Note 1: The general layout of the apparatus is shown in the diagram. The brass tubing and Swagelok fittings are not essential and can be replaced by silicone tubing. The precision metering valve is essential. Details for working and maintenance of the Beckman IR analyzer should be obtained from the manufacturer.
Note 2: It is best to do most operations (see below) in some form of protective hood or
111.4. SOLUBLE ORGANIC CARBON 155
"dry-box" to minimize dust contamination but the necessity for this is not absolute and will depend upon laboratory conditions.
D. SAMPLING PROCEDURE AND SAMPLE STORAGE
See Section G below for precautions for cleaning apparatus. Samples should be filtered through well-washed membrane or, preferably, glass filters. If only small volumes of sample are available use an ignited glass filter previously washed with a few milliliters of sample. Store the samples in small, clean glass-stoppered glass bottles or in ampules with their mouths covered by aluminum foil. If storage periods are to exceed a few hours freeze the samples at —20 C protected from the possibility of contamination by organic vapours.
E. SPECIAL REAGENTS
1. Cylinder nitrogen with reduction valve and pressure gauges.
2. Activated coconut charcoal, 8- to 12-mesh.
3. Chromatographic grade "molecular sieve", 60- to 80-mesh.
4. Ascarite soda-lime, 20- to 30-mesh.
5. POTASSIUM IODIDE SOLUTION
Dissolve 200 g potassium iodide, KI, in 500 ml of 10% v/v sulphuric acid, 1-1,SO4 .
6. ANHYDRONE (MAGNESIUM PERCHLORATE) Reject material passing a 10-mesh sieve.
7. ORGANIC-FREE WATER
This is an important reagent in this method. Start with a good quality distilled (not de-ionized) water and distill it once again in an all-glass apparatus. Reflux the distillate, about one liter at a time, with 10 g of potassium persulphate K2S208 and 1-2 ml of 85% phosphoric acid for 4 hr and then distill it into a receiver, protected from dust, rejecting the first 100 and last 200 milliliters. Store in a clean glass-stoppered bottle, which is never left open. This water, when analyzed as described below, should not contain more than 0.2 mg C/liter.
8. PHOSPHORIC ACID 3% v/v
Dilute 30 ml analytical reagent grade 85% (syrupy) phosphoric acid, 1-131304, to 1000 ml with organic-free water in a glass-stoppered glass bottle. Add 10 g of potassium persulphate, K 2S,08, and heat the bottle (with the stopper loosened) for 4 hr in a boiling water bath. Cool and store in the same stoppered glass bottle. This solution is stable indefinitely.
9. POTASSIUM PERSULPHATE
Add solid, analytical reagent grade potassium persulphate, K2S208 , to 500 ml of organic-free water at 70 C until no more solid dissolves (ca. 120 g). Keep the solution in a well-covered beaker for 2 hr, cool to room temperature, and filter the crystals onto the clean sintered-glass filter base of a Millipore filtration unit with no filter paper in place. Remove the crystals with a metal spatula into a clean glass dish and dry them in a dust-free atmosphere at 50 C for 12-24 hr. Pulverize the crystals to a fine powder in a clean glass mortar. Store in glass with a glass cover which is never left off unnecessarily.
156 A PRACTICAL HANDBOOK OF SEAWATER ANALYSI S F. EXPERIMENTAL
PROCEDUR E
Note : The Beckman IR spectrometer and recorder should be switched on about 60 min before measuring the first sample .
1 . Add 200 mg of potassium persulphate to an ampule with the special scoop and small glass funnel and then introduce, from the 10-ml hypodermic syringe, 5 .0 ml sample (Note a) . Finally, add 0 .25 ml of 3% phosphoric acid using the 1 .0 ml hypodermic syringe (Note b) .
2 . Place a 5- to 7-cm length of silicone tubing just over the top of the ampule and seal the open end of this tubing with a small plug of silicone grease (ca . 2-3 mm long) . Insert the steel cannula into the side of the tubing, at about mid-length, and poke it down through the neck of the ampule into the sample.
3 . Pass nitrogen through the solution for 5 min with a flow-rate of about 200 ml/min (Note c) . Remove the cannula and press down the plug of grease to reseal the tube .
4 . Immediately seal the ampule by rotating it with the top (as near as prac-ticable to the silicone tube) in a small blunt oxyhydrogen flame from a glassblower's torch (Note d) .
5 . Autoclave ampules at 130 C in steam (24 psi) for 40 min (Note e) . 6 . Cool the ampules to room temperature (Note f) . Place the ampule firmly into the tygon tube with the cannula sticking into the silicone tube, as shown in the diagram . Remove the ascarite guard tube at the end of the train and turn on the tap to the IR spectrometer. With about 5-10 psi at the cylinder adjust the micrometer metering valve until a flow-rate of 200 ml/min of nitrogen is obtained. Maintain this flow until the recorder reads zero (Note g) .
7 . Crush the end of the ampule through the tygon tubing, using a small pair of pliers, and quickly push the cannula down through the hole at the top of the ampule until it is well under the liquid (Note h) . The flow rate should return to 200 ml/min after a brief rise (Notes i and j) . Continue passing nitrogen until the recorder pen returns to zero . Remove the ampule after withdrawing the cannula, shake out any broken glass from the tygon tubing, and put in the next ampule . Repurge the system and break the new ampule, etc., as described above . After finishing a batch of samples remove the cannula completely, wash it with water, and replace it through the silicone tube down into a clean dry empty ampule left for this purpose . Purge the system and then turn off all taps and replace the ascarite tube at the end of the train (Note k) .
8 . Measure the peak height of the curve recorded from the infrared analyzer or, preferably, the area under this curve and calculate the amount of carbon (as mg C/liter) as described in Section H . Subtract a blank obtained as described in Section G.
NOTES
(a) The hypodermic syringe and needle should first be rinsed with sample. The sample and phosphoric acid syringes must not be greased . After use both syringes should be cleaned by
sucking up and expelling a little distilled water .
111.4. SOLUBLE ORGANIC CARBON 157 (b) Ampules should not be left for too long between filling and sealing to minimize con-tamination and are best filled and sealed one at a time.
(c) Use about 5-10 psi of nitrogen. The flow meter must first be calibrated if the manufac-turer's calibration is not available. Gas escaping out of the silicone grease plug hinders the re-entry of atmospheric carbon dioxide. The ampule should be resealed with the finger after the cannula is removed from the tubing.
(d) This is a critical stage in the method and some practice is necessary. The sealing should be done rapidly (a few seconds) to minimize the entry of atmospheric or combustion CO2. The seal should be somewhat tapered to facilitate breakage later (see F.7, above). The recognition of a suitable seal comes with experience.
(e) This time is not critical but should exceed 30 min after the autoclave has come to full temperature and pressure. We have found no evidence that prolonging the heating improves oxidation, indeed, the persulphate may be decomposed fairly rapidly. For this reason it is best to ensure that the ampules come to temperature as rapidly as possible. We have found no evidence that the persulphate decomposes, even after a few weeks, at room temperature but such a decomposition is possible, especially in the presence of certain metal ions, and for this reason we do not recommend that sealed ampules be stored for more than 1 or 2 days before being heated.
(f) This may be done at any convenient rate.
(g) The zero is adjusted by a small screw on one of the light paths of the analyzer. As the analyzer cells get more and more use this adjustment will have to be increased. Finally when zeroing is no longer possible the cells should be dismantled and cleaned. If peak height is measured exact zeroing is not necessary but it is required if peak area is measured (refer to Section H).
(h) This is another critical stage of the method which requires practice. As mentioned in Note d, the operation is facilitated by having the correct type of elongated seal. Check that the ampule has not cracked down to below the tygon tube. A good rapid break is essential if peak height is to be measured but this is not so critical if area is measured (see Section H).
(i) A constant and reproducible flow rate is essential at this stage. The rate need not be exactly 200 ml/min provided that it is reproduced for standards and samples to within a few per cent when measuring peak heights. If peak areas are used an exact reproducibility of flow rate is not so important.
(j) Be on the lookout for blockage of the cannula by rubber or for the hole in the silicone tube enlarging so much that leaks occur. Both these troubles will be detected by a drop of flow-rate at a given setting of the precision metering valve. If this occurs to a significant extent during the analysis of a sample (5% or more) the determination should be rejected. The silicone tube requires renewing frequently (about every 20-50 samples).
(k) It is necessary to change the potassium iodide solution (which absorbs the chlorine produced by the action of persulphate on sea water) every 20-30 samples. The anhydrone should be changed every day. Other desiccants have been found to absorb carbon dioxide.
G. DETERMINATION OF BLANKS
Note: All glassware, filters, reagent bottles, pipettes, etc. must be cleaned with hot chromic-sulphuric acid, rinsed with distilled water, and finally rinsed with organic-free water. The am-pules are first rinsed with distilled water, shaken nearly dry, and then ignited for 4 hr in a muffle furnace at 450-500 C. Any aluminum foil that may contact samples should first be ignited for a few hours at about 500 C.
A reagent blank is difficult to determine for this method because of the impossi-bility of getting water completely carbon-free. Carry out the method exactly as described in Section F, paragraphs 1-8, except make duplicate determinations on 1.0 ml, 2.5 ml, and 5.0 ml of low carbon sea water (e.g. a sample taken below 500 m). Convert mean peak heights or areas to mg C/liter from the calibration curve
158 A PRACTICAL HANDBOOK OF SEAWATER ANALYSIS
(Sect. H) and plot these carbon concentrations as a function of the number of milli-liters of sample. Extrapolate to determine the value at zero volume. This quantity should be equivalent to between
0.15and
0.3mg
C/literand must be redetermined every time new phosphoric acid or
persulphateis used.
H. CALIBRATION
1.
STANDARD GLUCOSE SOLUTION
Dissolve
1.25g of dry glucose in
250 mlof distilled water. Add a few crystals of mercuric chloride,
HgC12,and keep this stock solution in the refrigerator.
Dilute
5.0 mlof this solution to
100 mlwith organic-free water. If refrigerated, this solution may be left in glass for a few days. Finally dilute
5.0 mlof this latter solution to
100 mlwith organic-free water and use this working standard imme-diately.
1 ml 1