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ANALYTICAL EDITION
Industrial AND E N G I N E E R I N G
Chemistry
VOLUME7 NUMBER 2
PUE~LISHED B Y TEE AMERICAN CHEMICALSOCIETY HARRISON E. HOWE, EDITOR
MARCH 15, 1935
Calcium Hypochlorite as a Volumetric Oxidizing Agent Stability and Standardization of the Solution. Determination of Ammonia I. M. KOLTHOFF AND V. A. STENGER, School of Chemistry, University of Minnesota, Minneapolis, Minn.
I
T HAS come to the authors’ attention that “H*T* H.1”a Of
Solutions of hypochlorite have found little application in volumetric analysis since in general they have been found unstable. It was found lhat T.H ’ hypochlorite yielded solutions which were quite stable, B y adding a n excess of bromide to the sample to be titrated, the added hypochlorite behaves as hypobromite. A systematic study is being made of the application qf standard hypochlorite to the determination of IJUriOuS reducing substances. In this paper the standardization of the solution either in acid or uieakly alkaline medium, with Bordeaux as indicator, is described and its use in the determination of ammonia investigated.
opening soon became corroded, the material stopperedwas bottles. transferred The solid to glassis not stable, losing strength gradually whether or not it is exposed to light. J. J. Linganel of this laboratory, has analyzed a sample of uncertain age, robably over 4 years, with the f o f o ~ n results g expressed in percentages: moisture, 2.10; available chlorine (as OCI-), 31.5 (active chlorine, 43.4); total chlorine, 45.5; total calcium as Ca, 22.36; Na, 10.59; Fe208,0.54; free ca0, 3 . ~ 7 ; tacos, 6.08. The freshproductcontainsabout60per cent of available chlorine. To prepare approximately o.l solutions, about 16 grams of the solid are taken for 2 liters. It was found convenient to stir the solid into about 500 cc. of water, allow t o settle, and filter through paper in a Biichner funnel, then dilute the filtrate to about 2 liters and refilter if necessary. Calcium carbonate which may appear as a turbidity is not harmful, but soils burets rather uickly. The solution is preferably kept in a bottle painted hack on the outside, or at least protected from light as much as possible. A reagent prepared in the foregoing manner upon analysis was found to be e uivalent t o a solution 0.0267 M in calcium hypochlorite (0.1068% oxidizingstrength), 0.0019M in calcium hydroxide, 0.0086 M in calcium chloride, and 0.0306 M in sodium chloride. The ratio of sodium to calcium is variable in different solutions, as varying amounts of are removed in filtration. SODIUM ARSENITE. Pure arsenious oxide from the Bureau of standards was dissolved to make 0.1000 N sodium arsenite solutions according to the directions of Kolthoff (5)) and 0.0100 N solutions were prepared by appropriate dilution. G. F. Smith of the University of Illinois kindly INDICATORS. furnished samples of the indicators studied by himself and Bliss (10)for use in bromate titrations. These and various other indicators were used without further purification, in 0.2 per cent aqueous solutions to which equivalent amounts of sodium hydroxide were added if necessary. AMMONIUMCOMPOUNDS. Determinations of ammonia were made using stock solutions of ammonium hydroxide or ammonium chloride. The former were prepared from redistilled ammonia and were kept free from carbon dioxide, being standardized acidimetrically. The latter were prepared from carefully dried c. P. ammonium chloride. The other salts used, potassium bromide and the buffering
high available oxidizing strength manufactured by the Mathieson Alkali Works, New York, Yields stable solutions. This being the case, they have attempted to make use of the product in the quantitative oxidation of various reducing substances, I n general, hypobromite acts more rapidly in Oxidations than does hypochlobut is much less Since hypochlorite reacts quickly with bromide to form hypobromite, the advantages of both stability and rapid oxidation may be obtained by keeping a sto& solution of calcium hypochlorite? and adding an
of aka1i bromide to each before titration* By this means calcium hypochlorite ma,y be expected to become useful in any reaction a t present involving the use of sodium hypobromite or bromine-bromide solutions. (It may and lV*Krestoff, 4, be mentioned that K. sodium hypochlorite as a standard reagent. They kept the stock solution in clear glass bottles and found after 17 days of standing a decrease in strength of about 2 per cent. Possibly if protected the reagent have been found Inore from fight.) The authors are now engaged in a study of the application of calcium hypochlorite to the determination of various inorganic compounds; and it &ould be useful as well this paper are discussed in quantitative organic analysis. the stability and other general properties of Calcium hypochlorite solutions, indicators for use in weakly alkaline and acid media, and an application to the determination of ammonia. Subsequent papers will deal with the determination of several other inorganic substances.
REAGENTS
The solid “H. T. H.” was obtained CALCIUM HYPOCHLORITE. from the Mathieson Alkali Works in cans, but as these after 79
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 7, No. 2
agents, were c. P. quality, recrystallized if necessary. The potassium bromide should not contain over 0.01 er cent of iodide, as this is oxidized to iodate by hypohromite, &us causing an error if the solution is not subsequently acidified. Conductivity water was used in all experiments.
been rejected as bromination indicators since they react either too rapidly or too slowly with hypobromite in alkaline solution. None of the common reversible oxidation indicators have been found useful under the conditions employed. In any case the indicator must be used in aqueous rather than STANDARDIZATION OF CALCIUM HYPOCHLORITE SOLUTIONS alcoholic solution. The standardization may be made either of several ways, STABILITY OF CALCIUM HYPOCHLORITE SOLUTIONS corresponding to various methods which may be followed in actual determinations. With use of hypochlorite for direct Experiments have shown that the stability of calcium hypotitrations in acid solution, any of the nonreversible indicators chlorite solutions decreases with decreasing concentration of used in bromate titrations of sodium arsenite are satisfactory, the solutions, with exposure to light, and with frequent openprovided an excess of bromide is present. For use in indirect ing of the containers. Chapin (2) found that potassium determinations, the hypochlorite is best standardized by hypochlorite has its maximum stability a t a p H of 13.1. pipetting a known volume into excess potassium iodide, The 0.1 N solutions employed by the authors probably have a acidifying with 10 cc. of 4 N sulfuric acid, and titrating with pH between 11 and 12. In general, calcium hypochlorite standard sodium thiosulfate to the disappearance of the blue solutions are more stable than sodium hypochlorite and far more stable than hypobromite. The results in Table I1 color with starch. It is a particular advantage of hypochlorite that determinations may be made entirely in basic solutions, thus eliminating OF CALCIUM HYPOCHLORITE SOLUTIONS interference of many oxidizing agents such as chromate, TABLE11. STABILITY iodate, and bromate. The procedure for standardization is TIMEAFTER APPROXIMATELY 0.1 N APPROXIMATELY PREPARATION SOLUTIONS 0.01 N SOLETIONS simple, and the same whether direct or indirect titrations are Months D a m involved in the determinations. .... 0.0895 0.00919 0.00918a .. 51 0.1262 .... 0.1047 .... ..... ..... To 25 cc. of standard sodium arsenite in a 200-cc. Erlenmeyer .. 15 .... 0.1044 .... ..... ..... 1 .. .... 0.1042 .... ..... ..... flask add 10 cc. of a solution containing 10 per cent potassium 1 15 . . . 0.1041 . . . . 0.00905 0.00905 bromide and 5 per cent sodium bicarbonate. Titrate at a 2 0.1040 .... ..... ..... moderate speed with hypochlorite until near the expected end 3 :: o:i268 .... 0.0884 4 15 .... .... .... point, add 0.05 cc. of a 0.2 per cent solution of Bordeaux, and 6 .. .... 0.1035 0.0881 titrate dropwise with thorough stirring until the color fades. 8 19 .... .... .... Then add another 0.05 cc. of indicator and, if it does not fade, add 8 20 .... .... .... ..... 8 26 .... .... .... 0.00814b ..... more hypochlorite cautiously until a split drop causes the solution 9 3 . . . . . . . . . . . . 0.00808b ..... almost to % m h ” from pink to colorless or light yellow-green. 15 .. 0.1230 ..,. .... ..... ... Since these indicator reactions are not reversible, one must make 16 .. 0.1230 .... .... ..... ..... -~ 24 .. .... .... .... 0.00535 ..... certain that the end point has actually been reached when the This solution contained a larger excess of :alcium hydroxide than the color has faded; this should be tested by the addition of more and appeared to be more stable. indicator. It is advantageous to use as little indicator as pos- other b Determination made by E. B. Sandell. sible and, when the approximate end point is known, one can perceive the end point sharply with only one or two drops. The total amount used should be known and the indicator correction TABLE111. IODOMETRIC DETERMINATION OF AMMONIA made on the basis of the results below. NITROGEN TIMF,OF NITROQEN The standardization may also be made potentiometrically TAKEX BEFFER STANDINQ FOUND ERROR against arsenite in bicarbonate medium, using a bright platinum Mg. Gram Min. Mg. % electrode, agaFpotassium chloride salt bridge, and calomel half13.96 1 -0.4 14.01 cell. A sharp rise in potential appears in close agreement with 14.02 15 $0.1 14.01 5 14.01 14.01 0 the end point obtained with Bordeaux indicator. 14.04 5 14.01 +0.2a 5 14.02 14.01 +o. 1 5 14.05 14.01 fO.3b INDICATORS AND CORRECTIONS 14.14 5 +0.9 14.01 5 f1.4 0 . 7 1 0 0.700 As mentioned above, any of the common bromination india In presence of 30 mg. KNOa. cators may be used with hypochlorite in acid bromide solu1, Return of starch color after end point indicated nitrite formation. tion. Bordeaux serves very well; 0.10 cc. of 0.2 per cent Bordeaux solution requires 0.012 cc. of 0.1 N hypochlorite for decoloration, good proportionality holding for other amounts show the stability that may be expected when approximately ’ regardless of moderate change in dilution. 0.1 N and 0.01 N solutions of “H. T. H.” are kept in glassFor use in weakly alkaline solutions containing bromide the stoppered bottles painted black on the outside, a t an average indicators given in Table I have been chosen, largely from the temperature of 25” C. with occasional variations of * 5” C. list studied by Smith and Bliss (IO). A 0.01 N solution exposed to average room light lost 9 per cent of its strength in 2 days. FOR WEAKLY ALKALINE MEDIA TABLEI. INDICATORS BRITISH COLOUR APPLICATION TO DETERMINATION OF AMMONIA INDICATOR INDEXNo. COLORCHANGE Bordeaux 88 P i n k 4 v e r y faint yellow-green The iodometric determination of ammonia has been inBrilliant Ponceau 5 R 185 Pink+colorless Naphthol blue black 246 Blue4violet-pink+colorless vestigated thoroughly by several groups of workers (1, 3 , 6 , 7 , Congo Corinth 375 Pink+light yellow 8, 9, I I ) , the general conclusion being that the oxidation of Indigo carmine 1180 Blue--fgreen+light yellow ammonia to nitrogen and water Bordeaux is recommended most highly. For titrations in 2NHs 4- 30Br-+ Nz -!- 3Hz0 3Brsolutions containing 0.5 to 1.0 gram of potassium bromide and 0.5 gram of either sodium bicarbonate, disodium phosphate, takes place quantitatively in excess hypobromite a t a p H or sodium carbonate, the indicator correction for Bordeaux is close to 8. The authors have obtained good results using approximately 0.02 cc. of 0.1 N hypochlorite for each 0.10 cc. hypochlorite instead of hypobromite as standard solution, by of 0.2 per cent indicator, plus a constant amount of 0.01 cc. first adding an excess of potassium bromide to the weakly if the total volume is 50 cc. or 0.02 cc. if the volume is 100 cc. alkaline ammonia solution. The mixture is alIowed t o stand Nearly all of the common pH indicators as well as several 3 to 5 minutes with a slight excess of hypochlorite, then dyes have been investigated, but all except the above have treated with potassium iodide and acid, and back-titrated
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ANALYTICAL EDITION
March 15, 1935
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with standard thiosulfate. Table I11 shows the effect of varying conditions, 1.0 gram of potassium bromide having been used throughout. Teorell (11) has introduced a sensitive micromethod for ammonia, back-titrating the excess hypobromite with naphthyl red. It seems probable that calcium hypochlorite could profitably be substituted in this method as well, being more stable than the sodium hypobromite originally specified.
titration of the excess arsenite with standard hypochlorite solution) yielded excellent results and is especially advantageous in the presence of colored compounds or in the determination of extremely small amounts of ammonia with reagents 0.01 I\i or even more dilute. The data in Table V show that quantities of ammonia as small as 0.5 mg. in a volume of 25 cc. can be determined with a maximum error of about 2 per cent, using the general procedure described above. The iodometric procedure yields TABLE IV. DETERMINATION OF AMMONIA IN ALKALINE MEDIUM errors of the same magnitude hpt is preferable in the absence NITROQEN TIMEOF NITROGEN SALTS of other oxidizing substances, since the color change of the TAkEX BUFFER STANDING FOUNDERROR PRESENT iodine starch is sharper than that of Bordeaux in these very Grams Min. 1MQ. Me. MQ. % dilute solutions. Blanks should be run along with the de10.51 5 NaHCOt 3 10.61 11.0 ... 10.51 10.55 3 NaHCOa 3 +0.4 ... terminations, as traces of reducing substances are likely to be 10.51 10.61 1 NaHCOs 3 0.0 ... 10.51 10.52 1 NaHCOs 12 +o. 1 present in the water used as a solyent and some bromine or ... 14.06 15 14.01 0.5 NaHCOs 4-0.4 oxygen may be lost by decomposition of the hypobromite on 5 +0.2 14.04 14.01. 0.5 NaHCOs 14.05 14.01 0.5 NaHCOa 5 f0.3 30
