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Titration of cyanide and hexacyanoferrate(II) with silver nitrate—II
nhr~,
vol.
25, pp. 14>146
Pergamon
TITRATION
DETERMINATION
Press, 1978. P&cd
I” Great Britm
OF CYANIDE AND HEXACYANOFERRATE(I1) WITH SILVER NITRATE-II OF SODIUM CYANIDE AND SODIUM IN A COMPLEXING AGENT
HEXACYANOFERRATE(I1)
JAN ASPLUND Department of Analytical Chemistry, Abe Akademi, SF-20500 Abe 50, Finland (Received 10 February 1977. Revised 18 August 1977. Accepted 18 October 1977)
Summary-Cyanide and hexacyanoferrate(I1) can be titrated with silver nitrate in the presence of a complexing agent masked with a suitable metal ion. A method for determination of sodium cyanide and sodium hexacyanoferrate(I1) in the presence of sodium nitrilotriacetate ions is given as an example.
Cyanide is usually titrated with silver nitrate, to form dicyanoargentate. The end-point of the titration can be indicated by precipitation of silver dicyanoargentate. At pH values lower than 10, the cyanide participates in a side-reaction with hydrogen ions, the influence of which can be described in terms of the side-reaction coefficient,’ ~+~(u). In titration of cyanide ions in the presence of a complexing agent, L, the silver ions may also participate in side-reactions, which can be described by the side-reaction coefficient cxAgc_). Generally, CI,.++)denotes [Ag’]/[Ag+], where [Ag’] is the sum of the concentrations of all species containing Ag except the complex formed in the main reaction. The conditional constant for the 1:2 complex of Ag+ with CN- is
masked with magnesium
When the end-point is indicated by precipitation of silver dicyanoargentate the silver-ion concentration at the end-point, [Ag+],,d, can be calculated from (5)
The small amounts of cyanide present as impurity in a complexing agent can exist in various forms such as free cyanides (I-ICN, KCN, NaCN) and as relatively stable cyano-complexes such as hexacyanoferrates(II1). Cyanide and hexacyanoferrate(I1) ions can be titrated potentiometrically with silver nitrate’ when the initial concentration of free cyanide is lower than 5.00 x 10m4M. The end-point for the 2:l reaction between cyanide ions and silver ions can be indicated by precipitation of silver hexacyanoferrate(I1). The silver-ion concentration at this point can be calculated (1) from
CMCNLI
B;= [Ag’] [CN’]’
CAg+lm,=
or 8; =
82
(2)
GIAg(L)'&N(14)
In the titration of a cyanide solution of concentration ccN with silver nitrate in the presence of a complexing agent, L, the equivalence point can be calculated from the equation:
J
P2
J
.
(6)
CFc(CN)b-
Equations (5) and (6) can also be used for calculation of the silver-ion concentration at the end-points when the solutions do not contain the complexing agent L. The titration error, A[Ag], can be defined as
ACAd= IL&+1- C&l
1 3 CCN'dN(H)'~Ag(L)
[We, = 2
4 Ks~,Ae,F4W~
(7)
(3) and it is evident that lA[Ag] 1 has the lowest value when c(&(,_)= 1. In the titration of cyanide with silver nitrate the complexing agent L can be masked with metal ions which do not form strong cyano-complexes. The optimal conditions for masking of the complexing agent with the metal ion can be investigated as follows (charges are omitted).
In a potentiometric titration the silver-ion concentration, [Ag+], (or rather the silver-ion activity, {Ag+ )), is measured, and for that reason it is better to calculate [Ag”] than [Ag’]. The silver-ion concentration at the equivalence point for the silver-cyanide reaction can be calculated from (4)
aAg(L)=
143
1 + CLIKA~L
(8)
144
JAN ASPLUND
Table 1. The metal ion concentration, [M], for complete masking of ethylenediaminetetra-acetate, EDTA, and nitrilotriacetate, NTA, with lead, calcium and magnesium ions
in the titration of cyanide ions with silver nitrate. Values of constants are as given by Ringbom.’ The value used for gABNrAwas that given by Stary” Metal ion Lead Calcium Magnesium
EDTA NTA log &X -log [M] log Kh(r -log [M] 18.0 10.7 8.7
11.7 4.4 2.4
11.8 6.4 5.4
7.6 2.2 1.2
In the presence of an excess of metal ions the concentration of free complexing agent can be calculated from :
CMLI
cL1 = @ipg
(9)
From (8) and (9)
The metal-ion concentrations necessary for complete masking of ethylenediaminetetra-acetate and nitrilotriacetate with lead, calcium and magnesium ions in the titration of cyanide ions with silver nitrate are given in Table 1. The values in the table were calculated for [ML] = O.lM. Sodium cyanide is best titrated at a pH high enough for none of the cyanide to be protonated (c+ur = 1). In masking of a complexing agent with a metal ion, the precipitation of metal hydroxide usually determines the highest possible metal-ion concentration at any pH-value. Titration of cyanide ions with silver nitrate is possible even when c(&(L)> 1, but the conditions for the titration should be chosen so that the value ~A&_) is as low as possible. Other complexing agents can also be masked by metals. Complete masking of ethylene glycol bis(2aminoethyl ether)tetra-acetate, EGTA, in the titration of cyanide. is possible with [Pb”] > 10-7.2M, [Ca”] > 10-5.20M or [Mg’+ > 10°.68M, so lead and calcium are suitable for masking EGTA. The in-
fluence of the formation of Pb(CN):- (log /I4 = 10) can be neglected. The potentiometric titration of cyanide and hexacyanoferrate(I1) with silver nitrate in the presence of nitrilotriacetate masked with magnesium ions is discussed here. A suitable pH for the titration is -9.8 and a magnesium concentration [Mg2’] of lo-‘M is used. Log @AB(Lj is 0.83 when [MgNTA] = O.lM. A titration curve was calculated for a solution 10v4M in sodium cyanide and 1.5 x 10e5M in sodium hexacyanoferrate(I1). Table 2 shows calculated silver ion concentrations at the equivalence points and endpoints. The value -log[Ag+] = 8.85 gives the calculated silver-ion concentration at the equivalence point of the 1:2 reaction between silver ions and cyanide ions (a = 0.5). The value -log[Ag+] = 8.99 when A&Fe(CNJ6 begins to precipitate gives the calculated silver-ion concentration at the end-point of this reaction. At this point, a has a value very close to 0.5, so the equivalence point, and the end-point are in good agreement. The value -log[Ag+] = 8.04 gives the calculated silver-ion concentration at the equivalence point of the 4:l reaction between silver and hexacyanoferrate(I1) (a = 1.1). [If the titration ratio, a, is defined by a’CA8 cCN
and the ratio I is defined as r=-
cFe(CN)e
9
CCN
then in this case the ratio r has the value 0.15 (r = 1.5 x 10-5/10-4), and a = 0.5 + 4r = 1.1.1 The value -log[Ag,+] = 7.00, when Ag[Ag(CN),] begins to precipitate, gives the calculated silver-ion concentration at the end-point of the 4:l reaction between silver and hexacyanoferrate(I1). At this point, a again has a value very close to 1.1, so the equivalence point and the end-point are in good agreement. The silver-ion concentration at the equivalence point in the titration of hexacyanoferrate(I1) was cal-
Table 2. Values calculated for the silver-ion concentrations at the equivalence points and the end-points in the stepwise titration of cyanide and hexacyanoferrate(I1) ions, in the presence of nitrilotriacetate, with AgNO, at pH 9.80 in a solution 10m4M in NaCN and 1.5 x lo-‘M in Na,Fe(CN), (the NTA was masked with magnesium nitrate) Reaction Ag,Fe(CN)6 begins to precipitate. End-point in the titration of CN- to Ag(CN); Equivalence point in the titration of CN- to Ag(CN); Equivalence point in the titration of Fe(CN)t- to Ag,Fe(CN), Ag[Ag(CN),] begins to precipitate. End-point in the titration of Fe(CN)zto Ag,Fe(CN),
-log
CAg+l
-log CAg’l
8.99
8.16
8.85
8.02
8.04
7.21
7.00
6.17
Titration of cyanide and hexacyanoferrate(I1)
145
culated from the equation:
(11)
Cb+l = v-. EXPERIMENTAL Reagents
Sodium cyanide and sodium hexacyanoferrate(I1) were determined in technical-grade sodium nitrilotriacetate. Nitrilotriacetic acid of analytical grade was used as reference substance. The complexing agent was maSked with Mg(NO& ‘6HrO (Merck Suprapur with a maximum iron content of 5 x 10-60/0).All other reagents were of analytical quality. The pH of sample solutions was adjusted to 9.80 with sodium tetraborate and sodium hydroxide. RESULTS The total concentration of cyanide in different substances can be determined after steam distillation of hydrogen cyanide from acid solution. The hydrogen cyanide is absorbed in sodium hydroxide solution and the cyanide concentration is measured with an ionselective cyanide electrode. This method was used to determine the total concentration cyanide in the reagents used. Cyanide was not detected in the analytical grade nitrilotriacetic acid used as reference.
o$
IO ml
O.OlOM
AgNOS
Fig. 1. Titration curve for the potentiometric titration (--) of 100.0 ml of 5.00 x 10e4M NaCN in the presence of analytical-grade nitrilotriacetate at pH 9.80 with AgNO,. The NTA was masked with magnesium nitrate. The titration curve (0) for titration of NaCN in O.lOM NaNOs medium is also shown. Table 3. Results of determinations Solution CNSample, mg added, mg
2500 2500 2500 2500 2500
0.256 0.511 0.767 1.022
ml
0.0050M
8
AdNO
Fig. 2. Titration curve for the potentiometric titration of 100.0 ml of a solution of technical-grade sodium nitrilotri-
acetate containing NaCN and Na,Fe(CN), at pH 9.80 with AgNOS (2.12mg of NaCN added to the solution). The NTA was masked with magnesium nitrate. Figure 1 shows the observed titration curve for the potentiometric titration of sodium cyanide in the presence of analytical grade nitrilotriacetate at pH 9.80 with silver nitrate. The nitrilotriacetate was masked with magnesium nitrate. The corresponding titration curve for titration of sodium cyanide in O.lOM sodium nitrate medium at the same pH is also shown. The two titration curves coincide up to the equivalence point for the reaction between silver and dicyanoargentate ions. A difference of 6-7 mV can be explained by the fact that the ionic strength in the titrations is not the same. For determination of sodium cyanide and sodium hexacyanoferrate(I1) in technical-grade sodium nitrilotriacetate five 2.5-g samples were taken, 15.0 ml of l.OM magnesium nitrate were added the solutions were diluted to 100 ml and the pH was adjusted to 9.80. To four of the sample solutions various known amounts of sodium cyanide were added. The results of the determinations are given in Table 3. Figure 2 shows the potentiometric titration curve for the sample solution containing 2.12 mg of sodium cyanide.
of free and complex cyanide in technical-grade sodium nitrilotriacetate
CN- found in solution, mg Free
-o.eo_l
Complex
0.197 0.259 0.455 0.244 0.707 0.255 0.939 0.244 1.196 0.254 Mean value
CN- found in NTA, mglkg Free 78.8 79.6 78.4 68.8 69.6 75.0
Complex 103.6 97.6 102.0 97.6 101.6 100.5
Deviation from mean value, mslke
Total
Free
182.4 177.2 180.4 166.4 171.2 175.5
+3.8 +4.6 + 3.4 -6.2 -5.2
Complex +3.1 -2.9 + 1.5 -2.9 + 1.1
Total +6.9 + 1.7 +4.9 -9.1 -4.3
JAN ASPLUND
146
Table 4. Results of determinations of total cyanide in technical-grade sodium nitrilotriacetate Sample, *g
CN- found, *g/kg
Deviation from the mean value, *g/kg
100.0 150.0 250.0 500.0
180 178 173 173
+4 +2 -3 -3
Mean value:
176
The technical-grade sodium nitrilotriacetate used contains on average 0.0075% of free cyanide and 0.0101% of cyanide complexed with iron( or, on average, a total of 0.0176% CN-. The results of determinations of the total cyanide by steam-distillation are given in Table 4. The values are in good agreement with the values obtained by titration with silver nitrate. Tetrasodium ethylenediaminetetra-acetate was also examined. For the titration with silver nitrate it was masked with magnesium nitrate, and it was found that the technical-grade material did not contain detectable amounts of cyanides.
that comparatively high concentrations of impurities may have been introduced into the sample solutions by the reagents. Magnesium nitrate of analytical grade contains at most 5 x 10m4% of iron. When 3.85 g of it are used to mask 2.50 g of the Na,NTA, the iron content of the sample solution will increase by the Na,NTA contains about 20% because 3.75 x 10e3% of iron [as Fe(CN)z-1. With magnesium nitrate of Suprapur grade containing at most 5 x 10m6% of iron, the iron content of the sample solution will increase by only about 0.2%. If iron(I1) or iron ions are added to a solution containing cyanide ions, hexacyanoferrate(I1) or hexacyanoferrate(II1) ions are formed. The nitrilotriacetate examined contained small amounts of sodium cyanide and sodium hexacyanoferrate(I1). When both are to be determined a super pure reagent with a low iron content must be used. Mg(N0&.6Hz0 of Merck Suprapur grade is suitable. Acknowledgements-The
author is grateful to Mr. Ian McIntosh for checking the English of the manuscript submitted, Prof. Erkki Wlnninen for valuable discussion and advice and the Neste Oy Research Foundations for financial support. REFERENCES
DISCUSSION
The technical-grade sodium nitrilotriacetate examined contained only small amounts of cyanides, so large amounts of reagents had to be used. This means
A. Ringbom, Complex&ion in Analytical Wiley-Interscience, New York. 1963. 2. J. Asplund, Talanta 1978, 1978 25, 137. 3. J. Star$, Anal. Chim. Acta, 1963, 28, 132. 1.
Chemistry,
vol.
25, pp. 14>146
Pergamon
TITRATION
DETERMINATION
Press, 1978. P&cd
I” Great Britm
OF CYANIDE AND HEXACYANOFERRATE(I1) WITH SILVER NITRATE-II OF SODIUM CYANIDE AND SODIUM IN A COMPLEXING AGENT
HEXACYANOFERRATE(I1)
JAN ASPLUND Department of Analytical Chemistry, Abe Akademi, SF-20500 Abe 50, Finland (Received 10 February 1977. Revised 18 August 1977. Accepted 18 October 1977)
Summary-Cyanide and hexacyanoferrate(I1) can be titrated with silver nitrate in the presence of a complexing agent masked with a suitable metal ion. A method for determination of sodium cyanide and sodium hexacyanoferrate(I1) in the presence of sodium nitrilotriacetate ions is given as an example.
Cyanide is usually titrated with silver nitrate, to form dicyanoargentate. The end-point of the titration can be indicated by precipitation of silver dicyanoargentate. At pH values lower than 10, the cyanide participates in a side-reaction with hydrogen ions, the influence of which can be described in terms of the side-reaction coefficient,’ ~+~(u). In titration of cyanide ions in the presence of a complexing agent, L, the silver ions may also participate in side-reactions, which can be described by the side-reaction coefficient cxAgc_). Generally, CI,.++)denotes [Ag’]/[Ag+], where [Ag’] is the sum of the concentrations of all species containing Ag except the complex formed in the main reaction. The conditional constant for the 1:2 complex of Ag+ with CN- is
masked with magnesium
When the end-point is indicated by precipitation of silver dicyanoargentate the silver-ion concentration at the end-point, [Ag+],,d, can be calculated from (5)
The small amounts of cyanide present as impurity in a complexing agent can exist in various forms such as free cyanides (I-ICN, KCN, NaCN) and as relatively stable cyano-complexes such as hexacyanoferrates(II1). Cyanide and hexacyanoferrate(I1) ions can be titrated potentiometrically with silver nitrate’ when the initial concentration of free cyanide is lower than 5.00 x 10m4M. The end-point for the 2:l reaction between cyanide ions and silver ions can be indicated by precipitation of silver hexacyanoferrate(I1). The silver-ion concentration at this point can be calculated (1) from
CMCNLI
B;= [Ag’] [CN’]’
CAg+lm,=
or 8; =
82
(2)
GIAg(L)'&N(14)
In the titration of a cyanide solution of concentration ccN with silver nitrate in the presence of a complexing agent, L, the equivalence point can be calculated from the equation:
J
P2
J
.
(6)
CFc(CN)b-
Equations (5) and (6) can also be used for calculation of the silver-ion concentration at the end-points when the solutions do not contain the complexing agent L. The titration error, A[Ag], can be defined as
ACAd= IL&+1- C&l
1 3 CCN'dN(H)'~Ag(L)
[We, = 2
4 Ks~,Ae,F4W~
(7)
(3) and it is evident that lA[Ag] 1 has the lowest value when c(&(,_)= 1. In the titration of cyanide with silver nitrate the complexing agent L can be masked with metal ions which do not form strong cyano-complexes. The optimal conditions for masking of the complexing agent with the metal ion can be investigated as follows (charges are omitted).
In a potentiometric titration the silver-ion concentration, [Ag+], (or rather the silver-ion activity, {Ag+ )), is measured, and for that reason it is better to calculate [Ag”] than [Ag’]. The silver-ion concentration at the equivalence point for the silver-cyanide reaction can be calculated from (4)
aAg(L)=
143
1 + CLIKA~L
(8)
144
JAN ASPLUND
Table 1. The metal ion concentration, [M], for complete masking of ethylenediaminetetra-acetate, EDTA, and nitrilotriacetate, NTA, with lead, calcium and magnesium ions
in the titration of cyanide ions with silver nitrate. Values of constants are as given by Ringbom.’ The value used for gABNrAwas that given by Stary” Metal ion Lead Calcium Magnesium
EDTA NTA log &X -log [M] log Kh(r -log [M] 18.0 10.7 8.7
11.7 4.4 2.4
11.8 6.4 5.4
7.6 2.2 1.2
In the presence of an excess of metal ions the concentration of free complexing agent can be calculated from :
CMLI
cL1 = @ipg
(9)
From (8) and (9)
The metal-ion concentrations necessary for complete masking of ethylenediaminetetra-acetate and nitrilotriacetate with lead, calcium and magnesium ions in the titration of cyanide ions with silver nitrate are given in Table 1. The values in the table were calculated for [ML] = O.lM. Sodium cyanide is best titrated at a pH high enough for none of the cyanide to be protonated (c+ur = 1). In masking of a complexing agent with a metal ion, the precipitation of metal hydroxide usually determines the highest possible metal-ion concentration at any pH-value. Titration of cyanide ions with silver nitrate is possible even when c(&(L)> 1, but the conditions for the titration should be chosen so that the value ~A&_) is as low as possible. Other complexing agents can also be masked by metals. Complete masking of ethylene glycol bis(2aminoethyl ether)tetra-acetate, EGTA, in the titration of cyanide. is possible with [Pb”] > 10-7.2M, [Ca”] > 10-5.20M or [Mg’+ > 10°.68M, so lead and calcium are suitable for masking EGTA. The in-
fluence of the formation of Pb(CN):- (log /I4 = 10) can be neglected. The potentiometric titration of cyanide and hexacyanoferrate(I1) with silver nitrate in the presence of nitrilotriacetate masked with magnesium ions is discussed here. A suitable pH for the titration is -9.8 and a magnesium concentration [Mg2’] of lo-‘M is used. Log @AB(Lj is 0.83 when [MgNTA] = O.lM. A titration curve was calculated for a solution 10v4M in sodium cyanide and 1.5 x 10e5M in sodium hexacyanoferrate(I1). Table 2 shows calculated silver ion concentrations at the equivalence points and endpoints. The value -log[Ag+] = 8.85 gives the calculated silver-ion concentration at the equivalence point of the 1:2 reaction between silver ions and cyanide ions (a = 0.5). The value -log[Ag+] = 8.99 when A&Fe(CNJ6 begins to precipitate gives the calculated silver-ion concentration at the end-point of this reaction. At this point, a has a value very close to 0.5, so the equivalence point, and the end-point are in good agreement. The value -log[Ag+] = 8.04 gives the calculated silver-ion concentration at the equivalence point of the 4:l reaction between silver and hexacyanoferrate(I1) (a = 1.1). [If the titration ratio, a, is defined by a’CA8 cCN
and the ratio I is defined as r=-
cFe(CN)e
9
CCN
then in this case the ratio r has the value 0.15 (r = 1.5 x 10-5/10-4), and a = 0.5 + 4r = 1.1.1 The value -log[Ag,+] = 7.00, when Ag[Ag(CN),] begins to precipitate, gives the calculated silver-ion concentration at the end-point of the 4:l reaction between silver and hexacyanoferrate(I1). At this point, a again has a value very close to 1.1, so the equivalence point and the end-point are in good agreement. The silver-ion concentration at the equivalence point in the titration of hexacyanoferrate(I1) was cal-
Table 2. Values calculated for the silver-ion concentrations at the equivalence points and the end-points in the stepwise titration of cyanide and hexacyanoferrate(I1) ions, in the presence of nitrilotriacetate, with AgNO, at pH 9.80 in a solution 10m4M in NaCN and 1.5 x lo-‘M in Na,Fe(CN), (the NTA was masked with magnesium nitrate) Reaction Ag,Fe(CN)6 begins to precipitate. End-point in the titration of CN- to Ag(CN); Equivalence point in the titration of CN- to Ag(CN); Equivalence point in the titration of Fe(CN)t- to Ag,Fe(CN), Ag[Ag(CN),] begins to precipitate. End-point in the titration of Fe(CN)zto Ag,Fe(CN),
-log
CAg+l
-log CAg’l
8.99
8.16
8.85
8.02
8.04
7.21
7.00
6.17
Titration of cyanide and hexacyanoferrate(I1)
145
culated from the equation:
(11)
Cb+l = v-. EXPERIMENTAL Reagents
Sodium cyanide and sodium hexacyanoferrate(I1) were determined in technical-grade sodium nitrilotriacetate. Nitrilotriacetic acid of analytical grade was used as reference substance. The complexing agent was maSked with Mg(NO& ‘6HrO (Merck Suprapur with a maximum iron content of 5 x 10-60/0).All other reagents were of analytical quality. The pH of sample solutions was adjusted to 9.80 with sodium tetraborate and sodium hydroxide. RESULTS The total concentration of cyanide in different substances can be determined after steam distillation of hydrogen cyanide from acid solution. The hydrogen cyanide is absorbed in sodium hydroxide solution and the cyanide concentration is measured with an ionselective cyanide electrode. This method was used to determine the total concentration cyanide in the reagents used. Cyanide was not detected in the analytical grade nitrilotriacetic acid used as reference.
o$
IO ml
O.OlOM
AgNOS
Fig. 1. Titration curve for the potentiometric titration (--) of 100.0 ml of 5.00 x 10e4M NaCN in the presence of analytical-grade nitrilotriacetate at pH 9.80 with AgNO,. The NTA was masked with magnesium nitrate. The titration curve (0) for titration of NaCN in O.lOM NaNOs medium is also shown. Table 3. Results of determinations Solution CNSample, mg added, mg
2500 2500 2500 2500 2500
0.256 0.511 0.767 1.022
ml
0.0050M
8
AdNO
Fig. 2. Titration curve for the potentiometric titration of 100.0 ml of a solution of technical-grade sodium nitrilotri-
acetate containing NaCN and Na,Fe(CN), at pH 9.80 with AgNOS (2.12mg of NaCN added to the solution). The NTA was masked with magnesium nitrate. Figure 1 shows the observed titration curve for the potentiometric titration of sodium cyanide in the presence of analytical grade nitrilotriacetate at pH 9.80 with silver nitrate. The nitrilotriacetate was masked with magnesium nitrate. The corresponding titration curve for titration of sodium cyanide in O.lOM sodium nitrate medium at the same pH is also shown. The two titration curves coincide up to the equivalence point for the reaction between silver and dicyanoargentate ions. A difference of 6-7 mV can be explained by the fact that the ionic strength in the titrations is not the same. For determination of sodium cyanide and sodium hexacyanoferrate(I1) in technical-grade sodium nitrilotriacetate five 2.5-g samples were taken, 15.0 ml of l.OM magnesium nitrate were added the solutions were diluted to 100 ml and the pH was adjusted to 9.80. To four of the sample solutions various known amounts of sodium cyanide were added. The results of the determinations are given in Table 3. Figure 2 shows the potentiometric titration curve for the sample solution containing 2.12 mg of sodium cyanide.
of free and complex cyanide in technical-grade sodium nitrilotriacetate
CN- found in solution, mg Free
-o.eo_l
Complex
0.197 0.259 0.455 0.244 0.707 0.255 0.939 0.244 1.196 0.254 Mean value
CN- found in NTA, mglkg Free 78.8 79.6 78.4 68.8 69.6 75.0
Complex 103.6 97.6 102.0 97.6 101.6 100.5
Deviation from mean value, mslke
Total
Free
182.4 177.2 180.4 166.4 171.2 175.5
+3.8 +4.6 + 3.4 -6.2 -5.2
Complex +3.1 -2.9 + 1.5 -2.9 + 1.1
Total +6.9 + 1.7 +4.9 -9.1 -4.3
JAN ASPLUND
146
Table 4. Results of determinations of total cyanide in technical-grade sodium nitrilotriacetate Sample, *g
CN- found, *g/kg
Deviation from the mean value, *g/kg
100.0 150.0 250.0 500.0
180 178 173 173
+4 +2 -3 -3
Mean value:
176
The technical-grade sodium nitrilotriacetate used contains on average 0.0075% of free cyanide and 0.0101% of cyanide complexed with iron( or, on average, a total of 0.0176% CN-. The results of determinations of the total cyanide by steam-distillation are given in Table 4. The values are in good agreement with the values obtained by titration with silver nitrate. Tetrasodium ethylenediaminetetra-acetate was also examined. For the titration with silver nitrate it was masked with magnesium nitrate, and it was found that the technical-grade material did not contain detectable amounts of cyanides.
that comparatively high concentrations of impurities may have been introduced into the sample solutions by the reagents. Magnesium nitrate of analytical grade contains at most 5 x 10m4% of iron. When 3.85 g of it are used to mask 2.50 g of the Na,NTA, the iron content of the sample solution will increase by the Na,NTA contains about 20% because 3.75 x 10e3% of iron [as Fe(CN)z-1. With magnesium nitrate of Suprapur grade containing at most 5 x 10m6% of iron, the iron content of the sample solution will increase by only about 0.2%. If iron(I1) or iron ions are added to a solution containing cyanide ions, hexacyanoferrate(I1) or hexacyanoferrate(II1) ions are formed. The nitrilotriacetate examined contained small amounts of sodium cyanide and sodium hexacyanoferrate(I1). When both are to be determined a super pure reagent with a low iron content must be used. Mg(N0&.6Hz0 of Merck Suprapur grade is suitable. Acknowledgements-The
author is grateful to Mr. Ian McIntosh for checking the English of the manuscript submitted, Prof. Erkki Wlnninen for valuable discussion and advice and the Neste Oy Research Foundations for financial support. REFERENCES
DISCUSSION
The technical-grade sodium nitrilotriacetate examined contained only small amounts of cyanides, so large amounts of reagents had to be used. This means
A. Ringbom, Complex&ion in Analytical Wiley-Interscience, New York. 1963. 2. J. Asplund, Talanta 1978, 1978 25, 137. 3. J. Star$, Anal. Chim. Acta, 1963, 28, 132. 1.
Chemistry,