Determination of Carbon Dioxide in W a t e r D. S. MCKINNEY, Carnegie Institute of Technology, Pittsburgh, Pa., AND A. M.AMOROSI', Elliott Company, Jeannette, Pa. sample between two properly selected end points, acidifying and boiling off the carbon dioxide, cooling the sample, and retitrating between the same two end points. The difference between the two titrations gives the carbon dioxide in the sample. Titration between two end points is necessary to ensure the same effect of the interfering substances for the titration before and after boiling.
A n improved method for determining total carbon dioxide in water is capable of a precision of * 1 p.p.m. even in the presence of rather large amounts of interfering substances such as phosphates. It is believed that this i s the best precision that can be obtained using open flasks for the titration. The method uses apparatus readily available in any Iaboratory,'and requires no knowledge of the nature or concentration of interfering substances, except sulfides.
SELECTION OF END POINTS
D
URING the years 1931 to 1933, a number of papers (3,8, 6,7, 8, 9) were published on the determination and interpretation of alkalinity in boiler waters. This work led to the acceptance of the evolution method of Partridge and Schroeder (7) for carbon dioxide as a tentative standard by the American Society for Testing Materials ( 1 ) . The authors' experience indicates that this is the most precise method for determining carbon dioxide, but it suffers as a plant method from the fact that it is rather slow and requires special apparatus that is not easily portable nor always available. This, and the realization by steam plant operators that carbon dioxide may cause serious corrosion in condensate return lines, indicate the need for a rapid, accurate titration method requiring only apparatus readily available in any laboratory. With the exception of the evolution method ( I ) , titration methods, especially when used to determine small amounts of carbon dioxide (8),are defective in one or more of the following respects: (1) Indicators used do not change color at optimum pH values. (2) Titration is carried out by observing color change, and not to precise pH values. (3) Corrections due to the presence of interfering substances are uncertain. The method described overcomes these defects by titrating the 1
Present address, Navy Department, Bureau of Ships, Washington, D. C.
~
~
Table 1.
Results of Titration from p H 8.5 to p H 5
Known COz in Water
(Using thymol blue and bromocresol green indicators) Known Apparent COa PO' in Other Substances Unboiled Boiled Water in Water sample, A sample, B
P.p.m.
P.p.m.
P.p.m.
P.p.m.
P.p.m.
51 25.5 11.0 5.5 2.75
..............
80.1
0.0
47.5 23.75 47 33 16.5 47.5
40.2 40.3 20.5 11.5 37.5
30.4 14.9 29.7 16.4 9.3 31.6
7
6.0
47.5
37.8
8
6.6
47.5
71NaiSO4 10 Oxalic acid 10 Citric acid 71NanSO4 1ClOxalic acid 10 Citric acid
9
6.6
47.5
71 NazS.01 10 Oxalic acid 10 Citric acid
10 11
11.0 516.5
... ...
Test No.
12
13
Titration curves were calculated as described by Schroeder (8)for carbon dioxide ranging from 4.4 to 440 p.p.m. and for a mlxture of 44 p.p.m. of carbon dioxide with 95 p.p.m. of phosphate, using the values of the dissociation constants for carbon dioxide and phosphoric acid selected by Latimer ( 4 ) . Inflection points (most rapid change of pH per unit of titrant) for the bicarbonate stage in the titration were very close to pH 8.5 in the absence of phosphate. The corresponding inflection point for the carbon dioxide-phosphate mixture occurred a t pH 8.8. Inflection points for the free carbon dioxide stage shifted from pH 5 a t the lowest carbon dioxide concentration to pH 4 a t the highest, and were practically unaffected by phosphate. Since h g h precision is desired for small amounts of carbon dioxide, pH values 8.5 and 5 were selected. pH values 9 and 5 are slightly better in water containing phosphates. I n order to check the selection of pH 8.5 and 5.0 as end points, a number of samples were titrated using thymol blue and bromocresol green as indicators, with the results shown in Table I. Carbon dioxide was added as sodium carbonate and phosphate as potassium monohydrogen phosphate, with other substances as shown. End points were determined by comparison with LaMotte colorimetric standards. Titrations were conducted in open flasks, but as rapidly as possible to avoid contamination from the air. The sample volume was 100 ml. In all samples except No. 11, the agreement between the known carbon dioxide in the water and that found by analysis is within f 1 p.p.m. A few samples were titrated using a glass electrode for the endpoint determination. Repeated titrations differed by as much as 1 p.p.m. It is therefore believed that greater precision than * 1 p.p.m. cannot be obtained in open flasks, because of chance loss of carbon dioxide to or gain from the atmosphere. Erron caused by improper sampling and handling of the sample for analysis may greatly exceed the errors indicated above. Water samples for carbon dioxide should be analyzed promptly, and should be transferred from one Net vessql to another by siphoning rather than by Cot Error The siphon should be immersed well P.p.m. P.p.m. elow the water surface in the vessel being sam50.7 -0.3 pled and should deliver the sample to the bottom 25.3 -0.2 of the receiving vessel. 10.6 -0.4
7.25
44.3
... 6.0
.............. ..............
..............
gounng*
4.1 2.2 5.Q
-1.1 -0.55
31.2
6.6
+o.e
37.2
31.7
5.5
-1.1
37.2
30.4
6.8
+0.3
10.3 540,O
0.0 21.2
10.3 518.8
4-2.3
10 ml. of 0.02 NNaOH
70.0
63.3
6.7
-0.55
2 NaAlOa 30 NaaSOs 12 NaCl
57.7
12.4
45.3
f1.0
..............
71 NazS04 10 Oxalic acid 10 Citric acid
50NalSOr Contamination not known
+O.O
-0.7
Figures in column A and B corrected for the amount of acid necessary t o change pure water from pH 8.5 to pH 5.0, equivalent to 0.6 p.p.m. of CO,.
315
SELECTION OF INDICATORS
Because of the blue alkaline color of both thymol blue and bromocresol green, they cannot be used together in the same sample. Each determination of Table I required the titration of four samples. A search was t h e r e fore made for indicators that could be used together in the same sample, and that were sufEciently stable to resist the necessary boiling, in order that a determination could be made on a single sample. Considering only indicators showing intense color contrast, methyl red, bromocresol green, and methyl orange were selected for the pH range 4 to 5, and thymol blue, phenolphthalein, and 0-cresolphthalein for the pH range 8 to 9. Trials of pairs of these indicators led to the selection of methyl red mixed with o-cresolphthalein as the best pair, with methyl red mixed with phenolphthalein &s a close second choice.
INDUSTRIAL AND ENGINEERING CHEMISTRY
316
REAGENTS REOUIRED
MIXED INDICATOR SOLUTION. Dissolve 0.1 gram of methyl red and 0.1 gram of o-cresolphthalein in 200 ml. of 50% alcohol
(phenolphthalein may replace o-cresolphthalein). BUFFERSOLUTION.Prepare buffer solutions for pH 8.5 and 5.0. If preferred, a pH 9.0 buffer may replace pH 8.5buffer. ACID. Hydrochloric or sulfuric acid, 0.02 N . BASE. Approximately 0.02 N sodium hydroxide, carbonatefree, prepared by diluting a clear, saturated solution of sodium hydroxide with well-boiled distilled water, is satisfactory. Keep in a heavily waxed bottle, protected from atmospheric contamination by a guard tube of Ascarite or soda lime. PROCEDURE
1. Select two flasks of the size and type to be used in the titration (250-ml. Erlenmeyer flasks are satisfactory). Place 100 ml. of pH 8.5 buffer in one and 100 ml. of pH 5.0buffer in the other, to each add 0.4ml. of mixed methyl red-c-cresolphthalein indicator, and cork the flasks. After some practice, more or less indicator may be preferred, but the volume used in the buffered solutions should always be the same as that used in the sample. 2. I n a third flask, add 0.4ml. of mixed indicator to 100 ml. of the water to be tested. 3. Titrate to pH 8.5 (matching the buffered solution), using 0.02 N hydrochloric acid if the sample is more alkaline than pH 8.5 or 0.02 N sodium hydroxide if it is more acid. If sodium hydroxide is used, record the volume required as Vlx (see below, interfering substances in sodium hydroxide solution). 4. Titrate from pH 8.5 to pH 5 (again matching the buffered solution) with 0.02 ,V hydrochloric acid. Record the volume used as VI. 5. Acidify by adding 20% more of the 0.02 N hydrochloric acid than was required for VI, but in no case less than 5 drops of this solution. Boil the solution vigorously for 2 minutes over a strong flame. 6. Cool the flask rapidly in running water to room tempersture, add 0.02 N sodium hydroxide until the pH. is 8.5, and record . titrate the sample the volume of base required as V ~ X Now from pH 8.5 to pH 5 with 0.02 N hydrochloric acid and record the volume of acid used as V*. INTERFERING SUBSTANCES IN SODIUM HYDROXIDE SOLUTION
The sodium hydroxide solution is used only to adjust the pH of the sample to 8.5; hence, its exact normality need not be known. However, it may contain appreciable quantities of carbon dioxide, which would be titrated by the acid, and for which corrections must be made. The magnitude of the correction is determined as follows:
To 80 ml. of carbon dioxide-free distilled water add 0.4ml. of mixed indicator and sufficient 0.02 N sodium hydroxide to bring the pH to 8.5. Then titrate to pH 5 using 0.02 N hydrochloric acid. Let the volume of acid used be A . Add immediately 20 ml. of 0.02 N sodium, hydroxide and again titrate with 0.02 N hydrochloric acid, noting the volume re uired to change the pH from 8.5 to 5. Let this volume be B . %isregard the volume of acid required to titrate the 20 ml. of base to pH 8.5. The volume of acid required to titrate the carbon dioxide in 1 ml. of base is then: B 3 - X 20 CALCULAllON OF CARBON DIOXIDE
The carbon dioxide in the sample is calculated by the following formula:
COz = K X (p.p.m.1 where K = K = or V, = N =
lo00
v,X N
[(Vi
- V I XX X ) - (Vz - VZXX X I ]
45.56 if the titration from pH 8.5to 5 is used 5 is used sample volume in ml. (usually 100 ml.)
43.95 if the titration from pH 9 to
normality of HC1 solution
VI, VI = ml. of acid required to titrate, respectively, unboiled and boiled sample from pH 8.5 to 5 (or pH 9 to 5 if these end points are used) VI,, VZX= ml. of base used to adjust unboiled and boiled sample, respectively, to pH 8.5 (or 9)
Vol. 16, No. 5
X = ml. of acid required to titrate CO? in 1 ml. of base from pH 8.5 to 5 (pH 9 to 5) The numerical value of K differs slightly from 44 01 (the molecular weight of carbon dioxide) since not exactly one equivalent of carbon dioxide is titrated between the pH values chosen (3,6). To calculate K for a pH range other than those given above, the later tables of McKinney (8) should be used. I n many water supplies, the concentration of interfering substances remains practically constant, as evidenced by the constancy of the titration on the sample after expulsion of the carbon dioxide by boiling. I n such cases, titration of the boiled sample need only be run often enough to determine ( V , - V l x X X ) in the formula above. The time required to run a single complete test is approximately 10 minutes. When a series of tests is run, the average time per determination is approximately 5 minutes, since a second sample may be titrated while the first is boiling and cooling. ACID GASES OTHER THAN CARBON DIOXIDE
Water supplies may be encountered containing sulfur dioxide or hydrogen sulfide or their salts. In the p H range 8.5 to 5, sulfites are titrated from SO,-- to HSOa- and sulfides are titrated from HS- to HB. Sulfites, a t the low concentrations used in industrial water treatment, do not interfere with the carbon dioxide determination, as is indicated by sample 13, Table I. This is as expected, since the solubility of sulfur dioxide, corrected for ionization, is about 40 times that of carbon dioxide and in addition only approximately 5Yo of the sulfur dioxide is in the un-ionized form at pH 3 (the approximate pH a t which the carbon dioxide is boiled off). The sulfur dioxide is therefore not boiled off and is properly accounted for by the titration after boiling. The properties of hydrogen sulfide are similar to those of carbon dioxide. Its solubility is approximately 3 times that of carbon dioxide and a t pH 3 it is practically all in molecular form. Hence, it will be boiled off with the carbon dioxide, thus causing high results by the present method. The results may be corrected for hydrogen sulfide by the following procedure. Collect the distillate from step 5 of the procedure in cadmium chloride solution and determine hydrogen sulfide therein in the same manner as for sulfur in steel by the “evolution method”. Correct the carbon dioxide (p.p.m.) calculated from the formula given above by subtracting
44 01 X 0.9800 = HzS (p.p.m.) X 1.266 34.08 if the titration from pH 9 to 5 is used, or HzS (p.p.m.) X
HzS (p.p.m.) X
X 0.9618 = H2S (p.p.m.) X 1.242
if the titration from pH 8.5 to 5 is used. Samples containing large quantities of either hydrogen sulfide or sulfur dioxide cannot be successfully titrated by the procedure described in this paper, because of rapid reduction of the methyl red on boiling. LITERATURE CITED (1) . . Am. SOC.Testing Materials, Standards, Part 111, p. 1549 (1942) (D513-38T). (2) Collins, L. F., and Schroeder, W. C., IsID. ENQ. CHEM., ANAL. E D . , 4, 278 (1932). (3) Hecht, Max, and McKinney, D. S., Power Plant Eng., 35,602, 649 (June, 1931). (4) Latimer, W. M.,“Oxidation Potentials”, New York, Prentice Hall, 1938. (5) McKinney, D . S., IND.ENQ.CHEM.,ANAL.ED., 3, 192 (1931). (6) McKinney, D . S., Proc. Am. Sac. Testing Materiala, 41, 1290 (1941). (7) . . Partridge. E . P., and Schroeder, W. C., IND.E N Q .CEEM.,ANAL. ED.,^, 271,274 (1932). (8) Schroeder, W. C., Ibid., 5, 389 (1933). (9) Straub, F. G., Ibid., 4 , 2 9 0 (1932).
