1922
Anal. Chem. 1983, 55, 1922-1924
(25) Nishlkido, N.; Matuura, R. Bull. Chem. SOC. Jpn. 1977, 5 0 , 1690- 1694. (26) Aspila, K. 1.; Sastri, V. S.; Chakrabarti, C. L. Talanta 1969, 76, 1099-1102. (27) Joris, S. J.; Aspiia, K. I.; Chakrabartl, C. L. J . fhys. Chem. 1970, 74, 660-865. (28) Asplla, K. 1.; Chakrabarti, C. L.; Sastri, V. S. Anal. Chem. 1973, 45, 363-367. (29) Aspila, K. I.; Chakrabarti, C. L.; Sastri, V. S.Anal. Chem. 1975, 4 7 , 945-946.
(30) Heckley, P. R.; Holah, D. G.; Hughes, A. N.; Leh, F. Can. J . Chem. 1970, 4 8 , 3827-3830. (31) Joris, 647-651. S.J.; Aspila, K. I.; Chakrabarti, C. L. Anal. Chem. 1970, 4 2 , (32) Mukerjee, P.; Cardinal, J. R . J. f h y s . Chem. 1978, 82, 1620-1627.
RECEIVED for review January
25, 1983. Accepted June 27,
1983.
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 soluble 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 gravimetrically 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 photosynthesis 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 solutions, 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. Decomposition 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 nitrogen and passing it through a flame-ionizationdetector. 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 et 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 polysaccharides and higher molecular weight proteins sometimes produced low results. Some of these problems can be overcome by using a solution-phase oxidant and enclosing the
0003-2700/83/0355-1922$01.50/00 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% H3P04and 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 COPis passed through a NDIR analyzer whose signals are related t o 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 containing Carbosorb or soda asbestos to collect C02has found wide application for the determination of C as C02in 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, dependent 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 t o 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 C02removal 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 O2was 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 O2supply 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 KzS208was 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 t o a KI/H2S04scrubber (30 g of KI dissolved in 75 cm3 of 10% (v/v) H2S04)t o 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 cm3of 0.1 M HN03) scrubber bottle (13) to trap any 1,- carried over from (G) and to prevent clogging of the Mg(C104)2water 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)2absorption 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-freefilter. (ii) A 6-cm3sample is injected into a tared ampule (approximately 12 cm3 capacity) and 0.2 cm3 of 3% H3P04is added. (iii) O2 is bubbled through the solution for 2 to 3 min to remove C02from carbonates. (iv) A 0.6-g portion of solid K2S208is 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. R E S U L T S 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
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983
Table 111. The Effect of Storage on the Soluble Organic Material in Solution
Table I. Analysis of a Series of Organic Species at Various Concentration Levels, Both with and without NaCl Present
organic compd tested
range tested, ppm
mannitol acetic acid D-tartaric acid oxalic acid malic acid propan-2-01 acetylacetone ethanol glycine glycerol trisodium citrate salicylic acid
54-432 30-720 76-604 57-1140 36-716 45-180 232-465 208-417 10-382 10-392 123-490 42-406
no. of NaCl Sam% range, ples recovery ppm tested av 0-300 0-300 0-300 0-300 0-300 0-300 0-300 0-300 0-300 0-300 0-300 0-300
10 11 9 7 9
4 4 4
10 9 7 9
98.4 97.9 102.7 100.9 99.0 94.4 99.8 98.1 95.5 98.7 96.0 98.3
Table 11. Analysis of Natural Brine Solutions from a Solar Salt Field Illustrating the Possible High Results due to Soluble Organic Species from the Untreated Filter Paper sample pond A pond B pond C pond D pond E pond F pond G pond H pond I seawater inlet
soluble organic carbon, ppm unfiltered filtered 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 solutions 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 filtration. The results show an approximately constant higher value due to contamination from the filtration medium indicating that centrifrugal removal only of particulates is desirable. 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 to coz. 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. It is evident that for these two samples mi-
sample brine 1
brine 2
month of analysis
soluble organic carbon (ppm) on 4 aliquots high low average
February April June August October February April June August October
133 131 126 128 130 94 90 98 89 90
123 121 124 122 120 86 84 89 85 84
128 125 126 124 123 90 88 91 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
