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The spectrophotometric determination of some organic acids with copper benzidine
SHORT
394
COMMUNICATIONS
Higher sensitivity (3 p.p.m. or better)2 can bc achieved by using a much higher temperature such as a spark. Such an increase in sensitivity would be achieved only at the sacrifice of simplicity of technique, ease of manipulation, and low cost of instrumentation, thus detracting seriously from the merits of flame absorption spectroscopy. Further work is in progress, and details will be shortly published. ‘This work was sponsored by the United States Air Force (Contract No. AF 30(002)-, 2426) and the U.S. Public Health,Service (Contract No. Al?-00128-02). Kern-Tech Laboratories, Inc. Baton Rouge, Louisiana (U.S./l Coates Chemical Labo7uto7ie.s Louisiana State Univevsity Baton Rouge, I.ouisiana (USA Kern-Tech I_.abo7ato7ies, Inc. Baton Rouge, Louisiana(U.S.A
.)
.) .)
WALSII, Ahnnces it1 .Spcclrosco~y, vol. ii, cd. I-I. W. xgG1, 1’. Ij. 2 J. W. ROI~INSON , .49tnl. Cl&u. AcltC, 27 (1962) ‘l(i.5. :I -1. W. RO~I~NSON, Awut. C’ltew., 33 (1g61) 1067. 1 A.
Intcrscicncc,
‘~‘IlOhlI’SCtN,
New York,
01 J. W. ROI~INSON, Anul. C/mu., 32 ( I~Go) 17R. 6 A. C. MLSNZIES, Atd. Chcttt.. 32 (IgGo) 898. 0 J. A. DEAN, Flame Plrofomefry, McGraw-I-fill, New York, ~~#o,l>. 61, 7 A. G. GAYDON, Dissocinfiotz Etlevgies, 2nd Ed., Chapman and I-lall, London, 1953, 1’. 57. ” I’. E. COTI~ISLL, The Slrcttglh ofChemicaL Bonds, Ruttcrworth, London, 1954, p. 280. 0 J. W. ROII~NSON, Agzul. CJrim. AC/~, 23 (1960) 479. 10 J. W. ROISINSON AND L. J. ICEVAN, Anal. CJtim. AC/~, 28 (1963) 170.
Received
December
8,x962 Attnl.
The
spectrophotometric
determination of copper benzidine
CJiitn.
some
Aslu,
organic
28 (1963)
acids
392-394
with
A simple and rapid procedure for determining microgram quantities of various organic acids and their salts is often very useful in chemical studies of aqueous solutions after radiation treatment. In these cases, acidimetric titrations are not suitable and are inapplicable for salt solutions. The presence of hydrogen peroxide limits the use of titrations with potassium permanganate, and also often renders difficult the use of calorimetric methods. In our laboratory, in many cases, the method previously developed for oxalic acid1 has been used; this involves a spectrophotometric determination with copper-benzidine. The mechanism of this reaction is not quite clear although some salts of copperbenzidine, such as sulphates and acetates’ 2, have long been known. In the case of concentrated solutions of oxalic acid, it is assumed3 that the acid reacts with the A td.
Chim. Ado, 28 (19G3) 394-397
SHORT
COhlMUNlCATIONS
395
reagent (a 5% solution of benzidine in 5% acetic acid and 0.2 N copper acetate) to produce a compound of the following form: [CU~+(CI~H@HZ)Z),,]C~O~. Preliminary measurements showed that in spectrophotometric determinations, the molar ratio of copper acetate to benzidine should be larger than one, which is the opposite of what would be expected; from the results obtained the optimum ratio is three. Uenzidine itself has a very strong absorbance in the working region and its maximum concentration in the measured solution has to be limited to 1.25 . 10-3 M. Reugents Solzrtion A. Prepare a I.25 . xo- 3 M benzidine hydrochloride solution by carefully dissolving a mcasurccl quantity of the substance in 3o”/” acetic acid and then diluting with distilled water. The amount of acetic acid depends on the total amount of solution A; I ml is added for each xoo ml of the solution. Solutio~l B is 3.75 . x0-3 nd copper acetate, obtainecl by a clircct dissolution of the substance in water. For the rcagent solution, equal volumes of solutions A and R. Procedure Mcasurc an appropriate amount of the test solution into a 25-ml volumetric flask. Add 5 ml of the reagent solution and then dilute with water to the mark. Use 5 ml of the reagent solution diluted to 25 ml with water in the reference cell. If the solutions are prepared in smaller or larger flasks, proportional amounts of the reagent must be taken. Transfer the solutions to I-cm cells and measure the absorbance. Experimental data for the measurements are given in Tables 1 and II. Results
and discussion
The reaction with copper-bcnzidinc is instantaneous at room temperature. The solutions A and 13, as well as the mixed reagent are stable under normal laboratory conditions for at least xo days. ‘1’11~absorbance of the prepared sample solutions remains unchanged for 24 11. The temperature coefficients were measured at tcmperaturcs between 18 and 30~ and the data arc given in the last column in Tables I and II; as can be seen, the tem-
-.-__--
_._._ - _-..-
_. .---..-_._-
Work i jrr: wuvcleugllr (“W
.-Icid -.-----_--.. OXilliC
Tartaric Molonic Citric IACtiC
Glycolic Formic Succinic
._.. --_--..-
.- -. -. - -- -.
---._
...--.-
-_--
____
Molur clbsorbmce ccl a.#” . ..-_ _.._._-____-_
_.....-_,-
240
2.490
243
I.255
242 242
242 243 244 244
_.___.__-____
2.380 2.184 3gx 307 249 151
.__......_
_-
._.__
._...._._.
Vulidify I.uniberl-
region Beer’s
of luw
--
___-
._.. -____-_---
* 10-s4’ 10-a I.4 3 * 10-6-3.5 * x0-4
I.5 *
10-5-2.0
* x0-4
10-a-1.3
. 10-J
g * 1.5 . 1.5 * ‘2*4 .
IO-G- 5 * x0-4 10-L 8 * 10-4 * 10-S IO -4-1.2 IO’4-1.6 * 10-a
2
*
Ard.
luve cocfficiettl /or
(Af) _____.___.
.._.-- --_. Tcwaf~cvn-
Clrirn.
Acfa,
x0
( ‘%I)
-a7 --Q-7
-s4
-0.25
-0.7 --o..)
---0.8 --0.8 28 (rgG3) 394-397
SHORT
396
COnIMUNICATIONS
ISXI’ISRIMEtGTAL
Molar
C0?~1~011ml
DATA
uO-
sorbunce al 24”
FOR
SALTS
Validily region of LurheulBeer’s hw (filj
_--_Potassium oxalatc Potassium sodium tartratc Sodium formntc __._._ -_- _..^ --_-
I*.530
.__.
‘I.5
’ IO-“-.*.5
Ggr
_=)’
’ 77
‘I’ ,~,-‘I-
_..-- ..__ -.---_-~----.-
.. . _ .___ - _._. ----
to-fi-
.
* 10-G 5
* IO-.’
8 . 10-A
. .-. -.----
_.--.. --.I-
peraturc ahsorhancc coefficients are considerable and all mcasurcmcnts should be made at constant temperatures. It is important to equalize tlic temperatures of the sample and refcrencc solutions before the optical densities are read; otherwise, the measurecl absorbanccs do not become constant for some time, because the temperature coefficient of the reagent with the substance is different from that of the reference solution. For instance, with water as the rcfercncc solution, the temperature coefficient of the reagent with oxalic acid is ---o.s~/~ per IO, whereas that of the reagent alone with pure water is +0.8~/~ per 1’. Special attention should also be paid to the purity of the bcnzidinc hydrochloride. The molar absorbance dcpcnds greatly on the purity of benzidinc and may vary up to 10% even without any visible change in the absorption spectra. Renzidinc was purified by recrystallization from hot aqueous solution with the addition of concentrated hydrochloric acid to the cold solution: the benzidine hydrochloride obtained is more stable and suitable for work than benzidine itself. An occasional check of the molar absorban’cc is a good cheek of the purity of benzidine. The procedure described allows direct, individual spectrophotometric determination of the following organic acids and some of their salts: oxalic, tartaric ,malonic, citric, lactic, glycolic, formic and succinic acids. The absorption spectra of the acids and some salts are shown in Figs. I and z and further data are given in Tables I and II. It can be seen that the absorption spectra of salts arc different from tllose of the parent acids; they are broader and their maxima are shifted, the molar absorbanccs being smaller. This is certainly due to the presence of cations and the change in PH. It was also found that tlic molar absorbance decreases with increase in PH; for example, when the PH of potassium oxqlate k increased from 7 to 12, there is a IoO!~ decrease in the molar absorbance. As can be seen from Figs. I and 2, simultaneous determinations are not usually possible because the absorption spectra are similar and their absorption maxima very close. Tables T and II, however, show that the differl,llce in molar absorbances makes it possible in some cases to determine, with somewhat lower accuracy, some of the acids in the presence of others (for example, large amounts of oxalic acid in the presence of small concentrations of formic or succinic acicls, etc.). Oxalic, malonic, tartaric and citric acids, as well as the oxalates and tartrates, can be quite easily determined with an accuracy of -& 2% because of the somewhat larger AMI.
Clkr~t. AC&, 28 (x9G3) 334-397
SHORT COMMUNICATIONS
397
validity region of Lambert-Beer’s law and the higher molar absorbance. Determinations of lactic, @ycolic, formic and succinic acids, as well as the formates, are less accurate (& 5%). In the latter case, better results could probably be obtained by varying the concentrations of benzidine and copper acetate, as well as their propor,
A
>“- . \.\
OA.
0.3
0.2
0.1 \ I 01 220
240
260
20 WZ~velen~Y~
230
\ \
(rnp)
Absorption spwtra for acids. Oxalic (I .c) * IO-** nl) ;
b-ig
Trrrtnric (3.2 - 10-J M) ; Glvcolic (7.5 - IO-4 AT, : i\IAonic (;. ;H - IO-‘1 hi);
Fig.
2.
200
r\bsorption
spectra
for
sorue
salts.
1 i~ot;~ssium oxalatc (.I * 10-s) M); 2 l’ot;Lssiutn sodium tartratc (.+ * IO-.’ A,J); 3 Sotliuln formatc (r.1 * lo-3 Al).
Citric (1.25 - IO-4 AT): Succinic (I .G * I oe3 M) ; Lactic (4.7 * Io-*L M) : Formic (7.25 f Io-d RI).
tions. It should be mentioned also that the procedure described should be suitable for the determination of organic acids other than those mentioned. The method is sensitive, direct, simple and fast. The presence of hydrogen peroxide in concentrations smaller than 0.01 M does not affect the accuracy of the results, Complete details will bc publishccl which is important in radiation experiments. elsewhered. Boris Vi&a 1 Z.
Kidrich Imtitde (Yugoslavia) I>. DRAGANIC WELCHER, P. ICRESKOV,
a P. J.
3 A. 4 Z. 1). DRAGANIC,
Received
of N.&ear
Sciences,
%OKICA
1>.
DRAGANIC
UdI. Irut. h’ml. Sci. “Boris I
1.
G.
DRAGANIC.
0rgnuic Aualyfical
November
zoth,
rgGz AWAY.Chinr. Acln, 28 (1963) 394-397
394
COMMUNICATIONS
Higher sensitivity (3 p.p.m. or better)2 can bc achieved by using a much higher temperature such as a spark. Such an increase in sensitivity would be achieved only at the sacrifice of simplicity of technique, ease of manipulation, and low cost of instrumentation, thus detracting seriously from the merits of flame absorption spectroscopy. Further work is in progress, and details will be shortly published. ‘This work was sponsored by the United States Air Force (Contract No. AF 30(002)-, 2426) and the U.S. Public Health,Service (Contract No. Al?-00128-02). Kern-Tech Laboratories, Inc. Baton Rouge, Louisiana (U.S./l Coates Chemical Labo7uto7ie.s Louisiana State Univevsity Baton Rouge, I.ouisiana (USA Kern-Tech I_.abo7ato7ies, Inc. Baton Rouge, Louisiana(U.S.A
.)
.) .)
WALSII, Ahnnces it1 .Spcclrosco~y, vol. ii, cd. I-I. W. xgG1, 1’. Ij. 2 J. W. ROI~INSON , .49tnl. Cl&u. AcltC, 27 (1962) ‘l(i.5. :I -1. W. RO~I~NSON, Awut. C’ltew., 33 (1g61) 1067. 1 A.
Intcrscicncc,
‘~‘IlOhlI’SCtN,
New York,
01 J. W. ROI~INSON, Anul. C/mu., 32 ( I~Go) 17R. 6 A. C. MLSNZIES, Atd. Chcttt.. 32 (IgGo) 898. 0 J. A. DEAN, Flame Plrofomefry, McGraw-I-fill, New York, ~~#o,l>. 61, 7 A. G. GAYDON, Dissocinfiotz Etlevgies, 2nd Ed., Chapman and I-lall, London, 1953, 1’. 57. ” I’. E. COTI~ISLL, The Slrcttglh ofChemicaL Bonds, Ruttcrworth, London, 1954, p. 280. 0 J. W. ROII~NSON, Agzul. CJrim. AC/~, 23 (1960) 479. 10 J. W. ROISINSON AND L. J. ICEVAN, Anal. CJtim. AC/~, 28 (1963) 170.
Received
December
8,x962 Attnl.
The
spectrophotometric
determination of copper benzidine
CJiitn.
some
Aslu,
organic
28 (1963)
acids
392-394
with
A simple and rapid procedure for determining microgram quantities of various organic acids and their salts is often very useful in chemical studies of aqueous solutions after radiation treatment. In these cases, acidimetric titrations are not suitable and are inapplicable for salt solutions. The presence of hydrogen peroxide limits the use of titrations with potassium permanganate, and also often renders difficult the use of calorimetric methods. In our laboratory, in many cases, the method previously developed for oxalic acid1 has been used; this involves a spectrophotometric determination with copper-benzidine. The mechanism of this reaction is not quite clear although some salts of copperbenzidine, such as sulphates and acetates’ 2, have long been known. In the case of concentrated solutions of oxalic acid, it is assumed3 that the acid reacts with the A td.
Chim. Ado, 28 (19G3) 394-397
SHORT
COhlMUNlCATIONS
395
reagent (a 5% solution of benzidine in 5% acetic acid and 0.2 N copper acetate) to produce a compound of the following form: [CU~+(CI~H@HZ)Z),,]C~O~. Preliminary measurements showed that in spectrophotometric determinations, the molar ratio of copper acetate to benzidine should be larger than one, which is the opposite of what would be expected; from the results obtained the optimum ratio is three. Uenzidine itself has a very strong absorbance in the working region and its maximum concentration in the measured solution has to be limited to 1.25 . 10-3 M. Reugents Solzrtion A. Prepare a I.25 . xo- 3 M benzidine hydrochloride solution by carefully dissolving a mcasurccl quantity of the substance in 3o”/” acetic acid and then diluting with distilled water. The amount of acetic acid depends on the total amount of solution A; I ml is added for each xoo ml of the solution. Solutio~l B is 3.75 . x0-3 nd copper acetate, obtainecl by a clircct dissolution of the substance in water. For the rcagent solution, equal volumes of solutions A and R. Procedure Mcasurc an appropriate amount of the test solution into a 25-ml volumetric flask. Add 5 ml of the reagent solution and then dilute with water to the mark. Use 5 ml of the reagent solution diluted to 25 ml with water in the reference cell. If the solutions are prepared in smaller or larger flasks, proportional amounts of the reagent must be taken. Transfer the solutions to I-cm cells and measure the absorbance. Experimental data for the measurements are given in Tables 1 and II. Results
and discussion
The reaction with copper-bcnzidinc is instantaneous at room temperature. The solutions A and 13, as well as the mixed reagent are stable under normal laboratory conditions for at least xo days. ‘1’11~absorbance of the prepared sample solutions remains unchanged for 24 11. The temperature coefficients were measured at tcmperaturcs between 18 and 30~ and the data arc given in the last column in Tables I and II; as can be seen, the tem-
-.-__--
_._._ - _-..-
_. .---..-_._-
Work i jrr: wuvcleugllr (“W
.-Icid -.-----_--.. OXilliC
Tartaric Molonic Citric IACtiC
Glycolic Formic Succinic
._.. --_--..-
.- -. -. - -- -.
---._
...--.-
-_--
____
Molur clbsorbmce ccl a.#” . ..-_ _.._._-____-_
_.....-_,-
240
2.490
243
I.255
242 242
242 243 244 244
_.___.__-____
2.380 2.184 3gx 307 249 151
.__......_
_-
._.__
._...._._.
Vulidify I.uniberl-
region Beer’s
of luw
--
___-
._.. -____-_---
* 10-s4’ 10-a I.4 3 * 10-6-3.5 * x0-4
I.5 *
10-5-2.0
* x0-4
10-a-1.3
. 10-J
g * 1.5 . 1.5 * ‘2*4 .
IO-G- 5 * x0-4 10-L 8 * 10-4 * 10-S IO -4-1.2 IO’4-1.6 * 10-a
2
*
Ard.
luve cocfficiettl /or
(Af) _____.___.
.._.-- --_. Tcwaf~cvn-
Clrirn.
Acfa,
x0
( ‘%I)
-a7 --Q-7
-s4
-0.25
-0.7 --o..)
---0.8 --0.8 28 (rgG3) 394-397
SHORT
396
COnIMUNICATIONS
ISXI’ISRIMEtGTAL
Molar
C0?~1~011ml
DATA
uO-
sorbunce al 24”
FOR
SALTS
Validily region of LurheulBeer’s hw (filj
_--_Potassium oxalatc Potassium sodium tartratc Sodium formntc __._._ -_- _..^ --_-
I*.530
.__.
‘I.5
’ IO-“-.*.5
Ggr
_=)’
’ 77
‘I’ ,~,-‘I-
_..-- ..__ -.---_-~----.-
.. . _ .___ - _._. ----
to-fi-
.
* 10-G 5
* IO-.’
8 . 10-A
. .-. -.----
_.--.. --.I-
peraturc ahsorhancc coefficients are considerable and all mcasurcmcnts should be made at constant temperatures. It is important to equalize tlic temperatures of the sample and refcrencc solutions before the optical densities are read; otherwise, the measurecl absorbanccs do not become constant for some time, because the temperature coefficient of the reagent with the substance is different from that of the reference solution. For instance, with water as the rcfercncc solution, the temperature coefficient of the reagent with oxalic acid is ---o.s~/~ per IO, whereas that of the reagent alone with pure water is +0.8~/~ per 1’. Special attention should also be paid to the purity of the bcnzidinc hydrochloride. The molar absorbance dcpcnds greatly on the purity of benzidinc and may vary up to 10% even without any visible change in the absorption spectra. Renzidinc was purified by recrystallization from hot aqueous solution with the addition of concentrated hydrochloric acid to the cold solution: the benzidine hydrochloride obtained is more stable and suitable for work than benzidine itself. An occasional check of the molar absorban’cc is a good cheek of the purity of benzidine. The procedure described allows direct, individual spectrophotometric determination of the following organic acids and some of their salts: oxalic, tartaric ,malonic, citric, lactic, glycolic, formic and succinic acids. The absorption spectra of the acids and some salts are shown in Figs. I and z and further data are given in Tables I and II. It can be seen that the absorption spectra of salts arc different from tllose of the parent acids; they are broader and their maxima are shifted, the molar absorbanccs being smaller. This is certainly due to the presence of cations and the change in PH. It was also found that tlic molar absorbance decreases with increase in PH; for example, when the PH of potassium oxqlate k increased from 7 to 12, there is a IoO!~ decrease in the molar absorbance. As can be seen from Figs. I and 2, simultaneous determinations are not usually possible because the absorption spectra are similar and their absorption maxima very close. Tables T and II, however, show that the differl,llce in molar absorbances makes it possible in some cases to determine, with somewhat lower accuracy, some of the acids in the presence of others (for example, large amounts of oxalic acid in the presence of small concentrations of formic or succinic acicls, etc.). Oxalic, malonic, tartaric and citric acids, as well as the oxalates and tartrates, can be quite easily determined with an accuracy of -& 2% because of the somewhat larger AMI.
Clkr~t. AC&, 28 (x9G3) 334-397
SHORT COMMUNICATIONS
397
validity region of Lambert-Beer’s law and the higher molar absorbance. Determinations of lactic, @ycolic, formic and succinic acids, as well as the formates, are less accurate (& 5%). In the latter case, better results could probably be obtained by varying the concentrations of benzidine and copper acetate, as well as their propor,
A
>“- . \.\
OA.
0.3
0.2
0.1 \ I 01 220
240
260
20 WZ~velen~Y~
230
\ \
(rnp)
Absorption spwtra for acids. Oxalic (I .c) * IO-** nl) ;
b-ig
Trrrtnric (3.2 - 10-J M) ; Glvcolic (7.5 - IO-4 AT, : i\IAonic (;. ;H - IO-‘1 hi);
Fig.
2.
200
r\bsorption
spectra
for
sorue
salts.
1 i~ot;~ssium oxalatc (.I * 10-s) M); 2 l’ot;Lssiutn sodium tartratc (.+ * IO-.’ A,J); 3 Sotliuln formatc (r.1 * lo-3 Al).
Citric (1.25 - IO-4 AT): Succinic (I .G * I oe3 M) ; Lactic (4.7 * Io-*L M) : Formic (7.25 f Io-d RI).
tions. It should be mentioned also that the procedure described should be suitable for the determination of organic acids other than those mentioned. The method is sensitive, direct, simple and fast. The presence of hydrogen peroxide in concentrations smaller than 0.01 M does not affect the accuracy of the results, Complete details will bc publishccl which is important in radiation experiments. elsewhered. Boris Vi&a 1 Z.
Kidrich Imtitde (Yugoslavia) I>. DRAGANIC WELCHER, P. ICRESKOV,
a P. J.
3 A. 4 Z. 1). DRAGANIC,
Received
of N.&ear
Sciences,
%OKICA
1>.
DRAGANIC
UdI. Irut. h’ml. Sci. “Boris I
1.
G.
DRAGANIC.
0rgnuic Aualyfical
November
zoth,
rgGz AWAY.Chinr. Acln, 28 (1963) 394-397