TOC oxidation studies

To evaluate the TOC content in aqueous samples by a TOC analyzer, as far as the efficiency of the UV-persulphate oxidation method is concerned, a number of different aqueous organic solutions, with a known concentration were prepared and analyzed.
To reduce accidental interference due to organic impurities in the deionized water used and errors due to dilution or sample preparation, it was advisable to carry out the test using concentrated solutions, as 100 mg/l of C as TOC.
Preparation of the samples was made with a first solution at 1000 mg/L of TOC as "C" and then diluted to 100 mg/l C.
For each concentration three TOC measurements were performed and the data shown in the following table is the average of those results.
Note: The oxidation results of this experiment were made under normal working laboratory conditions.

We have taken all precautions to perform an accurate test , but those tests are not intended to be considered official research or evaluation of the performance of the analyzer used.
Their intent is only to give a general indication about the possible oxidation with UV-persulphate of several families of organic compounds.

For these initial studies we tested:

 # 1) LINEAR PRIMARY AMINES General Formula: R-NH2

# 2) LINEAR MONOCARBOXYLIC ACIDS General Formula: R-COOH

# 3) LINEAR PRIMARY ALCOHOL General Formula: R-OH

# 4) LINEAR KETONES General Formula: R-CO-R’

# 5) PHENOL, POLYHYDRIC PHENOLS, NITROPHENOLS, NAPHTHOLS.

# 6) PYRIDINE AND PICOLINES

#1) LINEAR PRIMARY AMINES General Formula: R-NH2

Since Linear Primary Amines are liquid and the amount used for the preparation of solutions is very low, it is necessary each time to evaluate the exact concentrations of those samples on the basis of real weighed amounts.

name

structural formula

   #of "C"

MW

theoretic TOC

observed TOC

% recovery
Ethyl amine CH3CH2NH2 2 45.08 99 == ==
n-Propylamine CH3CH2CH2NH2 3   105 99.5 95%
n-Butylamine CH3(CH2)3NH2 4 73.14 108 94.4 87%
n-Amylamine CH3(CH2)4NH2 5 87.17 138 125.4 91%
n-Heptylamine CH3(CH2)6NH2 7 115.2 108 99.7 92%

Amines containing more than 2 "C" atoms have a recovery percentage of between 87% and 95% of the theoretic value, while ethyl amine had a very long reaction time and the measured values have been considered not to be dependable. Amines are rather difficult to oxidize, especially as regards ethyl amine.

# 2) LINEAR MONOCARBOXYLIC ACIDS General Formula: R-COOH

Since Linear Monocarboxylic Acids are liquid and the amount used for the preparation of solutions is very low, it is necessary to evaluate each time the exact concentrations of those samples on the basis of real weighed amounts. There have been analyzed solutions of linear monocarboxylic acids containing a number of carbon atoms ranging from 1 to 10.

name

structural formula

#of "C"

MW

 theoretic
TOC 		observed
TOC

% recovery

  Formic Acid

HCOOH 1 46.03 100 95.0 95%
Acetic Acid CH3COOH 2 60.05 106 99.2 93%
Propionic Acid CH3CH2COOH 3 74.08 98 96.9 99%
n-Butyric Acid CH3(CH2)2COOH 4 88.12 100 97.6 98%
Valeric Acid CH3(CH3)3COOH 5

102.1

 125 117.3 94%
Caproic Acid CH3(CH2)4COOH 6 116.2 104 104.1 100%
Caprylic Acid CH3(CH2)6COOH 8 144.2 101 90.0 89%
Pelargonic Acid CH3(CH2)7COOH 9 158.2 116 105.1 91%
Capric Acid CH3(CH2)8COOH 10 172.3 115 106.2 92%

Values of relative oxidability vary between 89% and 100% of theoretic value. In the case of samples containing acids with more than 7-8 carbonic atoms, there is low solubility in water. This leads to a partial separation of the two phases, so the sample needs a good homogenization before the analysis takes place. Monocarboxylic acids are rapidly and accurately oxidized, producing CO2 in short times as the result of the demolition of acids in solution.

# 3) LINEAR PRIMARY ALCOHOL General Formula: R-OH

Since Linear Primary Alcohols are liquid and the amount used for the preparation of solutions is very low, it is necessary to evaluate each time the exact concentrations of those samples on the basis of real weighed amounts. There have been chosen the first six terms of the homologous series.

 

name

structural formula

   #of "C"

MW

theoretic TOC

observed TOC

% recovery
Methyl Alcohol  CH3OH 1 32.04 105 95.9 91%
Ethanol CH3CH2OH 2 46.0 103 91.3 89%
n-Propyl Alcohol CH3(CH2)2OH 3 60.10 107

92.4

86%
1-Butanol CH3(CH2)3OH 4 74.12 95 77.6 82%
1-Amyl Alcohol CH3(CH2)4OH 5 88.15 144 112.9 78%
n-Hexyl Alcohol CH3(CH2)5OH 6 102.2 109 74.7 68%

Relative oxidability values for each solution follow a regular and characteristic decreasing trend as the molecular weight increases, going from 91% for methyl alcohol to 68% for n-hexyl alcohol. These values are rather low, probably because of high volatility that may cause volatile organic losses during the inorganic carbon stripping.

# 4) LINEAR KETONES    General Formula: R-CO-R’

Since Linear Ketones are liquid and the amount used for the preparation of solutions is very low, it is necessary to evaluate each time the exact concentrations of those samples on the basis of real amounts weighed. The considered ketones have a number of carbonic atoms ranging from 3 to 7.

name

structural formula

   #of "C"

MW

theoretic TOC

observed TOC

% recovery
Acetone CH3COCH3 3 58.08 100 48.1 48%
Methyl Ethyl Ketone CH3COCH2CH3 4 72.11 105 41.3 39%
2-Pentanone CH3CO(CH2)2CH3 5 86.14 97 25.8   27%
Methyl-n-Amyl Ketone CH3CO(CH2)4CH3 7 114.2 104 5 5%

As shown in the table, recovery values are very low and decrease as the molecular weight increases. For methyl-n-amyl ketone the recovery is even negligible. Under these particular conditions, the oxidation of ketones does not reach sufficient efficiency levels , thus the result of the analysis can't be considered satisfactory. An explanation of such results may be the volatility of low molecular weight compounds as well as the low solubility of high molecular weight compounds.

# 5) PHENOL, POLYHYDRIC PHENOLS, NITROPHENOLS, NAPHTHOLS.

name

structural formula

   #of "C"

MW

theoretic TOC

observed TOC

% recovery
Phenol C6H5OH 6 94.11   106 102.8 97%
Hydroquinone C6H6O2 6 110.1   101 93.3 92%
Resorcinol C6H6O2 6 110.1 98 95.4 97%
Fluoroglucineo C6H6O3 6 126.1 98 74.2 76%
Nitrophenol C6H5N03 6 139.1 101 88.0 87%
p-Nitrophenol C6H5NO3 6 139.1 102 98.5 96%
2,4-Dinitrophenol C6H4N2O5 6 184.1 101 101.5 100%
2,5-Dinitrophenol C6H4N2O5 6 184.1 100 92.4 92%
2,6-Dinitrophenol C6H4N2O5 6 184.1 101 99.7 99%
Picric Acid C6H3N3O7 6 229.1 103 102.7 100%
1-Naphthol  C10H8O 10   144.2 101 = = = =
2-Naphthol C10H8O 10 144.2 100 100.4 100%

These are all aromatic substances; the process of oxidative demolition regarding these substances is much more complex than progressive oxidation occurring on carbon atoms present in aliphatic chains. Moreover in aromatic substances there is also present the photochemical effect of UV radiation (187 nm and 254 nm).

As shown in the table, there are four cases in which the recovery is total: 2-4 dinitrophenol, 2-6 dinitrophenol, picric acid, 2-naphthol. This is due to high instability towards oxidant agents, so they can be easily broken by sodium persulphate and totally revealed by the analyzer.

#6 ) PYRIDINE AND PICOLINES

Since Pyridine and Picolines are liquid and the amount used for the preparation of the solutions is very low, it is necessary to evaluate each time the exact concentrations of those samples on the basis of the real weighed amounts.

 

name

structural formula

   #of "C"

MW

theoretic TOC

observed TOC

% recovery
Pyridine C5H5N 5 79.10 106 107.0 100%
2-Picoline C6H7N 6 93.13 98 % 99.2 100%
3-Picoline C6H7N 6 93.13 103 104.9 100%
4-Picoline C6H7N 6 93.13 104 105.4 100%

In all cases recovery reaches 100%.

 

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