390
ANALYTICAL EDITION
It is evident that in general the shorter the periods of time during which the cooling unit runs or is at rest, the more constant the temperature will be. On the other hand, the longer the periods, the less will be the strain on the motor from starting. With very efficient stirring, it was found in test runs at 6' C. that the refrigerating unit ran for a period of about 3 minutes and was at rest for a somewhat longer period. The variation in temperature under these conditions did not exceed +0.015' and was usually within =tOo.0lo. By stirring less violently, by means of the solubility apparatus, the motor ran in periods of about 7 minutes and the variation in temperature was slightly greater, possibly *0.025'. When this variation is not objectionable, the slower stirring is preferable. The toluene-mercury regulator was a t first fastened directly to the tank. Owing to vibrations from the motor, the mercury surface was kept in constant motion, and the contact between mercury and platinum wire was continually inter-
Vol. 3, No. 4
rupted, causing the motor to run erratically. This wai corrected by fastening the regulator to a support which was not connected with the tank. The shape of the platinum tip making the connection a t the mercury surface was also found important in causing the motor to start and stop promptly. A smooth, flat point was found preferable to a needle pointq2 The special apparatus used in the construction consisted of a Kelvinator Condensing Unit L-53, a small relay running on direct current, and a larger Westinghouse relay running on alternating current. By suitable modifications in the wiring and the use of a heating coil, the apparatus can be used for high temperatures as well as low. The writers are particularly indebted to Professor Herbert S. Harned of this laboratory for advice and suggestions regarding the thermostat.
* The referee has suggested that a nickel wire might make a better contact than platinum, as the former is not wetted by mercury.
Determination of Iron in Milk and Other Biological Materials' Ralph Stugarf REEDAND CARNRICK,JERSEY CITY,N. J.
Methods employing large quantities of potassium or The iron content of milk and milk powder reported by various authorities varies widely, and this variation can be sodium hydroxide are unsatisfactory for the determination traced not so much to the contamination of the milk as to of very small amounts of iron. Potassium or sodium the methods of analysis employed and to the procedure hydroxide cannot be freed from iron by allowing a 40 per cent solution to stand several days and decanting the in preparing the material for analysis. The colorimetric method using ferrocyanide cannot be clear solution. Correction for the iron in the potassium used in the presence of appreciable amounts of phosphorus and sodium hydroxide is unsatisfactory owing to the fact that the iron is not completely precipitated, and blank and calcium. The sulfocyanate method employing amyl alcohol for determinations are unreliable because of the varying extracting the ferric sulfocyanate is reliable. The deter- proportions of iron precipitated. Methods using small quantities of alkali are unreliable mination involving the comparison of aqueous solutions is not permissible owing to the fading of the color. The due to the presence of iron in the sodium hydroxide use of ether for extracting the iron sulfocyanate is not and to the iron dissolved from the glass during heating. satisfactory and acetone does not afford a guarantee The results may indicate several times as much iron against fading. The iron content of milk cannot be deter- as actually present. A method in which alkali treatment is eliminated and the mined with sufficient accuracy by gravimetric or voluiron determined as ferric sulfocyanate is given in detail. metric methods. .. . . .. ., ., .. . . HE accurate determination of iron in milk has acquired greater importance on account of the recent work on nutritional anemia. As reported in the literature, the iron content of milk varies widely, but in an excellent review of the subject, Nottbohm and Weisswange (21) point out that this variation should be attributed less to the milk itself than to the inaccuracy of the method of analysis. This statement seems to apply also to the more recent data. Herapath (9) was the first to take advantage of the color of ferric sulfocyanate for the determination of small amounts of iron. Later, Thompson (26) used the same method with slightly different procedure and the method is frequently referred to as Thompson's method. Zega (27) used sulfocyanate for the determination of iron in water and stated that hydrochloric acid was not so good as nitric acid for acidifying. Mai (15) and Soxhlet (28) employed this method in determining iron in milk. Andrews (1) found that solutions in
T
1 Received April 16, 1930. Presented before the Division of Medicinal Chemistry a t the 79th Meeting of the American Chemical Society, Atlanta, Ga., April 7 to 11, 1930.
ethyl ether, amyl alcohol, and absolute ethyl alcohol were more intensely colored than aqueous solutions, indicating that dissociation or hydrolysis was responsible for the lighter color and the fading of aqueous solutions. Moore (18) noted that potassium sulfocyanate could not be used as a qualitative test for iron in the presence of phosphoric acid. I n the determination of iron in milk, Ewers (6) found it necessary to remove the phosphates from the milk ash, and also observed that sulfocyanate could not be used in the presence of pyrophosphates. Elvehjem and Hart (5) removed the interfering phosphates from milk ash but made no attempt to overcome the error due to fading in aqueous solutions. Ether was first used by Natanson (19) to make the test for iron with sulfocyanate more sensitive. Tatlock (25) used ether as a solvent for ferric sulfocyanate in the determination of iron in alum. Von K6ler and Lunge (10) used the procedure of Tatlock and pointed out that the determination involving the comparison of aqueous solutions was not permissible, owing to the fading of the color. Lachs and Fried-
October 15, 1931
INDUXTRIAL AND ENGINEERING CHEMISTRY
enthal (13) determined the iron in milk by a method similar to that of Tatlock. Marriott and Wolf (16) used acetone to reduce the dissociation of the ferric sulfocyanate, and this method has been used by several recent investigators. Stokes and Cain (24) pointed out the disadvantages of ether and acetone and suggested the use of amyl alcohol or a mixture of ether and amyl alcohol. Kugelmass ( I $ ) , Kennedy (11), and Elvehjem (4) also used amyl alcohol. For the determination of iron in milk, Fendler, Frank, and Stuber (7) employed titration with potassium permanganate, and Glikin (8), Langstein (14), and Edelstein and Czonka (3) used iodometric methods. The iron content of milk cannot be determined with sufficient accuracy by gravimetric or volumetric methods. The errors inherent in such methods are discussed by Nottbohm and Weisswange (81). Among the recent methods for the determination of iron in biological materials are those of Elvehjem and Hart (5), Kennedy (11), and Elvehjem (4). In the method proposed by Elvehjem and Hart, the phosphorus is removed as ammonium phosphomolybdate, and the iron present in the filtrate is precipitated by addition of a 40 per cent potassium hydroxide solution which is freed from iron by allowing to stand several days and decanting the clear solution. An attempt was made to employ this method in these laboratories early in 1929 in connection with studies on anemia, but as high and variable results were always obtained, it was obvious that iron was introduced somewhere in the procedure as an impurity. Accordingly, the 40 per cent potassium hydroxide solution was examined for its iron content and appreciable quantities were found. It was also observed that a small amount of a gelatinous precipitate was present in the solutions after boiling with 40 per cent potassium hydroxide solution, due to the action of the alkali on the glass. This action of the hot alkali on the glass vessel was another possible source of iron contamination. I n Table I will be found an analysis of nine samples of c. P. potassium hydroxide obtained from five leading chemical houses. The results represent the average of duplicate analyses of the clear solution after allowing a 40 per cent solution to stand three weeks. Table I-Iron in 40 Per Cent Potassium Hydroxide Solution SAMPLE IRON Mg.1100 cc. 0.157 0.109 0.102 0.235
0.400 0.207 0.122 0.093 0.060
Several determinations were made by the Elvehjem and Hart method using 5 grams of milk powder for analysis and 50 cc. of 40 per cent potassium hydroxide solution containing 0.16 mg. of iron per 100 cc. The amount of iron introduced by the potassium hydroxide solution, therefore, amounted to 16 mg. per kilo, or about three times the amount present in the milk powder. When corrected for the amount of iron in the potassium hydroxide solution, the results were variable, because the iron was not completely precipitated. Blank determinations cannot be relied upon, owing to the varying proportions of iron precipitated. The results of several blank determinations using 50 cc. of 40 per cent potassium hydroxide solution, with different amounts of calcium nitrate to act as a “collector” for the ferric hydroxide, are shown in Table 11. This method is thus seen to be unreliable for the determination of very small amounts of iron. Kennedy (11) used amyl alcohol to extract ferric sulfocyanate. He prepared the material for analysis by oxidation with sulfuric acid and perchloric acid. This method of ashing is not applicable to milk.
391
of Iron in 40 Per Cent Potassium Hydroxide Solutions CALCIUMNITRATE IRONIN ROK ADDED,10% SOLN. SOLN. IRON RECOVERED cc. Mg.llOO cc. Mg.1100 cc. % None 0.060 0.014 23.3 1.0 0.060 0.037 61.6 2.5 0.060 0.047 78.3 2.5 0.160 0.133 83.1 None 0.235 0.036 15.3 2.0 0.235 0.072 30.6 Table 11-Recovery
Elvehjem (4) later pointed out that as pyrophosphate interferes with the color development in the sulfocyanate method, low results are obtained on milk and milk powder when the procedure of dry ashing is used. I n order to overcome this difficulty, Elvehjem made the solution distinctly alkaline with sodium hydroxide and boiled for one hour to hydrolyze the pyrophosphate. The solution was then made acid with hydrochloric acid, potassium sulfocyanate added, and the ferric sulfocyanate extracted with amyl alcohol. I n this way high results were again obtained, owing to the presence of iron in the sodium hydroxide and to the iron dissolved from the glass. If the boiling is carried out in platinum and correction made for the iron in the sodium hydroxide, the results are practically identical with the proposed method ,where acid hydrolysis is used. The latter method is preferable as it is more convenient, can be carried out in glass, and no correction for iron in sodium hydroxide need be made. The amount of iron dissolved from glass by potassium and sodium hydroxide solutions varies with the kind of glass and the time of boiling. Porcelain yields more iron than glass, When using glass apparatus, fair checks can be obtained by the Elvehjem method if identical conditions are observed, but the results may indicate several times as much iron as actually present in the milk, owing to the iron derived from the glass. I n Table I11 data are presented to show the recovery of iron in solutions containing pyrophosphate, after acid and alkali hydrolysis. Table 111-Determination of Iron in Solutions Containing Pyrophosphate IRON IRON PYROINTROADDED PHOS- HYDROLYZING BOIL- DIJCED bEFORE IRON PHATE MEDIUM ING WITH VESSEL HYDROLYFOUND TAKENMaterial Amt. TIME NaOH USED sIs Mg. Cc. Minules Mg. Mg. Mg. 200 NaOH 4 60 0.0208 Platinum 0.050 0 0715 200 NaOH 3 60 0.0156 Platinum 0.050 0 0666 200 HC1 1 25 .... Pyrex 0,100 0.1000 200 HCl 5 25 .... Pyrex 0 010 0.0101 300 HCl 6 25 .... Pyrex 0.025 0.0252 200 HC1 1 Pyrex 0.050 0.0510 200 HC1 1 o: Pyrex 0.100 0.0150 200 HC1 1 a , Pyrex 0.050 0.0134 0 Potassium sulfocyanate added immediately without heating.
.... . ..
The accuracy of the method using acid for hydrolysis of pyrophosphate is shown in Table IV. No difference in results was noted when iron was added before or after ashing. The determination of small quantities of iron in organic materials relatively low in calcium and phosphorus offers no difficulties. The colorimetric method using ferrocyanide is simple and reliable if the unknown and standard solutions contain 0.05 to 0.1 mg. of iron and 0.5 to 1.5 cc. of free hydrochloric acid before development of the color. An acid concentration above this amount tends to produce fluorescence which renders color comparison difficult and results unreliable, whereas a lower concentration prevents madmum color development. Best results are obtained if the color is developed in approximately 5 or 10 cc. of the solution, followed by immediate dilution to 100 cc. In the presence of appreciable amounts of phosphorus and calcium this method cannot be used, but the sulfocyanate method proposed by Stokes and Cain (24), modified as outlined below, is convenient and reliable.
ANALYTlCAL EDITION
392
Modified Method MATERIAL-The accuracy of the results depends upon the proper preparation of the material for analysis. Because of the quantity of milk required for the determination of iron, wet ashing with sulfuric or nitric acid is unsatisfactory. Perchloric acid is also undesirable in the oxidation of large amounts of organic materials, owing not only to the danger of explosion, but also to the impurities it contains even after vacuum distillation. Dry ashing has been found reliable if due precautions are taken to prevent loss and fusion of the ash. Desiccated material is finely ground, placed in platinum dishes, dried at 110" C. to constant weight, and transferred to an electric muffle furnace. It is then allowed to carbonize very slowly in the open muffle. As carbonization proceeds, the door of the furnace is gradually closed and the contents allowed to remain overnight, with the temperature regulated so that the m d e is just below dull redness. No loss of material or fusion of the ash will occur if this procedure is followed. I n the case of milk, 100 cc. are evaporated to dryness on a water bath, and the dry material is ashed as described above. The ash is taken up ih just enough iron-free hydrochloric acid to effect solution, filtered, and the filtrate set aside. The residue, consisting largely of carbon, is ignited as before and taken up with a little dilute hydrochloric acid. The filtrate and washings are added to the original filtrate and made up to 50 cc. in a volumetric flask. METHOD-The details of the method are as follows: PREPARATION OF
IRON-FREE HYDROCHLORIC AcID-AI~ hydrochloric acid used should be freed from iron by distillation. STANDARD IRON SoLuTIoN-weigh accurately 0.5 gram of pure iron wire and dissolve in 20 per cent hydrochloric acid with the aid of 1 cc. of concentrated nitric acid. Carefully evaporate to dryness and dissolve the ferric chloride in hydrochloric acid, avoiding large excess. Dilute to 100 cc. and preserve carefully. Each cubic centimeter of this stock solution is equivalent to 5 mg. of iron, For a working standard, take 1 cc. of this stock solution and dilute to 100 cc., then each cubic centimeter is equivalent to 0.05 mg. of iron. It is convenient to make up two standards containing 0.01 and 0.02 mg. of iron. PRocmmE-Pipet an aliquot part of the unknown solution containing approximately 0.025 to 0.10 mg. of iron into a 150-cc. beaker. Add 5 cc. of concentrated hydrochloric acid and make up to approximately 25 cc. Hydrolyze by boiling 20 minutes, let cool, add one drop of concentrated nitric acid, and make up to 25 cc. in a volumetric flask. Transfer an aliquot portion of 10 cc. to a 30- or 50-cc. separatory funnel, and add 1 cc. of hydrochloric acid and enough 0.1 N potassium permanganate to produce a pink color which persists for 20 seconds (usually 1 or 2 drops are sufficient). Add 5 cc. of isoamyl alcohol accurately measured, then 5 cc. of 20 per cent potassium sulfocyanate solution. Shake vigorously for 30 seconds, allow to separate, and draw off the water. Transfer a suitable portion of the colored alcohol to the colorimeter cup, taking precautions to exclude any droplets of water, and compare with a standard prepared as follows: Pipet 1.0 and 2.0 cc. of the working standard equivalent to 0.05 and 0.10 mg. of iron, respectively, into each of two 50-cc. volumetric flasks. Add one drop of concentrated nitric acid, 10 cc. of hydrochloric acid, then make up to volume with distilled water. Transfer 10 cc. to a 30-cc. separatory funnel, add 1 cc. of hydrochloric acid, one or two drops of 0.1 N potassium permanganate, and mix Add 5 cc. of isoamyl alcohol, accurately measured, then 5 cc. of 20 per cent potassium sulfocyanate solution. Shake vigorously for 30 seconds and compare the colored alcohol with the unknown. In the amounts present in biological materials, cobalt, manganese, nickel, and zinc do not interfere with the color development. Copper, if present in appreciable quantity, should be removed.
By their earlier method, Elvehjem and Hart found milk powder to contain 0.0032 per cent iron. By his recent method, Elvehiem found milk powder to contain 0.00226 per cent iron. If the 40 per cent sodium hydroxide solution used contained 1 mg. of iron per liter, and 1 cc. were employed in hydrolyzing 3 grams of milk powder, the amount of iron in-
VOl. 3, No. 4
troduced per kilo of milk powder through the sodium hydroxide would be 0.33 mg. If the sodium hydroxide contained 16 mg. of iron per liter, then the amount of iron introduced by using 1 cc. of sodium hydroxide would be 5.33 mg. per kilo of milk powder. The high results obtainzd by Elvehjem and Hart, and Elvehjem, can be traced, no doubt, to the iron introduced with the potassium or sodium hydroxides and to the action of the hot alkali on glass. Table IV-Determination of Iron b y Proposed Method AMT.OR IRON IRON TIMEOF BOILING MATERIAL MATERIAL ADDED IRONFOUND RECOVERED Miautes Grams Mg. Mg. Mg./kilo % 15 Milk powder A 2.0 0.0098 4.90 30 Milk powder A 2.0 0.0099 4.95 45 Milk powder A 0.009s 4.90 2.0 60 2.0 Milk uowder A 0.0099 4.95 15 Milk powder A 1.0 0.02 0.0249 4.90 99.9 20 Milk powder A 1.0 99.3 0.03 0.0347 4.70 15 Milk powder B 2.0 0.009s 4.90 15 Milk powder B 2.0 0:oos 0.0177 4.85 99.4 0.0192 4.80 15 Milk powder C 4.0 30 0.0125 5.00 Milk powder D 2.5 20 Milk powder D 3.6 . . . 0.0179 4.97 20 Milk powder D 3.6 0.01 0.0279 4.97 99:s 15 Milk powder A 1.0 0.01 0.0148 4.80 99.1 100.5 15 Milk powder A 1.0 0.01 0.0150 5.00 15 Milk powder A 1.0 0.01 0.0150 5.00 100.5 20 Certified milk E 20 CC. 0.0088 0.44b 20 Certified milk F 20 cc. 0.0093 0.46 20 20 cc. Certified milk G 0.0106 0.53 20 Grade B past. 10 cc. 0.0073 0.73 milk H 20 Known solutions 0.2232 0.2193 98.2 20 Known solutiona 0.1036 0.1030 99.4 a Contained pyrophosphate, copper, zinc, manganese, cobalt, and nickel. Copper removed as coppet sulfide. b Mg. per liter.
... ... ... ...
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... .
In Table V is tabulated the iron content of cow's milk as reported by various authorities. The wide variations in the results obtained for the very small iron content of milk can be traced, no doubt, not so much to the contamination of the milk as to the methods of analysis employed and to the procedure in preparing the material for analysis. Content of Milk a n d Milk Powder Reported by Various Authorities IRON METHOD USED AUTHORITY Mg./liler
Table V-Iron
MILK
Gravimetric Iodometric Sulfocyanate in aque-
2.45 47.6-68.95 1.0
ous soh.
Titration with KMnOi Sulfocyanate in ether soh.
2.8-8.4 1.3
0.3-0.7coll. in glass 0.7-3.0commercial Iodometric milk \ 0.45 av. in uncon- ' taminated milk Iodometric 1.11 av. in mixed milk J 0.17-0.84av. of 26 Sulfocyanate in aquesamples 0.42 ous s o h . 0 . h Cupferron grecipitation and sulfocyanate in aaueous soh. 0.21-0.91b Same as above 1.2 Sulfocyanate in '- ----' amyl sa lroh nl alcohol 3.5 Sulfocyanate in aquesuit ous s o h after removal of phosphates 2.4,av. of 20 samples Same as above
1
0.23coll. in glass 1.4 market milk Mg./kilo
......
Bunge ( 2 ) Glikin (8) Mai (IS) Fendler, Frank, and Sttiber
(7)
Lachs (13)
and
Friedenthal
Langstein (14) Edelstein and Czonka (3) Soxhlet (23) Nottbohm and Weisswange (21) Nottbohm and Ddrr (20) Kugelmass ( I2) Elvehjem and Hart (5) Petersen and (22) Matsuo (17)
Elvehjem
MILK POWDER
Sulfocyanate in aqueous soh. after removal of phosphates Elvehjem and Hart (5) 22.6 Sulfocyanate in amyl alcohol after alkaline hydrolysis Elvehjem (4) Decimal point probably misplaced. b Most values between 0.21 and
32.0
5
0.49.
A number of the results in Table V are in fair agreement, although most of the methods used were lacking in accuracy. For example, Soxhlet found an average of 0.42 mg. of iron per liter of milk. Two determinations were made on pasteur-
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 15, 1931
ized milk, with an actual iron content of 0.73 mg. per liter, by the method used by Soxhlet. The results were 0.6 mg. and 0.8 mg. per liter. Kugelmass used amyl alcohol as solvent for ferric sulfocyanate; but his results were subject to error on account of the small amount of milk taken for analysis. Ashing in a glass tube seems also to result in contamination with iron since the same milk, containing 0.73 mg. of iron per liter, gave a result of 1.3 mg. by the method of Kugelmass. Acknowledgment
The writer wishes to express his indebtedness to Frank
R. Eldred for his suggestions and criticism in the course of this work, and to E. Passamaneck for carrying out many of the iron determinations. Literature Cited (1) Andrews, Chem. News, 70, 165 (1894). (2) Bunge, Z . Biol.,10, 295 (1874). (3) Edelstein and Czonka, Biochem. Z., 38, 14 (1912). (4) Elvehjem, J . Biol. Chem., 86, 463 (1930).
393
(5) Elvehjem and Hart, I b i d . , 67, 43 (1926). (6) Ewers, Chem. Zcntr., 69 (II), 605 (1898). (7) Fendler, Frank, and StLber, Z . Untcrsuch. Nahrungs-Gcnussm., 19, 369 (1910). (8) Glikin, Biochem. Z . , 21, 348 (1909). (9) Herapath, J . Chem. Soc., 5, 27 (1853). (IO) Kder, von, and Lunge, Z . angcw. Chem., 22, 670 (1894). (11) Kennedy, J . Biol. Chem., 74, 385 (1927). (12) Kugelmass, Bull. soc. chim. biol.. 4, 677 (1922). (13) Lachs and Friedenthal, Biochem. Z., 32, 130 (1911). (14) Langstein, Jahrb. Kinderheilk., 74, 536 (1911). (15) Mai, Z . Untersuch. Nahrungs-Genussm., 19, 21 (1910). (16) Marriott and Wolf. J. B i d . Chem., 1, 451 (1906). (17) Matsuo, Osaka J. Med., 28, 555-62 (1929). (18) Moore, Chem. News, 53, 209 (1886). (19) Natanson, Ann., 130, 246 (1864). (20) Nottbohm and Dorr, Z . Untersuch. Nahrungs-Genussm., 28, 417 (1914). (21) Nottbohm and Weisswange, Ibid., 23, 514 (1912). (22) Petersen and Elvehjem, J . Biol. Chem., 78, 215 (1928). (23) Soxhlet, MJnch. med. Wochschr., 59, 1529 (1912). (24) Stokes and Cain, J . A m . Chem. Soc., 29, 409 (1907). (25) Tatlock, J . SOC.Chem. Ind., 6 , 276 (1887). (26) Thompson, J . Chcm. Soc., 47, 495 (1885). (27) Zega, Chem.-Zbg., 85, 1564 (1893).
The Buffer Capacity of Sea Water’ Thomas G . Thompson and Robert U. Bonnar DEPARTMENT OF CHEMISTRY AND OCEANOGRAPHIC LABORATORIES, UNIVERSITY OF WASHINGTON. SEATTLE. WASH.
The buffer capacity of sea water is defined and the varying kpown quantities of principle of the determination demonstrated. The sea water may be deacid to the base-free sea water, buffer capacity is differentiated from such terms fined as the number of the latter being first diluted to as “alkaline reserve,” “methyl orange alkalinity,” millimoles of hydrogen ions a salt content equivalent to etc., because the effects upon the indicator by variawhich a unit volume of sea the waters being studied. tions in the concentration of the dissolved salts and of water will neutralize when an the excess acid are considered. excess of s t a n d a r d a c i d is Preparation of Base-Free The buffer capacity is shown to be a rather constant Sea Water added. The principle of the function of normal sea waters, and the value of the conmethod is to add a measured stant for the Puget Sound region and the San Juan Four liters of sea water are quantity of standard acid to Archipelago shown to be 0.1250 with chlorinity per liter boiled with a small excess of a known a m o u n t of freshly as unity. However, variations in conditions affecting sulfuric acid. Three or four sampled sea water, determine the sea water, such as land drainage, pollution, and drops of the concentrated acid the hydrogen-ion concentraphotosynthesis, cause noticeable deviations. are sufficient. After thorough tion of the resultant solution, The total carbon dioxide cannot be determined by boiling, 0.01 N sodium hyusing special standards for direct acidimetric titration methods. Sea water didroxide is added until a pH of comparison, and calculate the luted by streams tends to give a high value for the 6.6 is obtained, using bromoamount of acid neutralized. buffer capacity. Photosynthetic processes and industhymol blue and La Motte The buffer capacity is in the trial pollution of sulfite liquor give low buffer-capacity standards. The sea water is category of the “alkalinity” of values. again boiled, cooled, and the Ruppin as cited by HellandThe determination is one that may be easily and chlorinity (6) determined. Hansen (1). It is analogous quickly performed under rigorous field conditions. to the “alkaline reserve” of Knowing the latter and the McClendon (3) because no chlorinities of the samdes of difference was noted in the buffer capacity when the sea water sea water to be studied, the base-free sea water is dilute2 with was boiled, either before or after the addition of the acid. It is distilled water until analogous conditions of concentration are also similar to the “titrable alkalinity as bicarbonate” of Lucas obtained. The base-free sea water is used for the preparation and Hutchinson (d), if certain corrections are made for the end of standards within the range of bromophenol blue by the adpoints. However, the use of the term distinguishes it from dition of carefully measured amounts of standard 0.0100 N others that are analogous because recognition is made of the hydrochloric acid. It is necessary to prepare different sets of effects produced by varying the concentration of dissolved standards of the varying quantities of acid for different ranges salts and the excess of acid. The present paper is an extension of chlorinity. I n practice it is found more convenient to reand improvement of previous work reported from the authors’ standardize by comparison with the La Motte color standards. laboratories (6). Bromophenol blue is used because it is not sensitive to the Variations in the concentration of the dissolved salts oc- hydrogen ions of the carbonic acid formed, and thus the boiling curring in sea water have variable “salt effects” upon the in- of the solution is not necessary. dicators. Thus in the determination of the buffer capacity, The method for the determination of the buffer capacity has certain corrections must necessarily be applied. This is best been subjected to four modifications. The first, applicable to accomplished by the use of what is termed “base-free” sea waters having chlorinities between 14.50 O / o ~ and 18.00 O/w, water. Colorimetric standards may be prepared by adding was to add 25.00 ml. of 0.0100 N hydrochloric acid to 100 ml. of sea water and then take the pH of the resultant solution, 1 Received May 22, 1931.
HE buffer capacity of
T
