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LITERATURE CITED (1) Ahmann, C. F., and Hooker, H. D., Mo. Agr. Expt. Sta., Research Bull. 77, 5-39 (1925). (2) Assoc. Official Agr. Chem., Methods of Analysis, 1925. (3) Car& M. H , and Haynes, D., Biochem. J., 16, 60-9 (1922). (4) Dore, W. H., J. Am. Chem. Soc., 48, 232-6 (1926). (5) Fellers, C. R., Mass. Agr. Expt. Sta., Tech. Bull. 15, 218-51 (1 928). (6) Luers, H., and Lochmuller, K., Kollozd-Z., 42, 184-63 (1927).
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(7) Mehlita, A., Chem. Zelle Gewebe, 12, 353-61 (1926). (8) Myers, P. B., and Baker, G. L., Del. Agr. Expt. Sta., BuZ3. 160, 1-64 (1929). (9) Nelson, E. K., J . Am. Chem. Soc., 48, 2412-14 (1926). (10) Sucharipa, R., “Die Pektinstoffe,” Serger a n d Hempel, Braunschweig, 1925. RECBIVED September 9, 1931. Presented before the Division of Agricultural and Food Chemistry a t the 82nd Meeting of the American Chemical Society, Buffalo, N. Y , August 31 to September 4, 1931.
Determination of Hydroxide and Carbonate in Boiler Waters I. Methods EVERETT P. PARTRIDGE’ AND W. C. SCHROEDER,~ University of Michigan, Ann Arbor, Mich.
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H E determination of the composition of a boiler water involves two fundamental operations, the removal of a representative sample from the boiler, and the accurate analysis of this sample. I n discussing the removal of samples from a boiler, the writers have not made a complete survey of current practice but have indic a t e d t h e c o n d i t i o n s which s h o u l d b e fulfilled a n d h a v e shown a p r a c t i c a l s a m p l i n g system which meets these conditions. I n outlining the analytical methods which have been or might be applied to the determination of carbonate, hydroxide, and sulfate in boiler waters, references to important articles have been given but no attempt has been made to include the countless papers in which methods have been endlessly modified or rediscovered.
boiler water which actually contains little carbonate and considerable hydroxide may be reported as showing low hydroxide and high carbonate, or even a considerable concentration of bicarbonate. S u c h a n a l y t i c a l data, when used for control purposes, are dangerous a8 well as ridiculous. Better sampling practice a t the present time usually consists of cooling the sample under boiler pressure, introducing it directly into a bottle with a ground-glass stopper, filtering immediately in the laboratory, if possible without exposure to the air, and analyzing a t once. This procedure eliminates most of the d e f e c t s previously noted, but it leaves the suspended solids in contact with the water until they are removed by a laboratory filtration which almost always involves exposure of the sample to the air for several minutes. That evenbrief exposure may result in high values for carbonate is indicated by Ellms and Beneker (9) and Johnston (18),and proved by the results reported in the third part of the present paper. The ideal sampling procedure would involve the continuous removal of a small amount of water from the boiler, suspended solids being separated from the sample by means of a filter located within the boiler. The rate of flow and resistance of the filter should be such that no flashing of solution into vapor could take place because of decrease in pressure on passing through the filter. The hot solution free from suspended solids should then pass through a cooling coil maintained under boiler pressure by a throttling valve on the cold discharge side. The continuous stream of cold, filtered boiler water should be introduced a t the bottom of a glassstoppered sampling bottle from which it is allowed to overflvfl for a few minutes. On removal from the sampling line the bottle should be stoppered a t once. Although it is desirable to analyze the sample immediately because of the action of alkaline water upon glass, if bottles or glass-stoppered flasks
Titration methods commonly used for the determination of carbonate in boiler waters yield inaccurate values due to uncertainty i n end points and error in interpretation. The various methods available for determining carbonate are reviewed. The development and testing of a simple apparatus which gives accurate and consistent results for total carbon dioxide even at very low concentrations is described. Plant tests using a simple but egective method of Jiltering boiler-water samples at boiler temperature are described, and it is shown that large errors in carbonate, hydroxide, and sulfate m a y result *frompoor sampling procedure and the use of inadequate analytical methods. It is concluded that hydroxide m a y be determined in boiler water with suficient accuracy by either the titration with phenolphthalein and methyl orange or the Winkler barium chloride method, but that these methods do not give a n accurate value for carbonate.
SAMPLING OF BOILERWATERS It is still the practice in some plants to collect a sample from a boiler by draining water from a blow-off line or water-column connection into an open vessel. This sample is then cooled, frequently in contact with the air, filtered after the lapse of more or less time, and then analyzed. Such a procedure is obviously worthless for purposes of exact control, even when carried out rapidly. I n the first place, the hot solution obtained may have a composition differing widely from that of the boiler water a t the time of sampling. Even if it represents the boiler water a t the instant of removal, it soon will not, however. During cooling its composition will change owing to re-solution of suspended particles of calcium carbonate, calcium sulfate, or other substances having inverted solubility curves. Moreover, if it is left in contact with the air it will absorb carbon dioxide a t a rapid rate, so that a 1 Present address, Nonmetallic Minerals Experiment Btation, Bureau of Mines, New Brunswick, N. J. * Detroit Edison Fellow in Chemical Engineering, 1930-31.
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of resistant glass are used, the sample may probably be kept for a considerable time without serious change in composition. An internal filter combined with external cooling under pressure was first used in connection with boiler waters by Hall, Robb, and Coleman ( l a ) , and later by Partridge and White (33),in determining the solubility of calcium sulfate a t boiler temperatures; but it is believed that this type of sampling system has never been applied to an operating boiler, largely because of the difficulty of finding a filter medium which would stand up satisfactorily within a boiler, and the difficulty of introducing an internal filter without violating the provisions of the boiler code. I n the third part of this paper there is described a sampling system of rather simple construction which fulfils all the requirements previously noted, although the filter is located just outside the boiler rather than within the boiler drum. This system has demonstrated its practical value in plant service. SURVEYOB ANALYTICAL METHODS Accurate knowledge of the carbonate, hydroxide, and sulfate concentrations within a boiler is necessary in any plant where the sulfate-carbonate ratio is controlled to prevent the deposition of sulfate scale,s and where the sodium sulfatealkalinity ratio is controlled in an effort to obviate “caustic embrittlement.” The routine control methods used in the determination of the stoichiometric concentrations of these ions in boiler water have been inherited by the boiler operator from the investigator interested in the sanitary examination of water supplies. Like many legacies, these methods are rather out of place in their new surroundings, particularly that for the determination of carbonate and hydroxide discussed under the heading of “alkalinity” in the A. P. H. A. Manual (1). This method, which is the familiar titration with phenolphthalein and methyl orange, and the barium chloride method of C. Winkler4 are the only ones which have been used extensively in boiler-water control. Examination of the literature shows a number of other methods which have been, or might be, applied to the determination of carbonate and hydroxide in solutions. A number of these are briefly discussed in the following sections. Only the more important papers dealing with the various methods are noted, for a complete list of the modifications and rediscoveries would contain many hundred items. The various analytical methods for carbonate may be cIassified in two main groups, one including methods based on titration of the original sample, the other including methods involving an initial evolution of the carbon dioxide content of the sample. TITRATION OF ORIGINAL SAMPLE. The methods available for the determination of carbonate and hydroxide in boiler waters by titration of the original sample depend either upon the color change of indicators or upon changes in electrical potential as measuring devices. Any method involving titration of the original sample with the aid of color indicators is subject to the effect of neutral salts upon these indicators, to uncertainty in end points, and to error in interpretation when phosphates, aluminates, silicates, or salts of weak organic acids are present. The large effect of neutral salts upon phenolphthalein is shown by the data of Rosenstein (58) and of Kolthoff (20). Methyl orange, on the other hand, is nearly free from “salt error” (20, 21). On the score of definition of end points in the titration of carbonate-hydroxide solutions, 8 Where phosphate is used as a conditioning chemical, the POa conoentra tion must similarly oe known. 4 The writers have been unable t o procure a copy of Winkler’s “Massanalyse,” in which this method uas first described. For discussion of the method, see KIIster (13).
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phenolphthalein again shows less desirable properties than methyl orange. The “sliding end point” of phenolphthalein in such solutions has been noted by Thomson ( 4 4 ,Kuster (23), Hildebrand (16), Brubaker (3), and Truog (47’). While the methyl orange end point is frequently considered to be uncertain,. good precision may be attained by the use of a comparison standard containing the same concentration of indicator and saturated with carbon dioxide, as suggested by Kuster in his thorough study (23). The theoretical limit of accuracy in determining the halfway point in the titration of carbonic acid has been estimated by Noyes (sa), Tizard and Boeree (45),and Bjerrum (2) as greater than 1 per cent. An error of several per cent would probably be present in most routine analyses of boiler waters. The utmost care in titration technic would be of no avail, however, if part of the effect attributed to carbonate by the ordinary method of interpretation were actually due to the presence in the boiler water of ions of acids with dissociation constants in the same range as those of carbonic acid. The use of indicators other than phenolphthalein and methyl orange obviously offers little chance of improvement in accuracy. As far back as 1897 Kuster concluded that- the titration with phenolphthalein and methyl orange was not an accurate method for determining carbonate in carbonate-hydroxide solutions. After a painstaking investigation he recommended the Winkler barium chloride method for the determination of hydroxide, and the titration with methyl orange for the determination of total alkali. I n searching for an accurate method for the control of boiler-water conditioning with soda ash, Hall (12) discarded the phenolphthalein-methyl orange titration because it gave high values for carbonate which he attributed to the presence in the boiler waters of salts of weak organic acids. He adapted the Winkler barium chloride method by using phenolphthalein as indicator in the titration of duplicate samples, to one of which barium chloride was added to precipitate carbonate. Experiments by McKinney (15, 50) question the accuracy of this method for carbonate, and Larson (2.4) has recently announced that it yields unreliable carbonate values in the presence of sulfate. Poethke and Manicke (36)in their very complete investigation of the Winkler barium chloride method found that the phenolphthalein used as indicator was very readily removed from solution by adsorption on the precipitate of barium carbonate. When the method is applied to a water containing sulfate, i t is possible that the barium sulfate precipitated with the barium carbonate may remove the indicator from solution so rapidly as to prevent the satisfactory location of the titration end point. The error introduced by the use of the Winkler method on solutions containing sulfate has been independently observed by the present writers and is discussed in the second part of this paper. The excess-acid method was apparently first used with phenolphthalein by Warder (53). Its application in connection with water softening is mentioned by Handy ( l e ) ,and Hall (11) compared it with the phenolphthalein-methyl orange titration and modified Winkler method for the determination of carbonate in boiler waters. Hall’s data show that the values obtained with the excess-acid method were quite consistently somewhat lower than those from the other titrations, In the light of present knowledge, this may indicate that the excess-acid method yields more accurate results for carbonate in boiler waters than either the phenolphthaleinmethyl orange titration or the modified Winkler method. Further investigation seems justified. Electrometric titration has not, to the writers’ knowledge, been applied to actual boiler-water analysis, although Hildebrand (16), Greenfield and Buswell ( I O ) , and Davis, Oakes, and Salisbury (8) obtained satisfactory breaks in their titration curves for sodium carbonate solutions. Cox (Y),using a
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differential method, located the end points more definitely. Zhukov and Gortikov (67),also using the differential method, found that small amounts of carbonate showed up clearly in the titration of sodium hydroxide, so that this method may have some promise for use on boiler waters not containing phosphate, aluminate, silicate, or similar ions in amounts sufficient to prevent interpretation of the titration curve. The potentiometric method has also been applied to carbonatehydroxide mixtures by Little and Durand (2S), whereas Kolthoff (22) found the conductimetric method usable within limits The method recently suggested by McKinney (30) for the determination of carbonate by titration between definite p H values and the application of dissociation data is subject to the errors of any other titration method if the pH reference levels are determined colorimetrically. More accurate measurement of pH by the electrometric method in the control of boiler-water conditions is quite possible, but to date this method has not been applied outside of a few special investigations. McKinney’s proposal to calculate hydroxide concentrations from pH measurements is perfectly logical, but since the pH value of most boiler waters is close to or above 11, and since colorimetric methods employed in a routine manner will frequently not yield values accurate to within 0.2 unit, the hydroxide values calculated from such measurements will be only approximate. Electrometric measurement of pH here again offers increased accuracy. EVOLUTION OF CARBON DIOXIDE FROM SAMPLE. The total carbon dioxide content of a solution may be determined in a variety of ways, all of which depend upon the initial evolution of the carbon dioxide from the original sample. The various methods utilize measurement# of gas pressure or volume, the freezing-out of solid carbon dioxide, and absorption in various solid or liquid media. Many of these methods have been developed in fields rather distant from boiler-water chemistry. Their possible application to the latter depends upon how simply and how accurately they may be used for the determination of the small amounts of carbon dioxide present in boiler waters. The absorption of carbon dioxide in potassium or sodium hydroxide is old enough to pass for a tradition. Petterson and Palmquist (95) adapted this process to the determination of the small carbon dioxide content of air by measuring the volume of a sample before and after absorption of carbon dioxide. 1,. W. Winkler (54)translated the gas-volumetric method to the realm of solutions, Van Slyke (48) added the use of vacuum to remove the carbon dioxide from the solution without heating, and McClendon (29) extended the use of Van Slyke’s apparatus to the determination of the total carbon dioxide in sea water. Further applications of the general method to solutions were made by Shaw (41) and Hall (11), the latter being specifically interested in boiler waters. Van Slyke and Keill (49) also developed a manometric method to measure the carbon dioxide evolved from a sample. Determination of carbon dioxide by freezing it out from a gas mixture has been used by Theis in solid-gas equilibrium studies (49). Yensen (56) also used this method with subsequent expansion of the carbon dioxide into a known volume whose pressure was measured. The determination of evolved carbon dioxide by absorption offers an embarrassing variety of media and methods. The use of solid absorbents in connection with the determination of carbon in steel has become so well standardized that it need not be discussed. The problem here, as in the use of potassium hydroxide for absorption in weighing bottles, is to remove all of the carbon dioxide from the gas stream without gain or loss of water in the absorption unit which is weighed. I
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L. W. Winkler (56) avoided heating his water samples by using hydrogen generated in them by the action of acid on zinc to sweep the carbon dioxide into the absorption bulb. Waggaman (61) has described an arrangement which might prove applicable to boiler-water analysis. Absorption methods involving determination of carbon dioxide by titration seem, however, to be more generally applicable to boiler-water control than gravimetric methods. When potassium or sodium hydroxide is used as an absorbent, the subsequent titration of the carbonate-hydroxide mixture will be more accurate if the barium chloride method of C. Winkler is used to determine residual hydroxide than if the titration is made with phenolphthalein and methyl orange, owing to the uncertainty in the phenolphthalein end point in the latter case. The advantages of replacing potassium or sodium with calcium hydroxide, or better still, barium hydroxide, have been periodically rediscovered for the past century, if it is true that Dalton originated the method, as claimed by Letts and Blake (26). The latter investigators made a comprehensive survey of the method, which is usually associated with the name of Pettenkofer (34). They concluded that the barium carbonate precipitate produced by absorption of carbon dioxide should not be removed by filtration, but that the residual barium hydroxide should be titrated in the presence of the precipitate with phenolphthalein as an indicator. Nearly all of the possible variations of the methods have been tried. J. Walker (52), Nishi @ I ) , and Johnston and A. C. Walker (19) have preferred to filter out the barium carbonate precipitate before titrating the excess barium hydroxide. J. R. Cain (4)and Schollenberger (39), on the other hand, recovered the barium carbonate, the former titrating it with methyl orange as indicator, the latter weighing it. In general, however, titration of the residual barium hydroxide without removal of the barium carbonate has been found satisfactory (26, 27, 4 7 ) . When acids other than carbon dioxide may be absorbed also, it is possible t o titrate both the residual barium hydroxide and the precipitated barium carbonate. An entirely different method of measurement was used by Itano (I?‘), who determined the amount of carbon dioxide absorbed by the decrease in the conductivity of the barium hydroxide solution. Spohr and McGee (42) and Raymond and Winegarten (37) have also adopted this method. Lindner (27) has pointed out possible sources of error in the barium hydroxide absorption, whereas Schollenberger (40) has stated that thymolphthalein yields a sharper end point than phenolphthalein in the titration of the excess adsorbent. Many devices have been used to insure complete absorption of carbon dioxide in the barium hydroxide solutions used. Vesterberg (60) found that a 10-bulb tube was satisfactory. Truog (47) devised a simple and very ingenious scrubbing tower. T. L. B. Cain (6) added gelatin to his barium hydroxide solution to increase the time of retention of gas bubbles a t its surface. It was left for Constantino (6)to make the important improvement of circulating a small volume of gas through a closed system containing two flasks, in one of which the carbon dioxide was evolved from the sample, and in the other it was absorbed in barium hydroxide. Lescoeur and Manjean (25) subsequently used the same idea. The accuracy attainable in the measurement of small quantities of carbon dioxide by absorption in barium hydroxide and titration of the excess absorbent has attracted many investigators in widely different fields of research. I n searching for an accurate means of determining carbonate concentrations in boiler waters, the writers saw more promise in this method than in any of the others noted. This led to the development of two types of apparatus which are described in the second part of this paper.
