1922 Anal. Chem. 1983, 55, 1922-1924

(25) Nishlkido, N.; Matuura, R. Bull. Chem. SOC. Jpn. 1977, 50, (30) Heckley, P. R.; Holah, D. G.; Hughes, A. N.; Leh, F. Can. J . Chem.

(26) Aspila, K. 1.; Sastri, V. S.; Chakrabarti, C. L. Talanta 1969, 76, (31) Joris, 647-651. S. J.; Aspila, K. I.; Chakrabarti, C. L. Anal. Chem. 1970, 42,

(27) Joris, S. J.; Aspiia, K. I.; Chakrabartl, C. L. J . fhys. Chem. 1970, 74, (32) Mukerjee, P.; Cardinal, J. R. J. fhys . Chem. 1978, 82, 1620-1627.

(28) Asplla, K. 1.; Chakrabarti, C. L.; Sastri, V. S. Anal. Chem. 1973, 45,

1970, 48, 3827-3830. 1690- 1694.

1099-1102.

660-865.

363-367. (29) Aspila, K. I.; Chakrabarti, C. L.; Sastri, V. S. Anal. Chem. 1975, 47 ,

945-946. 1983. RECEIVED for review January 25, 1983. Accepted June 27,

Determination of Dissolved Organic Carbon in Concentrated Brine Solutions

Philip Hamaker*

Department of Geology, School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia 3052

Alan S. Buchanan C.R.A. Technology, 55 Collins Street, Melbourne, Victoria, Australia 3001

An absolute method Is reported for the determlnatlon of sol- uble organic carbon In concentrated brine solutlons. Wet oxidation wlth K,S208 Is used In a sealed ampule at 130 "C, followed by hot CuO treatment of the gas stream, to fully oxldize organic species to CO,. The COP Is measured gra- vimetrically after gas purlflcation. Results are presented for a wlde range of soluble organlc specles, both wlth and without NaCl present. This procedure now allows for the accurate determlnatlon of organlc carbon In brines over a range from about 5 ppm to values In excess of 1000 ppm. The technique overcomes the dlfflcultles of callbration curvature, catalytic ciogglng, and instrumental fogging, often encountered In modern Instrumental methods, when applied to concentrated brlne solutions.

The development of an accurate analytical technique for the determination of the dissolved organic content of water samples is of importance in elucidating the processes occurring in natural and industrial water systems. The extent of pho- tosynthesis and the fate of photosynthetic products are of particular significance in many situations.

The soluble organic content of brines is a problem in salt production, because in many solar salt fields whether based upon evaporation of seawater or underground brine, sufficient organic matter is often present to produce quality control problems. Organic matter is responsible for the discoloration of the salt crystals resulting in a vast color range under the present manufacturing procedures. Concentrated saline so- lutions, if sufficient phosphate is present, can support algal growth and in particular the alga Dunaliella salina is a very frequent inhabitant of brines exposed to sunlight. Decom- position products of this and other algae may be colored and become absorbed by the growing salt.

Brine solutions near or a t saturation with NaCl present a very difficult problem to the analytical chemist. Apart from high concentrations of Na, these solutions generally contain substantial amounts of Mg, K, and Ca, with a vast array of trace elements in a chloride matrix. This paper investigates the possibility of reliably determining the soluble organic

carbon content of such brine solutions. Several papers have been published on the determination

of the total organic content of low salinity water (1-3). Generally the methods involve injection of the sample into a high-temperature furnace containing CuO so that the organic constituents are converted to COz. The COz is then reduced to CH, which is measured with a flame ionization detector.

Eggertsen and Stross (4) have measured organic compounds in low salinity water by heating a sample in a stream of ni- trogen and passing it through a flame-ionization detector. The sample is first heated to 150 "C and then to 500 "C so that the volatile and nonvolatile compounds are distinguished.

Van Hall and Stenger ( 5 ) inject 20 L of sample into a high-temperature furnace containing a catalyst to promote oxidation of carbon compounds to CO, which is then passed into a nondispersive infrared (NDIR) analyzer. The carbonate interference can be determined by passing an acidified portion of the sample through a low-temperature furnace (6-8).

One of the best known commercial instruments developed for organic carbon determinations is the Beckman total carbon analyzer which utilzes an analysis scheme developed by Van Hall et al. (9). Another instrument developed by the Precision Scientific Co. was based upon the work of Stenger and Van Hall (IO).

The techniques mentioned above have been developed for the analysis of natural waters and waste industrial waters of relatively low salinity. Experience has shown, however, that application to concentrated or saturated brine solutions leads to erratic and unreliable results. There are several possible reasons for this: (a) the catalyst will rapidly become loaded with NaCl, (b) oxidation of C1- to C1, will occur, (c) volatile organics may not all be trapped by the solid catalyst.

Van Hall e t al. (9) have also pointed out that strong brines interfere with the method by producing "fogs" which may be counted as COz, while in cases where the flame ionization detector is being used, large spikes appear in the recorded curve ( 4 ) .

Low volatility natural organic material such as poly- saccharides and higher molecular weight proteins sometimes produced low results. Some of these problems can be over- come by using a solution-phase oxidant and enclosing the

0003-2700/83/0355-1922$01.50/0 0 1983 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983 192:)

U

Figure 1. Arrangement of analytical equipment. The items are expanded in the text.

system in a sealed tube. In this way all of the constituents are fully contained and eKposed to oxidation and, moreover, oxidation of the organic matter to C 0 2 is complete for the greater majority of compounds.

Various methods for the wet oxidation of organic carbon have been published (11, 12). The method of Menzel and Vaccaro has been useful for determining the dissolved organic content of seawater samples and the oxidation has been shown to be essentailly complete (13). Seawater is first freed of inorganic carbon by treatment with a small volume of 3% H3P04 and the organic carbon is then oxidized in sealed glass ampules in an autoclave a t 130 "C using K&08 as an oxidant. The resulting COP is passed through a NDIR analyzer whose signals are related to milligrams of C in the sample.

EXPERIMENTAL SECTION In the present study the procedure followed by Menxel and

Vaccaro (12) was considered to be a possible solution for the determination of the organic composition of the more difficult to analyze brine solutions. However the IR method used to measure the evolved COS from the ampule was replaced by a Carbosorb absorption tube. The use of absorption tubes con- taining Carbosorb or soda asbestos to collect C02 has found wide application for the determination of C as C02 in geological samples such as rocks and sediments (14, 15).

This modification was considered desirable for the following reasons: (a) the possible high organic concentrations of brine solutions might be beyond the linearity range of the instrumental detectors, (b) the organic levels will be very variable and Carbosorb absorption has a capacity to deal with wide ranges, (c) other instrumental methods require the accurate measurement of peak height or peak area.

An initial series of experiments was performed by using the procedure followed by Menzel and Vaccaro with the exception that the evolved C 0 2 was determined by absorption. Low values were obtained for distilled water and for brine solutions spiked with acetic acid. In brine solutions the results were found to be in error by up to 60%.

The use of a NDIR analyzer instead of an absorption tube assumes that all the carbon is oxidized to CO,; if however, other carbon species are being produced, errors can be expected, de- pendent upon the ratio of the species produced.

Gordon (16) pointed out that the oxidation products produced in the case of acetic acid are dependent upon the concentrations of persulfate, acctic acid, the solution composition, and acidity. Therefore, the analysis of brine solutions will present difficultnes due to the high solid loading and variable composition.

Further modificatione to the procedure were found to be necessary when low results were also obtained for organic species such as glycine and oxalic acid, possibly lost during C02 removal in acid conditions.

Equipment and Technique. The experimental technique reported here is based on the Menzel and Vaccaro procedure with the following differences. (1) Purified O2 was used as the carrier and flushing gas rather than N2 to maintain a highly oxidizing environmental at all times. (2) An additional purification bubbler bottle containing AgN03 was inserted after the KI bubbler bottle to provide an indication of the efficiency of KI in trapping C1,

and also to trap any 13- carried with the O2 supply in a fine spray. (3) A silica tube of CuO maintained at a temperature of 600 "C was added to ensure complete conversion of carbon species to C02, thus eliminating possible errors due to the formation of CHI. (41) An absorption tube containing Carbosorb was used rather than a NDIR detector to enable the direct measurement of COz over a vast concentration range. (5) The concentration of KzS208 was increased by a factor of 6, to ensure ample oxidizing power in thle presence of higher concentrations of Cl- and organic species. (6) The carrier gas flow was reduced to 0.5 L/min to ensure complete purification of the gas stream and capture of the resulting COlz. (7 ) Measurement was conducted over a 20-min period to ensuire complete C02 movement through the equipment.

Apparatus Arrangement. The equipment was arranged as shown in Figure 1. The apparatus allows finely regulated O2 ('4, B, C) to be purified by passing it through a tube of Carbosorb and Mg(C104), (D). The gas is then passed through a hypodermic needle (E) connected to a glass ampule (F), using silicone tubing for all connections. The needle is inserted through silicone tubing and should reach to the bottom of the ampule. The silicone tubing should be clear to allow for visible manipulation of the needle. The silicone tubing is connected to a KI/H2S04 scrubber (30 g of KI dissolved in 75 cm3 of 10% (v/v) H2S04) to complex any C1, produced by the wet oxidation of the brine sample. This scrubber bottle (G) is connected to a AgN03/HN03 (0.1 g of AgNO, dissolved in 75 cm3 of 0.1 M HN03) scrubber bottle (13) to trap any 1,- carried over from (G) and to prevent clogging of the Mg(C104)2 water removal bottle (I). The drying bottle (I) is connected to a silica glass tube containing CuO, maintained at a temperature of 600 "C (J), to ensure conversion of carblon products to C02. This tube is connected to the Carbosorb/M[g- (C104)2 absorption tube (K) inserted to trap the resulting C02 produced from the oxidation procedure. The system is sealed from the atmosphere by allowing the exit gas to bubble through concentrated H2S04 (L).

Procedure. Part 1. Initial Treatment. (i) Particulate matter is removed from a fresh brine sample by use of a high speed centrifuge or an organic-free filter. (ii) A 6-cm3 sample is injected into a tared ampule (approximately 12 cm3 capacity) and 0.2 cm3 of 3% H3P04 is added. (iii) O2 is bubbled through the solution for 2 to 3 min to remove C02 from carbonates. (iv) A 0.6-g portion of solid K2S208 is added and the inside of the ampule is flushed with OF (v) Silicone lpease is placed over the end of the ampule, and the ampule is sealed in an 02-gas flame, taking care not to trap carbon from the flame. (vi) The sample is autoclaved at 1.30 "C for 30 min.

Part 2. Measurement. (i) The neck of the ampule is placed in the clear silicone tubing below the needle and the system purged with 0 2 for 5 min. (ii) The Al-covered absorption tube is removed, weighed after 5 min, and then replaced. (iii) The O2 flow is decreased to 0.5 L/min,the ampule tip crushed, and the needle inserted in the solution for 20 min. (iv) The absorption tube is wrapped in A1 foil to minimize charge effects and weighed after 5 min.

RESULTS AND DISCUSSION By use of the procedure outlined above with the equipent

assembled as shown in Figure 1, a series of organic species were measured a t various concentration levels. From the results

1924 ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983

Table I. Analysis of a Series of Organic Species at Various Concentration Levels, Both with and without NaCl Present

no. of range NaCl Sam- %

tested, range, ples recovery organic compd tested ppm ppm tested av

mannitol 54-432 0-300 10 98.4 acetic acid 30-720 0-300 11 97.9 D-tartaric acid 76-604 0-300 9 102.7 oxalic acid 57-1140 0-300 7 100.9 malic acid 36-716 0-300 9 99.0 propan-2-01 45-180 0-300 4 94.4 acetylacetone 232-465 0-300 4 99.8 ethanol 208-417 0-300 4 98.1 glycine 10-382 0-300 10 95.5 glycerol 10-392 0-300 9 98.7

salicylic acid 42-406 0-300 9 98.3 trisodium citrate 123-490 0-300 7 96.0

Table 11. Solar Salt Field Illustrating the Possible High Results due to Soluble Organic Species from the Untreated Filter Paper

Analysis of Natural Brine Solutions from a

soluble organic carbon, ppm sample unfiltered filtered

pond A pond B pond C pond D pond E pond F pond G pond H pond I seawater inlet

26 40 46 47 59 44 66 70

8 23

29 42 49 49 50 48 67 78 11 26

presented in Table I, it can be seen that very close agreement was obtained for a variety of organic compounds both with and without NaCl present.

Table I1 presents data for a number of natural brine solu- tions measured by using the same apparatus arrangement.

Each sample was centrifuged and split into two. The first portion was analyzed without filtration, the second with fil- tration. The results show an approximately constant higher value due to contamination from the filtration medium in- dicating that centrifrugal removal only of particulates is de- sirable. Also, in the determination of urea it has been reported that it is inadvisable to filter the samples since many filters contain urea (17).

Because of the nature of the brine solution, the use of spikes of organic standards added to the brine may be unreliable if the brine is kept for any appreciable period before analysis. This is due to the bacterial conversion of the added material

Samples with phosphorus present provide a nutrient source for microorganisms in solution. Therefore, if analysis cannot be completed a t once, brine samples should be stored in the dark. Strickland and Parsons (I&?), however, have suggested that quick deep-freezing stabilizes samples for many months.

Table I11 provides measurements of the effect of storage on the soluble organic material in the brines used in the present study. I t is evident that for these two samples mi-

to coz.

Table 111. Material in Solution

The Effect of Storage on the Soluble Organic

soluble organic carbon (ppm) on 4 aliquots month of

sample analysis high low average

brine 1 February April June August October

April June August October

brine 2 February

133 123 131 121 126 124 128 122 130 120

94 86 90 84 98 89 89 85 90 84

128 125 126 124 123 90 88 9 1 87 88

croorganism activity is not affecting the results to any marked degree over a period of 9 months. The samples were stored in brown glass bottles in a dark cupboard and their phosphorus content was determined as <0.4 ppm by ICP-AES analysis.

The wet oxidation analysis of concentrated brine solutions for soluble organic carbon, described in this paper, has proved to produce reliable results in contrast to other established procedures. Care in storage and pretreatment is necessary when there are high levels of phosphorus, particulate matter, or microorganism activity.

Detection limits are dependent upon the volume of sample measured and the accuracy of the balance used. With a five or six decimal place balance, levels down to approximately 5 ppm may be determined provided great care is used in the weighing procedure. Improved accuracy a t lower levels may be obtained by increasing the volume of brine measured and the quantity of KzSzOs used.

ACKNOWLEDGMENT The work reported here was carried out in the Department

Registry No. Carbon, 7440-44-0; water, 7732-18-5.

LITERATURE CITED

of Geology, University of Melbourne.

(1) Dobbs, R. A,; Wise, R. H.; Dean, R. B. Anal. Chem. 1967, 39, 1255. (2) Cropper, F. R.; Heinekey, D. M.; Westwell, A. Analyst (London) 1987,

92, 436 and 443. (3) Croll, B. T. Chem. Ind. (London) 1972, 386. (4) Eggertsen, F. T.; Stross, F. H. Anal. Chem. 1972, 44, 709. (5) Van Hall, C. E.; Stenger, V. A. Anal. Chem. 1967, 39, 503. (6 ) Golterman, H. L., Ed. "Methods for Chemical Analysis of Freshwater";

Blackwell: Oxford, 1969; pp 133-143. (7) "Water Pollution Research Laboratory, Notes on Water Pollutlon, No.

59"; H.M.S.O.: London, 1972. (8) Hill, H. N. Instrum. Techno/. 1969, 16, 33. (9) Van Hall, C. E.; Safranko, J.; Stenger, V. A. Anal. Cbem. 1983, 35,

- A - J 13. Stenger, V.; Van Hall, C. E. Anal. Chem. 1989, 39, 206. Armstrong, F. A. J.; Wllliams, P. M.; Strickland, J. D. H. Nature (Lon- don) 1968, 277 (5048), 481-483. Menzel, D. W.; Vaccaro, R. F. Limnol. Oceanogr. 1964, 9, 138-142. Williams, P. M. Limnol. Oceanogr. 1969, 74, 297-298. Hughes, T. C.; Hannaker, P. Chem. Geol. 1978, 22, 331-339. Riley, J. P. Analyst (London) 1958, 83, 42-49. Gordon, M. A. J. Phys. Chem. 1914, 78, 55-66. Grasshoff, K., "Methods of Seawater Analysis"; Verlag Chemie: We- inheim, 1976; p 146. Strickland, J. D. H.; Parsons, T. R. Bull., Fish. Res. Board Can. 1988, No. 767.

RECEIVED for review November, 30, 1982. Accepted May 19, 1983.