Determination of Chloride in Water Improved Colorimetric and Titrimetric Methods
.
FRANK E. CLARKE Chemical Engineering Laboratory, U.S . Naval Engineering Experiment Station, Annapolis, Md. This paper describes improved colorimetric and titrimetric methods for determination of chloride ion in water. I n both methods, dilute mercuric nitrate solution is added to acidified water in the presence of diphenylcarbazone indicator. At the mercury-chloride equivalence point, a blue-violet, mercurydiphenylcarbazone complex forms, which is proportional in intensity to the excess of mercury ion present. The colorimetric test is restricted to clear uncolored waters without significant heavy metal contamination. The titrimetric procedure is independent of practically all common interference.
SI MPIJ :, reliable method
A
101 ~ L L I It I11A I I \ e tlcter niiiiat i c i n of chloride ion in water has been sought foi many j e i t i i The need for such a test is particularly acute in the Kavj , R her c practically all fresh water needs must be supplied by evaporation Qf sea water, at the risk of chloride contamination by leakage or carryover, and where many tests must be conducted by personnel without laboratory experience. Ideally, a chloride test for naval use should involve a minimum of manipulations and provide a Sharp, well defined end-point signal. It should be equally applicable to the vanishingly low chloride concentrations of se'c water distillate and to the relatively high concentrations of chloride in contaminated boiler water. The traditional silvei nitrate-potassium chromate titration procedure (hfohr) now in general use is unsatisfactory, because of its subtile end-point color change and its insensitivity to low concentrations of chloride. Only the mercuric nitrate-diphenylcarbazide and mercuric nitrate-diphenylcarbazone methods described by Dubsky and Trtilek (1,2 ) showed promise of meeting these exacting requirements. The latter proved to be superior from the standpoint of stability of indicator and discernibility of end point. Accordingly, this investigation rvas confined to development of improved merruric nitrate-diphenvlcarbazone methods.
Table I.
pH OF SOLUTION
Figure 1
('over it wide range of chloride roriceiitrations with a given increment, of mercuric ion. This deficiency was overcome by using multiple increments of the mercuric nitrate reagent (1 ml., 2 ml., etc.), so that a single set of color standards sufficed for several orders of chloride concentrations. .4 slide comparator kit including nine color standards in permanent form was prepared for this laboratory by R. A. Taylor & Company, Baltimore, Md., and was used successfully in routine chloride analyses. This colorimetric method is applicable only to clear waters which contain neither actual nor potential color interference. Most heavy. niet:il ions cause color interference. It differs from most colorimetric methods in that color intensity varies inversely with the iwiiixw tration of the ion bring mcnsured.
-
Sensitivity and Reproducibility of RIercur? Diphenylcarbazone Complex
Hg++ Concentration Me./141. X Ids 16.9 11.7 6.4 1.3
Run 1 6.7 9.9 22.9 63.8
T Transmiseion, 520 Mu Coyernan Spectrophotometer Run 2 Run 3 Run 4 Run 5 7.0 6.9 6.2 7.2 10.0 9.3 9.9 12.3 23.9 23.8 22.0 20.0 63.0 iO.0 67.0 63.0
Ar. 6.8
10.3 22.5 65.2
DEVELOPMENT OF COLORIMETRIC METHOD DEVELOPMEhT OF TITRIMETRIC METHOD
Diphenylcarbazone is an orange crystal which dissolves 111 ethyl alcohol to form a clear red solution. This solution reacts with mercuric ion to form an intense blue-violet complex. The complex has a single absorption maximum a t 520 millimicroni and is color-stable for periods up to 30 minutes. Table I shon 5 the order of its reproducibility and sensitivity. In acidic chloride solutions, the formation of weakly ionized mercuric chloride prevents development of the mercury-diphenylcarbazone complex until the mercury-chloride equivalence point is reached. I t was assumed that chloride ion might be determined by adding acid, mercuric ion, and diphenylcarbazone to chloride solution and measuring the excess of mercuric ion in terms of color intensity. Tests confirmed the general soundness of this hypothesis. However, the rapid increase in color intensity with increase in the excess of mercuric ion made it difficult to
The mercuric nitrate-diphenylcitrbazone chloride titration pioposed by Dubsky and Trtilek ( I , 2 ) suffers from the deficiencies of creeping end point and significant variation of accuracy with variation in pH and concentration of diphenylcarbazone indicator. Both high pH and high concentration of indicator ~ i e l dlow chloride values. The problem of creeping end point was overcome by masking the premature blue with a tolerable background color. All background colors in the range from grceu to orange improved the sharpness of end point, but best results were obtained with the greenish yellow, acid color of bromophenol blue indicator. With this background color end point. could be reproduced within 0.1 nil. of 0.025 A: mercuric nitratr solution. The optimum pH was determind by titrating acidified solu-
553
ANALYTICAL CHEMISTRY
554 tions of potassium chloride with 0.025 iV mercuric nitrate, using diphenylcarbazone indicator in the presence of bromophenol blue. Figure 1 shows that maximum accuracy was obtained in the pH range 3 . 0 t o 3.5. Conveniently, bromophenol blue indicator covers the same p H range (3.0 to 3.6) and makes the first detectable change from its alkaline blue to acid yellow a t approximately p H 3.6. I t therefore can serve the dual purpose of masking premature color and adjusting pH. A relatively wide range of indicator concentrations proved to be tolerable a t the optimum p H of 3.3. Best results were obtained with 5 to 10 drops each of 0.5% diphenylcarbazone and 0.05% bromophenol blue, both by weight in 95% ethyl alcohol. A mixed alcoholic indicator containing these concentrations of the chemicals yielded the same results. The stability and performance of this mived indicator have not been affected by 4 months of storage in a clear bottle under normal laboratory light. Roberts ( 3 ) used bromophenol blue in the determination of chloride with mercuric nitrate and diphenylcarbazide. He recommended that the samples first be alkalized to the blue color DE bromophenol blue and then adjusted to the p H range 1.5 to 2.0 with a relatively large addition of acid. I-Ie made no mention of the beneficial effect of the yellow, acid color of bromophenol blue, but stated that this color did not interfere with the titration. APPARATUS
Both the colorimetric and the titrimetric chloride te-ts ritn be conducted with the glassware normally available in an analytical laboratory. The colorimetric test can be applied more conveniently with a colorimeter or a spectrophotometer. I t is evpected that slide-comparator equipment xi11 be marketed for simplified application of this test in the field. REAGENTS
Potassium chloride stock solution, 1000 p.p.m. of chloride ion. Prepared by dissolving 2.103 grams of C.P. special potassium chloride in 500 ml. of reagent grade water and diluting to 1 liter. Potassium chloride standard solutions, 2, 4, 6, and 8 p.p.m. of chloride ion. Prepared by accurate dilution of pot,assium chloride stock solution n-ith reagent grade water. Nitric acid, approximately 0.05 N. Prepared by diluting 3.2 ml. of C.P. nitric acid (specific gravity 1.42) to 1 liter with reagent grade water. Sodium hydroxide solution, approximately 0.05 X. Prepared by dissolving 1 gram of C.P. sodium hydroxide in reagent grade water and diluting to 1 liter therewith. Diphenylcarbazone indicator, 1.0% by weight. Prepared by dissolving 1 gram of C.P. crystalline diphenylcarbazone in 75 ml. of 95% pure ethyl alcohol and diluting to 100 ml. therewith. Store in a brown bottle for maximum stability. (Pure methanol can be used if 95% ethyl alcohol is not available.) Diphenylcarbazone-bromophenol blue mixed indicator. Pre.pared by dissolving 0.5 gram of C.P. crystalline diphenylcarbazone and 0.05 gram of c.P., crystalline bromophenol blue in 75 ml. of 95% pure ethyl alcohol and diluting to 100 ml. therewit'h. I t is stored in a brown bottle for maximum stability. (Application has been made for letters patent covering diphenylcarbazonebromophenol blue mixed indicator. ) Mercuric nitrate solution, approximately 0.025 S. Prepared by dissolving 4.1710 grams of C.P. mercuric nitrate, HglTO3.H20,in reagent grade water and diluting to 1 liter therewith. I t is accurately standardized against the potassium chloride stock solution by titration in the presence of diphenylcarbazonebromophenol blue indicator a t pH 3.2 to 3.4. The pH is adjusted with 0.05 N nitric acid. Standard mercuric nitrate solution can be prepared directly from mercuric oxide according to Roberts ( 3 ) . Mercuric nitrate solution, 0.0141 X, is prepared from the above standardized mercuric nitrate solution by accurate dilution with reagent grade water. Each milliliter of this solution is equivalent to 10 p.p.m. of chloride ion. PROCEDURE
Colorimetric Determination of Chloride. Pour 50-nil. portions of the four standard potassium chloride solutions and the water to be tested into separate beakers or Erlenmeyer flasks. Add 5 drops of 1% diphenylcarbazone indicator to each and then
Table 11.
Reliability of Mercuric Nitrate-Diphenylcarbazone Colorimetric Test
Chloride Present, P.P.RI. 2
Skilled technician 2
Chloride Found, P.P.M. Technicians A B C 2 2 2
4
4
4
4
6 8 10 12 14 16 18
6
6 8 10 12 14 15
6 8 8 12 14 16
8 10
..
14 16
4
'i
10 13 14 15 17
18
Laboratory messenger 2 4
6 8 12 1.1 16 18
Table 111. Reliability of Mercuric Nitrate-Diphenylcarbazone Titrimetric Test Chloride Found, P.P.lMM. Chloride Present. P.P.M.
Skilled technician
Unskilled technician
Laboratory messenger
Powerplant operator
n
n.R.
n i
0.5
0fi
2
1.95 3.99 5.95
2.13 3.73 5,9l 7.90
2.66 4.35 6.22
4.44 6.84
4 6 8 10
7.90
8.61
9.85 39.34
2.31 8.44
10.31 10.90 20.42 19.09 39.8 41.56 fin 58.6 60.67 59.4 79 7 78.32 80.07 99,27 100 99.46 98.48 103.01 199.62 196.44 201.04 196.5 200 4988 4977 4981 4973 5000 a Titration made directly with 0.25 S mercuric nitrate solution and 50-ml. buret.
20 40
9.87 19.71 40.14 60.03 80.38
19.00
add 0.025 S sodium hydroxide dropwise until the solutions become orange in color. This addition of sodium hydroxide can be omitted from alkaline waters which turn orange immediately on addition of the indicator. To each orange solution add 0.05 N nitric acid dropwise until the color changes to yellow and add 1-ml. excess. Finally add 1 ml. of 0.0141 AT mercuric nitrate to each yellow solution and stir or shake to develop the blue-violet, mercury-diphenylcarbazone complex. If the water sample fails to develop the color coniplex with 1 ml. of mercuric nitrate solution, it contains more than 8 p p.m. of chloride ion. In this case, continue to add 0.0141 S mercuric nitrate in 0.5-ml. increments, with shaking, until the first persistence of blue-violet color, and record the quantity of mercuric nitrate reagent required. Transfer portions of the blue water sample and chloride standards to 5-ml Kessler tubes and compare the color of the water with those of the standards by diametrical comparison, using a daylight lamp or a good source of north daylight. The former is preferable. JVhen a color match has been obtained, determine the chloride content of the water by the following equation: C1-, p.p.m. = C
+ l O V - 10
where C = p.p.m. of chloride in the matching standard, and V = ml. of mercuric nitrate consumed. If the chloride content of the water exceeds 30 p.p.m., reduce it below that concentration by measured dilution of the sample with reagent grade water. Perform the determination on the dilute solution and calculate the chloride content by the following equation : 5oc 5oov - 500 __ C1-, p.p.m. =
+
s
where S = ml. of sample r a t e r in the 50-nil. portion. Titrimetric Determination of Chloride. Pour into a porcelain casserole 100 ml. or less of sample water containing not more than 20 mg. of chloride ion, and dilute to a final volume of approximately 100 ml. Add 5 drops of diphenylcarbazone-bromophenol blue mixed indicator and stir the sample. If a blue-violet or red color develops, add 0.05 N nitric acid dropwise until the color changes to yellow and then add 1 ml. excess of the nitric acid. If a yellow or orange color forms immediately on the addition of the indicator, develop the blue-violet color by adding 0.025 N sodium hydroxide solution dropwise and then proceed with the acidification. To the yellow, acidified solution, add 0.025 N mercuric nitrate solution dropwise from a 10-ml. or 25-ml. buret (0.05-ml. or 0.1-ml. divisions) until a blue-violet color persists throughout the solution. Read the mercuric nitrate buret and calculate the chloride content by the following equations: C1-, mg. = V X 35.46 -V
V O L U M E 22, NO. 4, A P R I L 1 9 5 0
555
V X 35,460.YS
ierence, the coloriiiietric I est htts an estimated accuracy of r O . 5 p.p.m. The titrimetric test has the same accuracy up to 200 p.p.m. Khen extended beyond that range by dilution of the sample, the estimated error in parts per million is 50/S.
C1-, p,p.m. C1-, e.p.ni. =
T.'
x
____-
1000 s
S
wheie V = milliliters of mercuric nitrate consumed, S = normality of mercuric nitrate, and S = milliliters of sample water. If titration must be made on solutions which consistently eyceed 1000 p.p.m. of chloride, it will be preferable to employ a higher concentration of mercuric nitrate. EXPERILMENTAL DATA
The precision and accuracy of the colorimetric and titrimetric tests 1%ere evaluated by various laboratory personnel, using gravimetrically standardized solutions of C.P. potassium chloride in distilled water. The results are shown in Tables I1 and 111. On completion of these analyses of pure chloride solutions, additional titrimetric tests Rere made on solutions contaminated Kith positive and negative ions apt to be encountered in industrial naters. KOeffort was made to overcome effects of the ions by modification of the titration. When used individually, 1000 p.p.m, of nitrate, sulfate, phosphate, magnesium, calcium, and aluminum did not significantly affect the titration. The heavy metal ions, zinc, lead, ferrous, nickel, and chromium, affected solution colors, but 100 p.p.m. concentrations of these ions did not reduce accuracy. cupric ion was tolerable in concentrations to 50 P,P.m. On the Other hand, chromate and ferric ions ruined the titration in concentrations as low as 10 p.p,m. These two ions must be removed Or Otherwise counteracted before determination of chloride by this method. In the absence of inter-
CONCLUSIONS
The improved analytical methods described in t,his papel' are applicable to the determination of chloride ion in all types of waters. They require only standard laboratory equipment and can be effectively applied by personnel with very limited laboratory experience. When implemented with slide-comparator equipment, the colorimetric test is ideally suited to field testing. The titrimetric method is independent of practically all common interferences. ACKNOWLEDGMENT
Acknowledgment is made to Genevieve Ziurys, who conducted most of the development tests concerned with the colorimetric method, and to Mary Q.Garner, who conducted the performance tests connected with the t,itrinietric method. The service of W. A. Taylor & Company in preparing a slide comparator for use with the colorimetric method, also is acknowledged. LITERATURE CITED
(1) Dubsky, J. Lr., and Trtilek, J., Mikrochemk 12, 315-20 (1933). (2) Ibid., 15, 302 (1934). (3) Roberts, Irving, IND.ENG.CHEM.,ANAL.ED.,8, 365-7 (1936). RECEIVED October 31, 1949.
The opinions expressed in this paper are those
of the author and are not necessarily official opinions of the U.S.N. Engineering Experiment Station or the yavyDepartment.
Water Content of Hydrocarbons Modi$ed Karl Fischer Method W. S. HANNA AXD A. B. JOHNSON California Research Corporation, Richmond, Calif. .4 niodification of the Karl Fischer method applicable to the determination of
water in hydrocarbons or petroleum fractions is described. The method involves extraction of the water from the hydrocarbon by dry ethylene glycol and subsequent titration of the glycol extract with Fischer reagent. Under normal conditions, over 90% of the water present is absorbed in one extraction. The method is elastic in that, when properly used, equally accurate determinations can be made on very dry or relatively very w e t stocks.
N
U?\IEROUSanalytical methods haye been deyeloped for the determination of Rater in hpdrocarbons, These Illethods mav be roughly classified into those that employ physical means for measurement and thosr that use chemical means. Some physical methods are: thc il.S.T.11. distillation procedure (4), t h r cloud 23), alld the electrical conductivity 11, method (6, 9). A fen- of the methods that employ chemical means are: titration of acid resulting from the action of water on arptyl chloride (15, 20) 2.i), measurement of the volunie of gas ]ibel.ated by the action of J7-ater on (lg, 26), methyl magnesium iodide (14, Z-$)>.01' metallic. sodium ( 8 ) , and the Karl Fischer method. With the exception of the A4.S.T.lI.distillation procedure, t,he Karl Fischer method is by far the most widely used because of its grcJatrr accuracy in the determination of Rater content over the range 0.0005 to 0.5%. There are, however, certain difficulties encountered in the conventional use of this method on petroleum frwtions.
The Fischer reagent is not miscible with hydrocarbons. This property in conjunction with the relatively low solubilitv of n ater in hydrocarbons produces an indefinite titration end point. The visua] end point is difficultto detect in colored fiactions. Because of the low solubility of water in hydrocarbons, t,he actual quantities of water are very small if samples of convenient size are taken. Some method of concentration of the water from :I large volume of sample into a relatively small volume of estract for analysis is required for accurate determinations. The conventional method cannot be emploged on stocks of high vapor pressure, such as butane, propane, isopentane, etc. Certain compounds, notably ketones, aldehydes, and mercaptans (thiols). interfere n-ith the determination bv combining directly with the Fischer reagent. In view of the above-mentioned difficulties various modifications of the Fischer method have been proposed: elimination of the two-phase condition in the t,itration by addition of a solvent to the hydrocarbon phase-e.g., chloroform, pyridine, methanol, or Gertain mixtures of these compounds (1, IS, 22), and elimination
