar to those found by Hitchcock .6 The method is therefore satisfactory for titration of nickel complexes of semicarbazide (Table I). However, copper(I1) introduces a considerable error (Fig. 2) which is dependent on the rate of addition of the titrant and on other conditions (Table II). Error curves (Fig, 3 and 4) showing the effect of copper ions on the titration in hydrochloric acid medium indicate definite minima at concentrations of acid ea. 2_7M, lower acidities having a more marked effect than higher ones. The curves (Fig. 4) for the dependence of the error on the copper-semicarbazide ratio show
Determination
TABL~I.-QUANTITATWE
NH,CONHNH,
Ni(sc),Br%
Ni(sc)SO&
2HI0
Ni(sc)J&,
2+H,O
NiW%
945
found,
%
NH,CONHNH,
40.3,, 404~,40.4~ (Av. 40.42, G 0.06) 5@4*, 5@3,, 50.49 (Av. 50.41, u @OS) 33*6,, 33.3,, 33.71, 33.2s, 32*9,, 33.2,,, 32*9p, 32.94, (Av. 33.25, CT0.28) 44.01,43*91,43*9*, 43.9, (Av. 43.95, (I 0.04) 56*6,, 565B, 56.68, 56.6, (Av. 56.65, Q 0.03) 68.44, 68*4,, 68.4, (Av. 68.42, u 0.02)
HI0
Ni(sc)$04,
hydrochloride
ESTJ~WIXON OF SEMICARBAZIDE (SC)IN NICKEL CQMPLEXES
Complex
Ni(s&Cl,,
of semicarbazide
4I-W
talc., %
40.72 50.42
32.7 44.02 56.30 68.63
(I = standard deviation.
16-
A
14-
6-
4-
-a- \.
-12-
\
. \
-16-
I 02
I 0.5 Final
, 2.0
I 50
1 IO
[HCL; “!A
2.-Dependence of the error on the hydrochloric acid concentration in presence of copper at [Cul: [SC]= 1. A-in absence of Br-; D-in presence of O*lM Br-; BS and CP +mves AC and BC from Fig. 1. FIG.
M. J. M. CAMPBELL,R. GRZ~SKOWIAKand B. Pzruwrr
946
TABLEII.-EFFECT OF rrrw ADDITION RATE ON THE TITRATION OF 0.6222 mnlOle OF SEMICAlWAZIDE HYDROCHLORIDE WITH 0.01666M BROMATE IN THE PRESENCE. OF COPPER(
ti(In: SPMICARBAZIDE MOLAR LATED TITRR 2489ml
RATIO 1:1. CALCU-
Titrant used, ml
ml/set
3.OM final [HCl]. Initially bromide-free
1.0&f final [HCl]. Initially bromide-free
050 0.10 002
24.47 24.37 24.28
21.26 2052 2006
Addition rate
l*OiWiinal [HCl]. Initial addition of bromide to give 0 1M final concentration 21.73 21.46 21.30
o-2 -
4-
-6 -
2.0
3-O
40
60
60
7.0
FInal [HCLI M
FIG. 3.-Dependence of error on the hydrochloric acid concentration for semicarbazide-bromate titrations at varying ratios of Cn(II) : SC. Curve A B c D E
Cn : sc ratio 0001 o-03 0.1
curve F G H .I
Cu : sc ratio 1 2 10 100
:::5
maximum errors at mole-ratios of Cu: SCbetween O-8and 1.6. This range corresponds to the presence of ionic species [CU(SC)]~+and [CU(SC),]~+in varying proportions. These errors arise from induced side-reactions competing with the main 4-electron oxidation reaction. That induced side-reactions operate is shown by the dependence of the titration error on the rate of titrant addition (Table II), a phenomenon characteristic of induced reactions.
Determination
of semicarbazide
hydrochloride
947
o-2 -
-4 -
-69 0
-8-
‘, k w -IO-
-12-
-14 -
1
I
I
001
oool
FIG. 4.-E&&
cu (II)
:“s:. mole
ratios
I
I
I
IO
IO
Ia0
of CL@) : SCratio on the error for semicarbazide-bromate constant hydrochloric acid concentration.
titrations at
The negative errors are attributed5 to the side-reactions 2NH,NHCONH,+ 2NH,NHCONH,+
+ 2H,O = 4NH,+ + 2C0, + Na + 2e-
+ 2Hz0 = 3NH,+ + HN, + CO2 + 3H+ + 4e-
An initial addition of bromide (Fig. 2) does not appreciably reduce the error, indicating that the slow generation of oxidant via a Hinshelwood-type reaction’ HCl + HBrO, + HClO + HBrO,
in media initially free from bromide is not an important factor. Also, as observed by Hitchcock, air-induced side-reactions, if present, in media free from bromide ion can be assumed to play only a minor role. It would appear therefore, that the sideeffects leading to negative error involve the reaction between copper-semicarbazide complex species and the bromate ion. The decrease in error as the hydrochloric acid concentration is increased from 1 to 2-W (Fig. 4) would indicate that these induced reactions involve mainly the [Cu(sc)Ja+ ion, since as the hydrogen-ion concentration rises the equilibrium. [Cu(sc),]a+ + Hf e [CU(SC)]~++ scH+ is displaced to the right, and there is an associated decrease in interference. At higher acidities (greater than 3M) the increase in error is concomitant with the formation of the yellow [CUCI,]~ ion. This error continues to increase with increasing hydrochloric acid content (Fig. 3) suggesting that the induced reactions may involve ion-pairs such as [scH$+ . . . . [CuCl,ls-. Addition of hydrochloric acid introduces simultaneously into the medium two ions, H+ and Cl-, which may both inlluence the reaction. Figures 5 and 6 show the effect of hydrogen and chloride ions respectively on the semicarbazide-bromate reaction where the concentration of one of the ions was varied independently of the 7
M. J. M. CAMPBELL,R.
948
GRZESKOWLU
and B. PERRETT
-2-
-4------_----7--._ -69 * . -8 5 = w -IO-
I 01
I 001 Final
[Cl-l
I IO
IO
M
FIG. K-The
effect of chloride ion concentration on the error for semicarbazidebromate titrations at constant hydrogen ion concentration. Final [H+]: A-2.5M; B--1*OM; C-S*OM; D-6,25M; E-9*OM.
2.0
30
5.0 Final
4k+I
60
70
M
6.-Dependence of the error on hydrogen ion concentration for semicarbazidebromate titrations at constant chloride concentration. Final [Cl-]: AAIM; -.2M; C-WM; D-l*OM, E-2*OM, F-3*OM.
FIG.
80
Determination
of semicarbazide
hydrochloride
949
other and the copper-semicarbazide ratio was maintained at 1: 1. The broken line curves in Fig. 5 correspond to the condition where the reaction proceeds so slowly that excess of bromate must be added to cause the electrode system to respond. At constant chloride ion concentration (Fig. 6) the negative error decreases on increasing the hydrogen ion concentration until a minimum error plateau is reached. Two effects are apparent: (i) the greater the chloride ion level the lower the hydrogen ion concentration at which the onset of the minimum error takes place, (ii) the lower the chloride ion concentration the lower the minimum error. The optimum conditions for determination of semicarbazide with bromate in the presence of copper(I1) are high acidity and low chloride ion concentration. Thus, as seen in Fig. 5, at 9M fInal hydrogen ion concentration the minimum error is approximately 0.1% for final chloride ion contents between 0405 and 0.05M. Presumably higher acid concentrations would reduce the error further, but the handling of the solution becomes less convenient. If the hydrogen ion concentration must be maintained at about 344 then an alternative method of eliminating the error is to add an excess of copper ions, as seen in Fig. 3. Zusammenfass&emicarbazid kann mit Kaliumbromat quantitativ in Gegenwart van Nickel, Kobalt(II) und Mangan(I1) titriert werden; Kupfer(II) dagegen stiirt erheblich. Der EintW von Kupferionen auf die Reaktion zwischen Kaliumbromat und Semicarbazid wird untersucht und die optimalen Redingungen angegeben, unter denen die Reaktion quantitativ verlluft. R&urnpeut titrer quantitativement le semicarbazide avec le bromate de potassium en la presence de nickel, cobalt(I1) et manganese(II), mais le cuivre(I1) g&e serueysement. Les influences des ions cuivre sur la reaction entm le bromate de potassium et le semicarbazide sont etudiees et l’on indique les conditions optimales dans lesquelles la reaction est quantitative. REFERENCES 1. 2. 3. 4. 5. 6. 7.
A. Kurtenacker and H. Kubma, Z. Ad. Chem., 1924,64,388. F. Jan=, 0. &&ova and J. Kijrbl, Co/l. Czech. Chem. Commun., 1959,24,2695. J. Vulterin, ibid., 1963, 28, 1391. J. Vulterin and J. Zfka, T&mu, 1963, 10,891. P. H. Hitchcock, Proc. Sot. Anal. Chem., 1968,5,203. M. J. Campbell and R. Grzeskowiak, J. Inorg. Nucl. Chem., 1968,30,1865. C. Hinshelwood, J. Chem. Sot., 1947, 698.