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Potentiometric determination of certain α-aminohydroxy compounds by using an ammonia gas-sensing electrode
0039-9140/82/l 1091l-05$03.00/0 Copyright 0 1982 Pergamon Press Ltd
~&mm, Vol. 29, pp. 911 to 915, 1982 Printedin Great Britain.All rightsreserved
POTENTIOMETRIC DETERMINATION OF CERTAIN a-AMINOHYDROXY COMPOUNDS BY USING AN AMMONIA GAS-SENSING ELECTRODE D. P. Laboratory
NIKOLELIS, C. E. EFSTATHIOU and T. P. HADJIIOANNOU
of Analytical
Chemistry. (Received
University
of Athens,
104 Solonos
St., Athens
144, Greece
14 April 1982. Accepted 7 June 1982)
Summary-A simple method is described for the determination of certain a-aminohydroxy compounds, based on the potentiometric measurement of ammonia released after oxidation with an excess of periodate. Ammonia is measured directly in the reaction mixture, with an ammonia gas-sensing electrode. Ethanolamine, diethanolamine, triethanolamine, serine, threonine, and glucosamine (0.3-6 pmole) can be determined with average errors of about l-2%.
gas-sensing electrode has been used extensively for the direct potentiometric determination of ammonia in a variety of matrices of clinical or environmental importance.’ It has also been used for the construction of bioselective sensors, such as enzyme electrodes, by taking advantage of the fact that ammonia is a product of biodegradation of many nitrogen-containing organic compounds.’ Ammonia is also obtained from the chemical degradation (e.g., hydrolysis) of many organic compounds, thus making possible their potentiometric determination. In a previous paper we described a simple method for assaying nicotinamide in multivitamin preparations by potentiometric measurement of ammonia released after alkaline hydrolysis of the sample.3 In this paper we describe a simple procedure for the determination of various a-aminohydroxy compounds, based on the determination of ammonia released after oxidation with an excess of periodate. It is known that periodate cleaves the carbon-carbon bond of a-aminohydroxy compounds smoothly.4 The general reaction scheme for the three a-amino alcohols, ethanolamine, diethanolamine and triethanolamine, is
The ammonia
(HOCH&HJ,NH,_, --+ 2nHCH0
+ nI04 + NH,
carbohydrates are present. The titrimetric determination of the ammonia released is not subject to these interferences but it involves tedious and time-consuming separation steps (e.g., by microdiffusion’). With the ammonia gas-sensing electrode, ammonia can be determined directly in the reaction mixture. No separation steps are necessary and the sensitivity of the analysis is greatly improved. To evaluate the method, it was applied to the determination of three ethanolamines, the a-hydroxyamino-acids serine and threonine, and the amino-sugar glucosamine. Microamounts of these compounds in the range 0.3-6 pmole were determined with average errors of about l-2%.
EXPERIMENTAL
Apparatus An Orion Model 95-10 ammonia gas-sensing electrode was used, and the potential measurements were made with an Orion Model 801 digital pH/mV meter. All measurements were made at 25°C k 0.1” in a lo-ml cell equipped with a magnetic stirrer. When not in use, the electrode was kept in O.OSM ammonium chloride.9 Reagents
+ nI0;
(n = l-3).
The optimum conditions (pH, temperature and reaction period) for completion of these reactions have been reported5 and various analytical schemes have been developed. Titrimetric methods are based on the iodometric determination of either the excess of periodate or the iodate produced, after masking of the excess of periodate with molybdate.6 Other schemes are based on the spectrophotometric measurement of the formaldehyde produced.’ These methods offer high accuracy and sensitivity but they lack selectivity when other compounds reacting with periodate, such as vicinal glycols, polyhydroxy compounds and
Analytical-grade materials and demineralized, distilled water were used throughout. Sodium metaperiodate, O.IOOM. Dissolve 21.4 g of reagent in water and dilute to 1 litre. Store in an amber bottle. Potassium carbonate, I.OOM solution. Sodium bicarbonate, 0.20M solution. Potassium hydroxide, 2.00M solution. cc-Aminohydroxy compounds. Prepare 0.0200M stock solutions of serine, threonine and glucosamine hydrochloride by dissolving the appropriate amount of each compound in water. Prepare 0.0200M stock solutions of ethanolamines by diluting 1M solutions, which have been titrated potentiometrically with standard I.OOOM hydrochloric acid solution. Prepare fresh standard solutions, 1.00 x 10m4M, 2.50 x 10m4M, 7.50 x 10e4M and 2.00 x 10m3M, as needed, by appropriate dilution of the stock solutions. 911
D. P.
912
bhKOLELIS
Procedures Monoethanolamine,
diethanolamine,
serine and threonine.
Pipette 4.00 ml of a 1:l mixture of O.lOOM sodium metaperiodate and l.OOM potassium carbonate (this mixture is stable for at least one day at room temperature) and 3.00 ml of the standard or unknown a-aminohydroxy compound solution into the measurement cell. Start the stirrer and read the potential when it has stabilized to within kO.1 mV (in about 334 min). Find the unknown concentration from a calibration graph of potential us. log [a-aminohydroxy compound]. Triethanolamine. Incubate the reaction mixture (prepared as before) in a lo-ml vial fitted with a well-fitting stopper, for 60 min, in a water-bath at 60”, cool to room temperature, transfer the contents of the vial into the measurement cell and measure as for monoethanolamine etc. Glucosamine.
Pipette 3.00 ml of a 2:l mixture of O.lOOM sodium metaperiodate and 0.20M sodium bicarbobonate (this mixture is stable for at least 15 min at room
temperature, but then disodium paraperiodate, NazHSI06, starts to precipitate) and 3.00 ml of the standard or unknown glucosamine solution into a IO-ml vial with a wellfitting stopper. Shake the vial and immerse it in a waterbath at 60” for 60 min. Cool the vial to room temperature, pipette 1.00 ml of 2.OOM potassium hydroxide into it, mix, and transfer the contents into the measurement cell. Measure as above. RESULTS Completion
Figure ammonia
I
of the
AND
DISCUSSION
reactions
1 shows recordings of the potential of the gas-sensing electrode during the course of
1
irthanolaminr
chloride
-I
min __c(
Time
-
Fig. 1. Records of the potential of the ammonia gas-sensing electrode during the reaction of periodate with a-aminohydroxy compounds. In a mixture of 4.00 ml of composite 0.05OOM periodate-0.500M carbonate solution and 3.00 ml of 6.67 x IO-‘M ammonium chloride, 100 ~1 of 0.0200M solution of the relevant a-aminohydroxy compound are injected. Ammonium chloride is also injected for comparison. 1: a-aminohvdroxy compound (or ammonium chloride) solution injection..Concentrations at the start of the reaction: NaIO,, 0.0282M; KXO,, 0.282M; NH,Cl, 2.82 x IOm5M (added in all cases -to keep the initial potential stable); a-aminohydroxy compound (or NH,Cl), 2.82 x 10m4M. Temperature: 25°C.
et al
the reaction of a-aminohydroxy compounds with periodate in alkaline (carbonate) solution. Since gassensing electrodes are slow-response concentration transducers, for comparison of speed of response an increase of ammonia concentration by addition of ammonium chloride has been included. The latter was added in an amount equivalent to that of the a-aminohydroxy compound. It has been reported that the ammonia electrode responds to lower organic amines.’ It can be seen from Fig. 1 that there is no direct response of the electrode to x-aminohydroxy compounds. This is probably because the hydroxyl groups make these compounds more hydrophilic and therefore less able to diffuse through the hydrophobic membrane of the ammonia gas-sensing electrode. Ethanolamine, diethanolamine, serine and threonine react quantitatively and almost instantaneously with periodate, and weakly alkaline (bicarbonate) media have been recommended4x6 for making the reaction quantitative. However, the reactions also proceed quickly in more alkaline (carbonate) solutions, and then practically all the ammonia released is in molecular form and thus is measured quantitatively with the ammonia gas-sensing electrode. On the other hand, triethanolamine and glucosamine react very slowly and incompletely. In fact, under all conditions tested (e.g., bicarbonate solutions, phosphate buffers of pH 7-9, carbonate and borate buffers of pH 9-10, use of Mn’+ as catalyst plus nitrilotriacetic acid as activator,” at temperatures from 25” to 75”) the ammonia released never exceeded 90% of the theoretical amount. These findings contradict those of other workers’ who claim complete oxidation, at least for triethanolamine. It is possible that the reaction is stoichiometric with regard to the periodate consumed or iodate produced per molecule of triethanolamine but that some of the ammonia condenses with the formaldehyde formed as one of the reaction products. Attempts to release ammonia quantitatively by addition of glycine as a scavenger for formaldehyde failed, because glycine is oxidized by periodate at elevated temperature and releases large amounts of ammonia. Plots of the percentage of ammonia released from triethanolamine and glucosamine us. reaction period, under a variety of conditions, are shown in Fig. 2. The ammonia released from triethanolamine and glucosamine was 80 and 88% of the theoretical amount, respectively, when the reaction with periodate took place in alkaline (carbonate) and weakly alkaline (bicarbonate) solutions respectively, at 60” for 60 min. Analytical
results
The potential of the ammonia gas-sensing electrode was found to be linearly dependent on the logarithm of the concentration of the a-aminohydroxy compound, according to the general equation. E =
constant
- S log[a-aminohydroxy
compound]
Potentiometric
determination
of a-aminohydroxy
100
I 0
I 60
I
30
I
120
Time,
c
GLUCOSAMI
I
I
90
913
compounds
I 30
0
I 60
Table
1. Results for the potentiometric Concentration,
min
Ethanolamine
1.00 2.50 7.50 20.0
1.02 2.43 7.32 20.4
Diethanolamine
1.00 2.50 7.50 20.0
0.99 2.39 7.56 20.2
Triethanolamine
1.00 2.50 7.50 20.0
1.04 2.52 7.41 20.4
Serine
1.00 2.50 7.50 20.0
2.42 7.62 20.2
1.00 2.50 7.50 20.0
1.00 2.54 7.55 19.9
1.00 2.50 7.50 20.0
1.02 2.43 7.65 20.2
*The lower concentration standard the calculation of the regression
of a-aminohydroxy
compounds
10m4M Found
1.02
and in A as C
olamine, diethanolamine and a-hydroxyamino-acids, obtained by plotting potential us. log[cc-aminohydroxy compound], practically coincide with a working curve obtained with standard solutions of ammonium chloride. Therefore, for these compounds the
determination
Taken
Compound
I 120
I
90
Fig. 2. Rate of release of ammonia for triethanolamine and glucosamine. Initial concentrations temperatures: A-NaIO,, O.O286M, K,C03 0.286M; triethanolamine, 3.21 x 10-4M; 25°C. B-as but at 60°C. C-NaIO,, 0.0286M; KzC03, 0.286M; glucosamine, 3.21 x 10m4M; 25°C. D-same but at 60°C. E-NaIO,, 0.0333M; NaHCO,, 0.0333M; glucosamine, 3.75 x 10e4M; 60°C. for the concentration range 1 x 10m4-2 x 10A3M. The working range can be extended to higher concentrations provided that enough periodate is added to oxidize the compound to be determined plus any other oxidizable compound. Working curves for ethan-
NE
Error,
%
+ 2.0 -2.8 -2.4 + 2.0 Av. 2.3 -1.0 - 4.4 +0.8 +1.0 Av. 1.8 f4.0 + 0.8 - 1.2 + 2.0 Av. 2.0 + 2.0 -3.2 + 1.6 + 1.0 Av. 2.0 + 1.6 +0.7 -0.5 Av. 0.7 + 2.0 -2.8 + 2.0 + 1.0 Av. 2.0
deviates from linearity equation.
Regression
equation
E = - 156.6 - 54.3 log C, r = -0.9993
E = - 167.8 - 55.9 log C, r = -0.9996
E = - 161.9 - 57.8 log C, r = -0.99948
E = - 172.0 - 57.8 log C, r = -0.99995
E = - 176.0 - 58.2 log C, r = -0.99994
E = - 155.3 - 53.4 log c, r = -0.9992
and has not been included
in
914
D. P. NIKOLELISet al. Table 2. Effect of various compounds on the determination hydroxy compounds Compound (5.00 x lo-‘%) Ethanolamine
Interferent Propylene glycol Glucose
Formaldehyde
Diethanolamine
Glycerol Formaldehyde
Triethanolamine
Glycerol
Glucosamine
Glucose
[Interferent] [Compound] 30/l loo/l 300/1* 3/l 10/l 30/l loo/l* 30/l 100/l 300/l 30/l 100/l 300/l? 30/l 100/l 300/1* 10/l 30/l 100/l 300/l 311 10/l 30/l 100/l
of cr-amino-
Error, % +0.4 - 0.4 - 12.0 -4.5 -5.5 - 10.4 -43.0 -2.8 - 7.4 -33.5 -0.8 - 2.0 -69.6 -1.6 - 5.9 - 36.9 - 1.6 -3.2 - 13.0 - 14.7 -3.8 -9.8 -32.4 - 46.2
*At these interference levels it took about 10 min to obtain a steady potential indication. tAt this interference level the potential was not stabilized even after 10 min, drifting continuously toward smaller values (smaller error). working curve can be based on standard ammonium chloride solutions. The working curves for triethanolamine and glucosamine were shifted slightly toward more positive potentials because of the incomplete release of ammonia. Table 1 gives typical analytical results for the determination of six a-aminohydroxy compounds, together with the regression equations of the corresponding working curves. The relative standard deviation for 5 x 10m4M ethanolamine was 2.0% (n = 12). Interferences
Positive analytical errors are to be expected in the presence of free ammonia or ammonium salts, or compounds releasing ammonia under the conditions of the measurement, such as amides or nitriles. Such interferences can be overcome by running blanks from which periodate is omitted. Negative analytical errors are to be expected in the presence of compounds that react with ammonia, such as aldehydes. Negative analytical errors are also to be expected in the presence of compounds that consume periodate, such as vicinal glycols, polyhydroxy compounds and carbohydrates. Such interferences can be minimized by using as much periodate as possible. Again the problem is only partially solved because aldehydes released from the cleavage of these compounds may react with some of the ammonia. The interference, expressed in terms of analytical
error, caused by such compounds in the analysis of various a-aminohydroxy compounds at various concentration ratios is shown in Table 2. It can be seen from Table 2 that a-aminohydroxy compounds which are oxidized at 25” by periodate in alkaline (carbonate) solution can be determined even in the presence of a hundredfold excess of propylene glycol and glycerol, because these interferents react very slowly with periodate in alkaline solutions. On the other hand, glucose reacts quickly with periodate, reducing its oxidative capacity. Formaldehyde masks ammonia only when it is present in large excess. Determination of triethanolamine and glucosamine results in more intense interference because of the higher temperature used for their determinations. At high interferent concentrations, it was noticed that the ammonia was released only slowly for the easily oxidized compounds, probably owing to the reduced availability of periodate because of the equilibrium existing between the free anion and its complex compound with the polyhydroxy compound, favoured by the alkaline PH.~ CONCLUSIONS
The proposed method for the determination of a-aminohydroxy compounds is simple, fast, sensitive and selective enough for many possible applications. The accuracy attained is similar to that of other direct potentiometric measurements. This method comple-
Potentiometric
determination
ments other methods for the determination of Uaminohydroxy compounds and broadens the spectrum of possible applications of the ammonia gassensing electrode. A possible use for the method would be the determination of total serine and threonine in the hydrolysis products of peptides and proteins, and of ethanolamines in petroleum plant effluents and cutting fluids. The possibility of the determination of glucosamine in serum glycoproteins is under consideration.
REFERENCES
1. G. J. Moody and J. D. R. Thomas, in Ion-Selective Ekctrodes in Analytical Chemistry, H. Freiser (ed.), Vol. 1, pp. 374, 400. Plenum Press, New York, 1978.
of z-aminohydroxy
compounds
915
2. D. N. Gray, M. H. Keys and B. Watson, Anal. Chem., 1977, 49, 1067A. 3. D. P. Nikolelis, C. E. Efstathiou and T. P. Hadjiioannou, Analyst, 1979, 104, 1181. 4. G. Dryhurst, Periodate Oxidation of Dial and Other Functional Groups. pp. 8, 35, 36. Pergamon Press, Oxford, 1970. 5. P. Fleury, J. Courtois and M. Grandschamp, Bull. Sot. Chim. France, 1949, 88. 6. A. Besada and Y. A. Gawargious, Talanta, 1974, 21, 1247. 7. E. Sawicki and C. R. Engel, Chemist-Analyst, 1967, 56, 7. 8. C. F. Burmaster, J. Biol. Chem., 1946, 165, 1. 9. Ammonia Electrode instruction Manual, Form IM 95510/679, Orion Research Inc., Cambridge, Mass., 1976. 10. C. E. Efstathiou and T. P. Hadjiioannou, Anal. Chem., 1977, 49, 414.
~&mm, Vol. 29, pp. 911 to 915, 1982 Printedin Great Britain.All rightsreserved
POTENTIOMETRIC DETERMINATION OF CERTAIN a-AMINOHYDROXY COMPOUNDS BY USING AN AMMONIA GAS-SENSING ELECTRODE D. P. Laboratory
NIKOLELIS, C. E. EFSTATHIOU and T. P. HADJIIOANNOU
of Analytical
Chemistry. (Received
University
of Athens,
104 Solonos
St., Athens
144, Greece
14 April 1982. Accepted 7 June 1982)
Summary-A simple method is described for the determination of certain a-aminohydroxy compounds, based on the potentiometric measurement of ammonia released after oxidation with an excess of periodate. Ammonia is measured directly in the reaction mixture, with an ammonia gas-sensing electrode. Ethanolamine, diethanolamine, triethanolamine, serine, threonine, and glucosamine (0.3-6 pmole) can be determined with average errors of about l-2%.
gas-sensing electrode has been used extensively for the direct potentiometric determination of ammonia in a variety of matrices of clinical or environmental importance.’ It has also been used for the construction of bioselective sensors, such as enzyme electrodes, by taking advantage of the fact that ammonia is a product of biodegradation of many nitrogen-containing organic compounds.’ Ammonia is also obtained from the chemical degradation (e.g., hydrolysis) of many organic compounds, thus making possible their potentiometric determination. In a previous paper we described a simple method for assaying nicotinamide in multivitamin preparations by potentiometric measurement of ammonia released after alkaline hydrolysis of the sample.3 In this paper we describe a simple procedure for the determination of various a-aminohydroxy compounds, based on the determination of ammonia released after oxidation with an excess of periodate. It is known that periodate cleaves the carbon-carbon bond of a-aminohydroxy compounds smoothly.4 The general reaction scheme for the three a-amino alcohols, ethanolamine, diethanolamine and triethanolamine, is
The ammonia
(HOCH&HJ,NH,_, --+ 2nHCH0
+ nI04 + NH,
carbohydrates are present. The titrimetric determination of the ammonia released is not subject to these interferences but it involves tedious and time-consuming separation steps (e.g., by microdiffusion’). With the ammonia gas-sensing electrode, ammonia can be determined directly in the reaction mixture. No separation steps are necessary and the sensitivity of the analysis is greatly improved. To evaluate the method, it was applied to the determination of three ethanolamines, the a-hydroxyamino-acids serine and threonine, and the amino-sugar glucosamine. Microamounts of these compounds in the range 0.3-6 pmole were determined with average errors of about l-2%.
EXPERIMENTAL
Apparatus An Orion Model 95-10 ammonia gas-sensing electrode was used, and the potential measurements were made with an Orion Model 801 digital pH/mV meter. All measurements were made at 25°C k 0.1” in a lo-ml cell equipped with a magnetic stirrer. When not in use, the electrode was kept in O.OSM ammonium chloride.9 Reagents
+ nI0;
(n = l-3).
The optimum conditions (pH, temperature and reaction period) for completion of these reactions have been reported5 and various analytical schemes have been developed. Titrimetric methods are based on the iodometric determination of either the excess of periodate or the iodate produced, after masking of the excess of periodate with molybdate.6 Other schemes are based on the spectrophotometric measurement of the formaldehyde produced.’ These methods offer high accuracy and sensitivity but they lack selectivity when other compounds reacting with periodate, such as vicinal glycols, polyhydroxy compounds and
Analytical-grade materials and demineralized, distilled water were used throughout. Sodium metaperiodate, O.IOOM. Dissolve 21.4 g of reagent in water and dilute to 1 litre. Store in an amber bottle. Potassium carbonate, I.OOM solution. Sodium bicarbonate, 0.20M solution. Potassium hydroxide, 2.00M solution. cc-Aminohydroxy compounds. Prepare 0.0200M stock solutions of serine, threonine and glucosamine hydrochloride by dissolving the appropriate amount of each compound in water. Prepare 0.0200M stock solutions of ethanolamines by diluting 1M solutions, which have been titrated potentiometrically with standard I.OOOM hydrochloric acid solution. Prepare fresh standard solutions, 1.00 x 10m4M, 2.50 x 10m4M, 7.50 x 10e4M and 2.00 x 10m3M, as needed, by appropriate dilution of the stock solutions. 911
D. P.
912
bhKOLELIS
Procedures Monoethanolamine,
diethanolamine,
serine and threonine.
Pipette 4.00 ml of a 1:l mixture of O.lOOM sodium metaperiodate and l.OOM potassium carbonate (this mixture is stable for at least one day at room temperature) and 3.00 ml of the standard or unknown a-aminohydroxy compound solution into the measurement cell. Start the stirrer and read the potential when it has stabilized to within kO.1 mV (in about 334 min). Find the unknown concentration from a calibration graph of potential us. log [a-aminohydroxy compound]. Triethanolamine. Incubate the reaction mixture (prepared as before) in a lo-ml vial fitted with a well-fitting stopper, for 60 min, in a water-bath at 60”, cool to room temperature, transfer the contents of the vial into the measurement cell and measure as for monoethanolamine etc. Glucosamine.
Pipette 3.00 ml of a 2:l mixture of O.lOOM sodium metaperiodate and 0.20M sodium bicarbobonate (this mixture is stable for at least 15 min at room
temperature, but then disodium paraperiodate, NazHSI06, starts to precipitate) and 3.00 ml of the standard or unknown glucosamine solution into a IO-ml vial with a wellfitting stopper. Shake the vial and immerse it in a waterbath at 60” for 60 min. Cool the vial to room temperature, pipette 1.00 ml of 2.OOM potassium hydroxide into it, mix, and transfer the contents into the measurement cell. Measure as above. RESULTS Completion
Figure ammonia
I
of the
AND
DISCUSSION
reactions
1 shows recordings of the potential of the gas-sensing electrode during the course of
1
irthanolaminr
chloride
-I
min __c(
Time
-
Fig. 1. Records of the potential of the ammonia gas-sensing electrode during the reaction of periodate with a-aminohydroxy compounds. In a mixture of 4.00 ml of composite 0.05OOM periodate-0.500M carbonate solution and 3.00 ml of 6.67 x IO-‘M ammonium chloride, 100 ~1 of 0.0200M solution of the relevant a-aminohydroxy compound are injected. Ammonium chloride is also injected for comparison. 1: a-aminohvdroxy compound (or ammonium chloride) solution injection..Concentrations at the start of the reaction: NaIO,, 0.0282M; KXO,, 0.282M; NH,Cl, 2.82 x IOm5M (added in all cases -to keep the initial potential stable); a-aminohydroxy compound (or NH,Cl), 2.82 x 10m4M. Temperature: 25°C.
et al
the reaction of a-aminohydroxy compounds with periodate in alkaline (carbonate) solution. Since gassensing electrodes are slow-response concentration transducers, for comparison of speed of response an increase of ammonia concentration by addition of ammonium chloride has been included. The latter was added in an amount equivalent to that of the a-aminohydroxy compound. It has been reported that the ammonia electrode responds to lower organic amines.’ It can be seen from Fig. 1 that there is no direct response of the electrode to x-aminohydroxy compounds. This is probably because the hydroxyl groups make these compounds more hydrophilic and therefore less able to diffuse through the hydrophobic membrane of the ammonia gas-sensing electrode. Ethanolamine, diethanolamine, serine and threonine react quantitatively and almost instantaneously with periodate, and weakly alkaline (bicarbonate) media have been recommended4x6 for making the reaction quantitative. However, the reactions also proceed quickly in more alkaline (carbonate) solutions, and then practically all the ammonia released is in molecular form and thus is measured quantitatively with the ammonia gas-sensing electrode. On the other hand, triethanolamine and glucosamine react very slowly and incompletely. In fact, under all conditions tested (e.g., bicarbonate solutions, phosphate buffers of pH 7-9, carbonate and borate buffers of pH 9-10, use of Mn’+ as catalyst plus nitrilotriacetic acid as activator,” at temperatures from 25” to 75”) the ammonia released never exceeded 90% of the theoretical amount. These findings contradict those of other workers’ who claim complete oxidation, at least for triethanolamine. It is possible that the reaction is stoichiometric with regard to the periodate consumed or iodate produced per molecule of triethanolamine but that some of the ammonia condenses with the formaldehyde formed as one of the reaction products. Attempts to release ammonia quantitatively by addition of glycine as a scavenger for formaldehyde failed, because glycine is oxidized by periodate at elevated temperature and releases large amounts of ammonia. Plots of the percentage of ammonia released from triethanolamine and glucosamine us. reaction period, under a variety of conditions, are shown in Fig. 2. The ammonia released from triethanolamine and glucosamine was 80 and 88% of the theoretical amount, respectively, when the reaction with periodate took place in alkaline (carbonate) and weakly alkaline (bicarbonate) solutions respectively, at 60” for 60 min. Analytical
results
The potential of the ammonia gas-sensing electrode was found to be linearly dependent on the logarithm of the concentration of the a-aminohydroxy compound, according to the general equation. E =
constant
- S log[a-aminohydroxy
compound]
Potentiometric
determination
of a-aminohydroxy
100
I 0
I 60
I
30
I
120
Time,
c
GLUCOSAMI
I
I
90
913
compounds
I 30
0
I 60
Table
1. Results for the potentiometric Concentration,
min
Ethanolamine
1.00 2.50 7.50 20.0
1.02 2.43 7.32 20.4
Diethanolamine
1.00 2.50 7.50 20.0
0.99 2.39 7.56 20.2
Triethanolamine
1.00 2.50 7.50 20.0
1.04 2.52 7.41 20.4
Serine
1.00 2.50 7.50 20.0
2.42 7.62 20.2
1.00 2.50 7.50 20.0
1.00 2.54 7.55 19.9
1.00 2.50 7.50 20.0
1.02 2.43 7.65 20.2
*The lower concentration standard the calculation of the regression
of a-aminohydroxy
compounds
10m4M Found
1.02
and in A as C
olamine, diethanolamine and a-hydroxyamino-acids, obtained by plotting potential us. log[cc-aminohydroxy compound], practically coincide with a working curve obtained with standard solutions of ammonium chloride. Therefore, for these compounds the
determination
Taken
Compound
I 120
I
90
Fig. 2. Rate of release of ammonia for triethanolamine and glucosamine. Initial concentrations temperatures: A-NaIO,, O.O286M, K,C03 0.286M; triethanolamine, 3.21 x 10-4M; 25°C. B-as but at 60°C. C-NaIO,, 0.0286M; KzC03, 0.286M; glucosamine, 3.21 x 10m4M; 25°C. D-same but at 60°C. E-NaIO,, 0.0333M; NaHCO,, 0.0333M; glucosamine, 3.75 x 10e4M; 60°C. for the concentration range 1 x 10m4-2 x 10A3M. The working range can be extended to higher concentrations provided that enough periodate is added to oxidize the compound to be determined plus any other oxidizable compound. Working curves for ethan-
NE
Error,
%
+ 2.0 -2.8 -2.4 + 2.0 Av. 2.3 -1.0 - 4.4 +0.8 +1.0 Av. 1.8 f4.0 + 0.8 - 1.2 + 2.0 Av. 2.0 + 2.0 -3.2 + 1.6 + 1.0 Av. 2.0 + 1.6 +0.7 -0.5 Av. 0.7 + 2.0 -2.8 + 2.0 + 1.0 Av. 2.0
deviates from linearity equation.
Regression
equation
E = - 156.6 - 54.3 log C, r = -0.9993
E = - 167.8 - 55.9 log C, r = -0.9996
E = - 161.9 - 57.8 log C, r = -0.99948
E = - 172.0 - 57.8 log C, r = -0.99995
E = - 176.0 - 58.2 log C, r = -0.99994
E = - 155.3 - 53.4 log c, r = -0.9992
and has not been included
in
914
D. P. NIKOLELISet al. Table 2. Effect of various compounds on the determination hydroxy compounds Compound (5.00 x lo-‘%) Ethanolamine
Interferent Propylene glycol Glucose
Formaldehyde
Diethanolamine
Glycerol Formaldehyde
Triethanolamine
Glycerol
Glucosamine
Glucose
[Interferent] [Compound] 30/l loo/l 300/1* 3/l 10/l 30/l loo/l* 30/l 100/l 300/l 30/l 100/l 300/l? 30/l 100/l 300/1* 10/l 30/l 100/l 300/l 311 10/l 30/l 100/l
of cr-amino-
Error, % +0.4 - 0.4 - 12.0 -4.5 -5.5 - 10.4 -43.0 -2.8 - 7.4 -33.5 -0.8 - 2.0 -69.6 -1.6 - 5.9 - 36.9 - 1.6 -3.2 - 13.0 - 14.7 -3.8 -9.8 -32.4 - 46.2
*At these interference levels it took about 10 min to obtain a steady potential indication. tAt this interference level the potential was not stabilized even after 10 min, drifting continuously toward smaller values (smaller error). working curve can be based on standard ammonium chloride solutions. The working curves for triethanolamine and glucosamine were shifted slightly toward more positive potentials because of the incomplete release of ammonia. Table 1 gives typical analytical results for the determination of six a-aminohydroxy compounds, together with the regression equations of the corresponding working curves. The relative standard deviation for 5 x 10m4M ethanolamine was 2.0% (n = 12). Interferences
Positive analytical errors are to be expected in the presence of free ammonia or ammonium salts, or compounds releasing ammonia under the conditions of the measurement, such as amides or nitriles. Such interferences can be overcome by running blanks from which periodate is omitted. Negative analytical errors are to be expected in the presence of compounds that react with ammonia, such as aldehydes. Negative analytical errors are also to be expected in the presence of compounds that consume periodate, such as vicinal glycols, polyhydroxy compounds and carbohydrates. Such interferences can be minimized by using as much periodate as possible. Again the problem is only partially solved because aldehydes released from the cleavage of these compounds may react with some of the ammonia. The interference, expressed in terms of analytical
error, caused by such compounds in the analysis of various a-aminohydroxy compounds at various concentration ratios is shown in Table 2. It can be seen from Table 2 that a-aminohydroxy compounds which are oxidized at 25” by periodate in alkaline (carbonate) solution can be determined even in the presence of a hundredfold excess of propylene glycol and glycerol, because these interferents react very slowly with periodate in alkaline solutions. On the other hand, glucose reacts quickly with periodate, reducing its oxidative capacity. Formaldehyde masks ammonia only when it is present in large excess. Determination of triethanolamine and glucosamine results in more intense interference because of the higher temperature used for their determinations. At high interferent concentrations, it was noticed that the ammonia was released only slowly for the easily oxidized compounds, probably owing to the reduced availability of periodate because of the equilibrium existing between the free anion and its complex compound with the polyhydroxy compound, favoured by the alkaline PH.~ CONCLUSIONS
The proposed method for the determination of a-aminohydroxy compounds is simple, fast, sensitive and selective enough for many possible applications. The accuracy attained is similar to that of other direct potentiometric measurements. This method comple-
Potentiometric
determination
ments other methods for the determination of Uaminohydroxy compounds and broadens the spectrum of possible applications of the ammonia gassensing electrode. A possible use for the method would be the determination of total serine and threonine in the hydrolysis products of peptides and proteins, and of ethanolamines in petroleum plant effluents and cutting fluids. The possibility of the determination of glucosamine in serum glycoproteins is under consideration.
REFERENCES
1. G. J. Moody and J. D. R. Thomas, in Ion-Selective Ekctrodes in Analytical Chemistry, H. Freiser (ed.), Vol. 1, pp. 374, 400. Plenum Press, New York, 1978.
of z-aminohydroxy
compounds
915
2. D. N. Gray, M. H. Keys and B. Watson, Anal. Chem., 1977, 49, 1067A. 3. D. P. Nikolelis, C. E. Efstathiou and T. P. Hadjiioannou, Analyst, 1979, 104, 1181. 4. G. Dryhurst, Periodate Oxidation of Dial and Other Functional Groups. pp. 8, 35, 36. Pergamon Press, Oxford, 1970. 5. P. Fleury, J. Courtois and M. Grandschamp, Bull. Sot. Chim. France, 1949, 88. 6. A. Besada and Y. A. Gawargious, Talanta, 1974, 21, 1247. 7. E. Sawicki and C. R. Engel, Chemist-Analyst, 1967, 56, 7. 8. C. F. Burmaster, J. Biol. Chem., 1946, 165, 1. 9. Ammonia Electrode instruction Manual, Form IM 95510/679, Orion Research Inc., Cambridge, Mass., 1976. 10. C. E. Efstathiou and T. P. Hadjiioannou, Anal. Chem., 1977, 49, 414.