V O L U M E , 2 6 , NO. 2, F E B R U A R Y 1 9 5 4 Table 111.
315
Efficiency of Color Removal during Centrifuging
C(F ( E = Total solids separating efficiency = F(C E1 = 42.5%; En = 28.8%) ~
Feed Skim Concentrate
%Total Solids 31.0(C) 9.8(S) 61.5(F)
-- S8)) X
100 = 80.8
Color Index 41.9 62.8 29.8
trate. Presumably this lost color merely accumulates in the bowl during the run and accounts for the yellon- regions that are usually observed when the bowl is taken apart for cleaning. Since the El value is based upon the yellow color actually remaining in the concentrate, it is considered the better one to use for expressing the efficiency of color removal. SUMMARY
A method which consists of milling the sample very thin and extracting it with acetone has been developed for determining the amount of natural yellow coloring matter in a sample of dry rubber or latex. The absorptivities of this acetone solution a t 360, 440, and 550 mp are then combined by an empirical equation to give a measure of the yellow coloring matter in the sample. The test may be carried out even in the presence of considerable
darkening of the sample due to oxidative processes. The oxidative products cause a strong absorption a t the shorter wave lengths of the visible range with a probable peak somewhere below 340 mp. In developing the method it wa8 shown that the absorption which is due to background effects and to the presumed oxidation products gives a curve which is similar to that for a-carotene, except that the peaks are shifted from 7 to 10 mp toward shorter wave lengths. This is taken as fairly good evidence that the yellow coloring matter of natural rubber is a carotenoid closely related to, but possibly not identical with, a-carotene. ACKNOWLEDGMENT
The author is grateful to A. I. Rand who submitted most of the crepe and centrifuge samples discussed in the application section of this paper, to Gerrit Verhaar for helpful criticisms and suggestions, and to the Firestone Plantations Co. for permission to publish this work. LITERATURE CITED
(1) Beachell, H. C., Fotis, P., and Hucks, J., J . Polymer Sci., 7, 353 (1951). (2) Gortner, R. A, and Gortner, K. A , , “Outlines of Biochemistry,” 3rd ed., p. 867, New York, John Wiley &- Sons, 1949. (3) Verhaar, G., private communication. RECEIVED for review April 3, 1953. Accepted October 26, 1953.
Determinations of Thiosulfate and Nitrite RAYMOND
H. PIERSON
Analytical Chemistry Branch,
U. S. Naval
Ordnance Test Station, lnyokern, China Lake, Calif.
Although methods are available for determining thiosulfate o r nitrite alone in ammoniacal solutions, none of these methods is applicable when both substances are present in the same sample. A convenient and accurate determination based on precipitation of the thiosulfate with silver nitrate has been developed for use with aqueous solutions not containing sulfide or sulfite. An improved method is presented for determining thiosulfate in ammoniacal sulfide solutions when nitrites and sulfites are absent. A test for sulfite in the presence of thiosulfate and nitrite has also been developed.
I
?J COSNECTION with some studies of rates of reaction, an
interest developed in the determination of both thiosulfate and nitrite occurring together in ammoniacal solutions (8). Well-known methods are available for the analysis of thiosulfate ( 4 , 6 ) or nitrite (1-3) alone, but none of these is applicable without modification when both ingredients are present in a sample. DETERMISATION IN AQUEOUS AMMONIACAL SOLUTIONS (SULFIDE AND SULFITE ABSENT)
A reliable and convenient procedure which has been devised for determining these two ingredients in aqueous ammoniacal solutions is based on a treatment with silver nitrate under carefully controlled p H conditions. Thiosulfate yields silver sulfide quantitatively and nitrite is unaffected. The thiosulfate determination is then completed by measuring the silver content of the silver sulfide precipitate by dissolving it in nitric acid and titrating with ammonium thiocyanate in accordance with the Volhard procedure and nitrite is determined in the filtrate from the silver nitrate treatment by cerate oxidimetry. The procedures described in this section are not applicable to
solutions which contain sulfide or sulfite. Application of the method to ammoniacal sulfide solutions (not containing an appreciable amount of sulfite) and a qualitative test for sulfite are also discussed in this paper. Chloride in moderately large amount will not interfere. APPARATUS AND RE-IGENTS
The apparatus used consists of a pH meter nith extension glass and calomel electrodes and a magnetic stirrer. Reagents. Ammonium acetate, 25% buffer solution, adjusted to pH 7.5 to 7.7 by addition of ammonium hydroxide. Acetic arid, 10% solution. Ammonium hydroxide, 2% solution. Silver nitrate, standard 0.131 solution (primary standard). Ammonium thiocyanate, standard 0.lM solution. Kitric acid, concentrated reagent grade. Sulfuric acid, dilute, 1 to 3. Ferric ammonium sulfate, saturated solution, indicator. Ceric ammonium sulfate, standard 0.1M solution in 0.5M sulfuric acid. Standardized against the 0 . l M ferrous ammonium sulfate using ferroin as indicator. Ferrous ammonium sulfate, standard 0.lM solution in sulfuric acid of about 2.0111‘ (dilute sulfuric acid, 1 to 9). May be standardized against Bureau of Standards potassium dichromate in acidic solution using sodium diphenylbenxidine sulfonate as indicator. Bureau of Standards potassium dichromate, sample 136, dried at 110” C. for 1 hour and cooled in a desiccator. Lead acetate test paper. Nitrogen, cylinder of compressed gas, oxygen-free. PROCEDURE
Pipet a sample of suitable size ( 2 to 50 ml. depending on thiosulfate and nitrite content expected) into a 600-ml. beaker containing 20 ml. of ammonium acetate buffer solution and about 100 ml. of distilled water. Insert a stirring bar in the solution,, p!ace it on a magnetic stirrer, and apply moderately vigorous stirring. Also insert calomel and glass extension electrodes connected with a pH meter. Adjust the p H of the solution to 7.5 to 8.0 by addi-
ANALYTICAL CHEMISTRY
316
tion of 10% acetic acid or 2% ammonium hydroxide. Place above the beaker two burets, one containing 0.1M silver nitrate, the other filled with 2% ammonium hydroxide. Begin adding the silver nitrate solution, introducing a few milliliters a t a time, and as the pH decreases add a few drops of the ammonium hydroxide solution so as to maintain the solution within the pH range 7.1 to 7.5. Continue adding silver nitrate and ammonium hydroxide in small increments until all thiosulfate has been reacted. A t first a white or yellowish precipitate will appear, but this darkens and soon has the black color characteristic of the silver sulEde which is being formed. When a moderate excess (about 2 to 5% excess) of the silver nitrate has been added the silver sulfide will have coagulated well and will settle rapidly u on stopping the stirring device. Test the su ernatant liquid &r excess silver nitrate by placing a few drops o A t on a clear glass or black spot plate and adding a drop of 0.1M ammonium thiocyanate solution (white precipitate). When a positive test for excess silver nitrate is obtained, add an excess of about 5% above the amount of the reagent already introduced. The entire treatment with silver nitrate and ammonium hydroxide requires only a short time (5 to 10 minutes) if the pH is properly controlled. KO heating is required, but the pH is critical. After precipitation is complete continue stirring for about 5 to 10 minutes, then remove the electrodes and stirring bar, wiping them with a small piece of filter paper to remove adhering traces of silver sulfide. Place the wiping paper in the solution containing the silver sulfide precipitate, add 2 ml. of 2% ammonium hydroxide solution, and let stand for an additional 10 to 15 minutes. Filter through a KO.1 Whatman paper and wash five or six times with distilled water. Reserve the filtrate for the determination of nitrite and use the precipitate aa a measure of thiosulfate by the following procedure. Thiosulfate Determination. With a jet of distilled water wash the sulfide precipitate back into the beaker in which precipitation took place. Make the volume of the liquid up to about 100 ml. and bring the liquid to boiling. Let cool for about 15 minutes and filter through the filter paper previously used for the original sulfide filtration. Wash with distilled water until the liquid coming through is free from silver, as shown by a negative test with ammonium thiocyanate or hydrochloric acid solution. Transfer the filter paper and silver sulfide precipitate to the beaker in which precipitation took place. Add 10 to 15 ml. of concentrated nitric acid and an e ual amount of distilled water. Heat the mixture to incipient bojing and maintain a t this temerature until all silver sulfide has dissolved (15 to 20 minutes). hen add about 100 ml. of distilled water and boil gently for 30 minutes. Make the total volume up to about 150 ml. and let stand for about 10 minutes, filter the solution through a Whatman Xo. 1 paper, and wash several times with distilled water. [The digestion step may cause the filter paper to become finely subdivided, and the filtration step will then be very slow by the simple filter paper process. Filtration may be hastened by applying suction. For this o eration a Fisher Filtrator in conjunction with a double-wallex slotted porcelain funnel (such as Fisher Scientific Co., No. 10-352, catalog No. 111) may be convenient. -4 still more rapid filtration and washing procedure has been described elsewhere (9).] Add a few drops of ferric ammonium sulfate indicator and titrate with 0.1M ammonium thiocyanate to a faint pink end point.
8
iilthough silver thiosulfate is undoubtedly formed a t the beginning of the titration (white or yellowish precipitate), the stoichiometry of the completed reaction yielding silver sulfide can be simply expressed by the following equation: 2.1gX08
+ Ka2S203+ H20 = Ag2S + 2 N a S 0 3 + H,SOa
Thus 1 mole of S203-- requires 2 moles of Ag+. One milliliter of 0.1M ammonium thiocyanate = 1 ml. of 0 . M silver nitrate = 0.5 ml. of 0.1M S20j-- = 5.606 mg. &Oa--. 5.606 X M X T
% s,o3--
=
TI’
where M is the molarity of the ammonium thiocyanate solution, T is the milliliter of titrant (ammonium thiocyanate solution), and W is the weight of sample in grams. Nitrite Determination. Place 30 ml. of distilled water, 30 ml. of sulfuric acid (1 to 3), and a measured amount of standard ceric ammonium sulfate solution (in moderate excess of that reuired to react with the nitrite present) in a 750-ml. Erlenmeyer 8ask. Transfer the filtrate from the silver sulfide filtration to a
large buret or separatory funnel equipped with an extension tube (about 10 inches of glass tubing connected to the lower end of the buret or separatory funnel by means of rubber tubing) whlch reaches below the surface of the ceric solution in the Erlenmeyer flask. Introduce the nitrite solution into the ceric solution slowly (5 to 10 minutes) and with constant agitation. Rinse the buret or separatory funnel twice with small portions of distilled water, allowing the washings to run into the ceric solution. Titrate the excess ceric ammonium sulfate with standard ferrous ammonium sulfate solution using ferroin as indicator. One milliliter of 0.1M ceric solution = 1 ml. 0.1M ferrous ammonium sulfate = 0.5 ml. 0.1M NOz- solution = 2.300 mg.
NO, -.
%NO*-
=
2.300 X M X ( B
-
T)
W
where M is the molarity of ferrous ammonium sulfate, B is the volume of ferrous ammonium sulfate to titrate the amount of ceric solution used as a blank (no nitrite present), in milliliters, T is the volume of ferrous ammonium sulfate required in the back titration of a sample, in milliliters, and W is the weight of the sample in grams. ( B - T )is a measure of ceric ammonium sulfate consumed by the nitrite in terms of standard ferrous ammonium sulfate. DISCUSSION
The success of these procedures depends heavily on close pH control. Thiosulfate and nitrite may coexist in alkaline solutions for relatively long periods without appreciable interaction, but they react with each other in solutions which are only weakly acidic. Even when acidified with acetic acid to a pH of about 5, a yellow color will develop in 5 minutes in solutions containing nitrite and thiosulfate and the reaction rate increases with decreasing pH. Mineral acids when present in only small amounts will decompose either nitrite or thiosulfate and must therefore be avoided in their analyses. In all adjustments of p H using dilute acetic acid care must be taken to avoid making the solutions less alkaline than pH 7.1. On the other hand, if the pH is appreciably above 8.0 the removal of thiosulfate may be incomplete and the silver sulfide precipitate will fail to coagulate readily a t room temperature and be difficult to filter. Hence the prescribed pH range of 7.1 to 7.5 should be adhered to closely. A reliable p H meter should be employed and its adjustment should be checked with standard buffer solutions. In early experimental work some fairly precise determinations of thiosulfate and nitrite were obtained by using Accutint p H papers and “cut-and-try” amounts of ammonium hydroxide added in one portion before the addition of an excess of silver nitrate. These results were largely fortuitous and did not lead to a method applicable to unknown solutions and describable in simple terms. Upon adoption of in situ electrodes and the pH meter, reliable results were obtained at once and a procedure which is applicable to unknown solutions became apparent. Silver sulfide tends to occlude silver nitrate, but the silver nitrate is effectively removed by boiling with water as outlined in the procedure. The solutions are filtered cool and the precipitate washed with cold water because silver sulfide is considerably more soluble in hot water than in cold water. Silver nitrite does not precipitate n-ith the silver sulfide. Moderately large amounts of chloride do not interfere. Chloride is precipitated by the excess silver nitrate and silver chloride is so insoluble under the conditions of digestion with nitric acid (to dissolve silver sulfide) that very little silver nitrate will be formed therefrom. Only in case chloride is extremely high might some consideration be necessary regarding the possibility of appreciable occlusion of silver nitrate by the silver chloride and subsequent release of some of the occluded salt in the boiling nitric acid step. Gradual addition of the silver nitrate in the original precipitation step in accordance with the directions given will tend to decrease the possibility of a serious amount of such occlusion.
V O L U M E 26, NO. 2, F E B R U A R Y 1 9 5 4
317
Table I. Recovery Values for Thiosulfate and Nitrite of Various Concentrations i n Aqueous Solutions" Thiosulfate (&OS--) Xitrite ( ~ O Z - ) 1Iillimoles RecovPresent, Found, ery, Present, Found, Deviamg. mg. nig. mg. tion % 14.1 9.2 14.0 +o. 1 1 0 1 . 0 9.1 14.1 14.0 101.0 45.9 +o. 1 45.8 14.1 91.4 14.0 +o. 1 1 0 1 . 0 9 1 . 6 28.1 28.0 +0.1 4.6 100.4 4.6 140.3 100.2 +0.3 4.6 4.6 140.0 281.2 +1.1 4.6 4.6 280.1 100 4 140.8 45.8 45.6 f0.8 140.0 100 6 279,s 91.6 91.1 280.1 -0.6 99.8 0 Determined values shown are averages. b Series No. 7 represents sixfold replication; all others were twofold. in Table 11. Series N0.b
-~
Deviation -0.1 -0.1 -0.2 0.0 0.0
0.0 -0.2 -0.5
in Sample
Tested -___
%
sz02--
sop-
98.9 100.2 99.8 100.0 100.0 100.0 99.6 99.5
0.1 0.1 0.1 0.2 1.0 2.0 1.0 2.0
0.2 1 0 2 0 0 1 0 1 0.1 1.0 2.0
Individual values for series 7 are qhown
REAGENTS
-
~
Table 11. Recovery Values for Thiosulfate and Nitrite (Sixfold replication a t one content level of each ingredient in aLqueous solution, 140 mg. &Os-- and 45.8 mg. Pion- present) Thiosulfate Found, Deviation, mg. mg. 140.8 +0.8 139.7 -0.3 141.4 +1.4 141.4 +1.4 140.6 f0.6 141.1 c1.1 Av. 140.83 0.93 Stand. deviation0
Recovery,
0.1% by weight of thiosulfate ion (about 0.01M in thiosulfate) and as much as 18% by weight of sulfide ion. The method is based on removal of the sulfide, polysulfide, and hydrosulfide as hydrogen sulfide and sulfur by bubbling nitrogen through the buffered solution. The procedure described here is not applicable to solutions containing nitrite or sulfite.
Xitrite Recov100.6 99.8 101.0 101.0 100.4 100.8
Found, mg. 45.5 45.6 45.7 45.6 45.7 45.7
100.60
45.63
erya,
%
De\iation, mg. -0.3 -04 -0.3 -0 4 -0.3 -0.3 0 33
Recoveryb,
%
99.3 99 6 99 8 99.6 99.8 99 8 9'3.63
0.46 0 20 For thiosulfate t h e 95% confidence limits for per cent recovery = 100.60 & 0.48 = 100.12 101.08. b For nitrite t h e 95% confidence limits for per cent recovery = 99.65 I. 0.21 = 99.44 99.86. 6 Standard deviation for per cent recovery was based on the equation Standard deviation = d/5(.~ - Z)z/(n 1).
-
-
-
The reliability of the method was tested by applying it to solutions containing varying amounts of sodium thiosulfate and sodium nitrite. Recovery values in the range of 99 to 101% for both ingredients were consistently obtained (Tables I and 11). DETERMIN.4TION O F THIOSULFATE IN AMiMONIURI SULFIDE SOLUTIONS (NITRITE AND SULFlTE ABSENT)
Ammonium a c e t a t , e . ~25% buffer solution, adjusted to pH 7.5 to 7.7 by addition of ammonium hydroxide. Acetic acid, 10% solution. Standard 0.01 to 0.1M iodine solution (the strength chosen depends on Ihe thiosulfate content of the samples). Starch indicator solution. Lead acetate test paper. Litmus paper. Nitrogen, commercial cylinder of compressed gas (oxygenfree). I V
PROCEDURE
Pipet a Sam le of suitable size (2 to 50 ml. depending on thicsulfate content? into a 300-ml. Erlenmeyer flask containing 20 nil. of ammonium acetate buffer solution and about 40 to 50 ml. of distilled water. By means of a glass delivery tube (either a simple glass tube or a coarse fritted glass bubbling stick) bubble nitrogen through the solution a t a moderately vigorous rate and gradually raise the temperature of the solution to 60" to 65' C. Maintain the temperature in this range (thermometer in the solution) and continue the nitrogen treatment until hydrogen sulfide is no longer evolved, as indicated by testing with lead acetate paper. This purging process usually requires 60 to 90 minutes. Wash down the nitrogen delivery tube and thermometer 1%-ithdistilled water and remove them from the flask. Cool the flask to room temperature and filter its contents through a Whab man No. 1 or KO.42 filter paper, choosing the No. 1 if the sulfur appears to be well coagulated and the No. 42 otherwise. The filtrate must be perfectly clear. (In most cases the sulfur nil1 filter easily on the No. 1 paper.) Wash the precipitate four or five times with distilled water. Make the filtrate acidic Kith 10% acetic acid (litmus paper) and then add about 50 ml. excess of acid. Titrate the thiosulfate iodimetrically using standard iodine and starch as indicator. One milliliter 0.131 iodine solution = 1 ml. O . l J 1 S203-- = 11.21 mg. S203--. 11.21 x -11 x T
Well-known methods (4, 5, 10) are available for the determination of thiosulfate in ammonium sulfide solutions, which are satisfactory when the solutions are relatively high in thiosulfate % s203-- = -___ and low in sulfide. Of the various methods investigated by the author, the one employing freshly precipitated cadmium carwhere h1 is the molarity of the iodine solution, T is the volume of titrant used, in milliliters, and W is the weight of sample, grams. bonate (4)proved to be the most efficient and reliable. Kone of the methods are considered suitable for the determination of small amounts of thiosulTable 111. Recovery Values for Thiosulfate Added i n Known Amounts to Several fate in the presence of large Types of Ammonium Sulfide Solutions amounts of sulfide. The reS201-Concn.in moval of sulfide from such a Thiosulfate, hlg. RecovKind In ery, No. Sample solution with cadmium carboSeries of Sample, original Total Total DeviaTotal RepliTested, nate becomes impractical beNo. (NH,)@ RI1. sample Added present found tion &Os--,% cations G./100 hll 1 A 2 . 4 0 , . . . . 2 20 . . . 0.012 cause of the Iaige bulk of 4.8 2 40 .4 69.9 73.9 -0:s 74.7 98.9 2 0.187 2 . 4 1 20 1 3 9 . 8 1 4 1 . 6 142.2 9 9 . 6 A 3 0 . 6 0.711 cadmium sulfide produced and 1.2 1 10 279.6 281.4 A 280,s 100.2 4 +0.6 2.808 difficulties encountered in tryB 27.2 2 20 0 5 0 136 2 7 . 2 B 20 1 3 9 , s 1Iii:o 167: 1 +o: 1 100: 1 6 6 0 835 ing to filter it and wash it 27.2 B 303.7 20 279.6 306.8 -3.1 99.0 2 1.534 7 29.6 C 0 20 2 0.148 8 free from thiosulfate. Using 59.2 40 2 C 126.9 69.9 1%: 1 100:s 0.323 40:s 9 an aliquot of the supernatant 29.6 C 69.9 99.5 98.6 1 20 99.1 -0.9 0,498 10 29 6 20 100.4 169.4 170.0 C 139,s 1 0.847 11 4-0.6 liquid from such a precipita153. .5 139.8 14.8 -1.1 10 99.3 154.6 12 C 1 1.546 100.5 279.6 14.8 C 10 296.0 1 2.944 294.4 13 +l.6 tion is preferable to filtration 41.8 D 20 0 2 0.209 14 but still is not very satisfactory. 41.8 D 20 144.2 186:o 186:6 1&:3 0.930 2 15 +0:6 20.9 D 144.2 163.7 99.2 10 165,l -1.4 1 1.651 16 A simple procedure has been a A = colorless freshly made solution 20% in terms of HzS. B = light-colored (yellow) commercial reagent devised which avoids this probabout 19% in ter& of HB; C = dark-cblored (red brown) com'mercisl reagent, about 18% in terms of HzS: D dark-colored (red brown) very old commercial reagent, about 15% in terms of HzS. lem. It is applicable to solutions which contain as little as
w
ANALYTICAL CHEMISTRY
318 Alternatively the end point may be determined by the method of Knowles and Lowden (7). DISCUSSION
The reliability of the method described was tested by applying it to various kinds of ammonium sulfide solutions to which known amounts of thiosulfate were added and was consistently found to yield recovery values in the range of 99 to 101% (Table 111). In Table 111, series 1, 5, 8, and 14 report the amount of thiosulfate present in the original sample; the values shown were obtained by the procedure described herein. The range for the six reDlicates of series 6 was 99.3 to 101.0% recovery. The standard deviation of these values was 0.70% recovery, based on the equation, standard deviation = d x (z - e)*/(%- 1) (see Table V). In terms of molarity the concentrations of thiosulfate shown in the last column of Table I11 range from 0.017M (series 2) to 0.263M (series 13). Although the concentration of thiosulfate in samples used for the results shown under series 2 was lower than 0.02M, a 40-ml. aliquot yielded a titration of 6.8 ml. of 0.1M iodine solution, a value of sufficient magnitude to represent good precision at this level. Tables IV, V,and VI recapitulate the data of Table I11 Xvith a few additions and deletions. Table IV gives a condensed comparison for five levels of thiosulfate concentration on a single stock (sample C of Table 111)to which varying amounts of thiosulfate were added. Table V gives in detail the sixfold replication data on one sample (sample B of Table 111) a t one level of thiosulfate content. Table VI provides a condensation of typical values for four different sulfide solutions and two levels after introduction of known amounts of thiosulfate.
Table IV. Data for Five Levels of Thiosulfate Concentration (Sample C , a dark colored ammonium sulfide solution) R.O. - -
Concn. SI08-
Series 0 1
2 3 4 5
-
G./lOd hll. 0.148 0.323 0.498 0,847 1.546 2.944
Sample, M1. 20 40 20 20 10 10
Prqsent 1n Aliquot,
Found,
Jlg.
Jig.
l2Q:l 99.5 169.4 154.6 294.4
29.6 129.9 98.6 170.0 153.5 296.0
szo*--
Deviation
Recovery,
$0:8 -0.9 $-0.6 -1.1
100:6 99.1 100.4 99.3 100.5
t1.6
%
Table Y. Six Replications at One Level of Thiosulfate Content (20-hfl. aliquot sample B, 167.0 mg. StOs-- present) SZOS-- Found, Recovery b , llg. Deviationn % 99.3 -1.2 165.8 99.5 -0.8 166.2 99.8 -0.3 166.7 99.8 -0.3 166.7 100.8 168.3 4-1.3 101 .o +1.7 168.7 Av. 167.1 0.93 100.03
-
LI Standard deviation = d Z ( z - e ) % / ( % 1) = 0.70% recovery. b 95% confidence limits for per cent recovery = 100.03 & 0.73 = 99.30100.76.
By bubbling oxygen-free nitrogen through the solutions, sulfide, polysulfide, and hydrosulfide are completely removed, partly as hydrogen sulfide and partly as sulfur. Sulfite is not completely removed by the nitrogen treatment and hence interferes with the iodimetric determination of the thiosulfate. Ammonium sulfide solutions exposed to air or oxygen undergo oxidation with the production of thiosulfate. Hence, air should not be substituted for the oxygen-free nitrogen specified for the purging operation. Carbon dioxide is also undesirable for this step, even though made oxygen-free, because of its acidic character.
Table VI. Comparison of Results on Four Ammonium Sulfide Solutions saos - Sample Series A 0 1 2 B 0 1
c D
2 0
1
2 0 1 2
Concn., Present G. in SzOa-S I O S - - / Sample, Aliquot, Found, 100 311. hI1. 31g. hlg. 0.012 20 2.4 20 142:2 141.6 0,711 281.4 2.808 10 280.8 0.136 20 27.2 20 167:O 167.1 0,835 20 306.8 303.7 1.531 0.148 20 ... 29.6 20 99.5 98.6 0,498 10 294.4 296.0 2.914 0.209 20 41.8 186.6 0,930 20 lS6:O 163.7 1.651 10 165.1
Deviation
...
-0.6 +0.6
.,.
+O.l -3.1
...
Reoovery, %
...
99.6 100.2 10O:l 99.0
...
-0.9 +1.6
99.1 100.5
+O:6
100:3 99.2
-1.4
Although the method has been found to be highly satisfactory for all the solutions tested by the author, it should be noted that in applying the procedure to ammonium sulfide solutions whose history is unknown, a test for reliability would be desirable. Such a test may be made by first determining thiosulfate content of the original sample and then that of a sample to which a known amount of thiosulfate has been added. A few preliminary experiments indicate the method could be extended to the analysis of sodium or potassium sulfide solutions. EFFECT OF NITRITE ON DETERMINATION OF THIOSULFATE IN AMMONIUM SULFIDE SOLUTIONS Attempts to determine thiosulfate and nitrite in ammonium sulfide solutions by combining the nitrogen-purging portion of the procedure given in the preceding section of this paper Ivith the silver nitrate separation and subsequent steps for thiosulfate and nitrite of the first part of the paper were successful for both thiosulfate and nitrite when applied immediately to freshly prepared ammonium sulfide solutions (prepared by passing hydrogen sulfide into cooled ammonium hydroxide solution). Recoveries in the range 99 to 101% for thiosulfate and 98 to 99.5% for nitrite were obtained on such solutions when known amounts of the two ingredients were added. These solutions contained very little or no polysulfide and were colorless or only faintly yelloiv. When applied to yellow or brown ammonium sulfide solutions containing appreciable amounts of polysulfide, low recoveries of added nitrite were obtained and recoveries of added thiosulfate were a little more variable than those obtained on fresh solutions. The darker the color of the solution the greater became the discrepancies in the nitrite recovery values. It has been reported (8) that in ammoniacal solutions the reaction of nitrite ion with sulfides or polysulfides does not produce thiosulfate. This finding, which is not in agreement with some earlier publications (6, l l ) ,was confirmed by experiments conducted during the development of the procedures described herein. Table VI1 is illustrative of values obtained on freshly prepared ammonium sulfide solutions. Table VI11 presents data obtained with three commercial solutions of ammonium sulfide. The following description of the combined procedures for determining thiosulfate and nitrite in ammonium sulfide solutions was found to be reasonably accurate with regard to thiosulfate on all solutions tested, but for nitrite the method is considered applicable to very fresh, light-colored samples only. Sulfite is an interference. A qualitative test for this ion is given in the last section of the paper. PROCEDURE
Pipet a sample of suitable size (2 to 50 ml. depending on thiosulfate and nitrite content) into a 300-ml. Erlenmeyer flask containing 20 ml. of ammonium acetate buffer solution and about 40 to 50 ml. of distilled water. By means of a glass delivery tube
V O L U M E 2 6 , NO. 2, F E B R U A R Y 1 9 5 4 Table VII.
Recovery Values for Thiosulfate and Nitrite in Fresh Ammonium Sulfide Solutions Thiosulfate
In orig. sulfide, mg.
319
Total mg.
presents,
Found, mg.
Deviation, mg.
Recorery, % of total
Nitrite Deviation, mg.
Present, Found, mg. mg.
77.7 78.6 $0.9 101.1 92.0 91.4 83.2 83.6 +0.4 100.5 92.0 91.6 5.5 149.9 148.6 -1.3 99.1 46.0 45.5 154.2 -1.2 46.0 99.2 45.7 I!.! 155.4 a.o 294.3 295.5 +1.2 100.4 23.0 22.7 11.0 299.8 299.8 0.0 100.0 23.0 22.7 0 Sum of milligrams present in original sulfide aliquot and milligrams added. b Approximate values. 5.5 11.0
Table VIII.
%
99.4 99.6 98.9 99.3 98.7 98.7
Millimoles in Sample Testedb
KOZ-
&OS--
0.7 0.7 1.3 1.4
2.6 2.7
2.0 2.0 1.0 1.0 0.5 0.5
Recovery Values for Thiosulfate and Nitrite i n Various Commercial Ammonium Sulfide Solutions Thiosulfate
In orig. Sulfide sulfide, Typea mg. B 3.5 7.0 3.5 7.0
-0.6 -0.4 -0.5 -0.3 -0.3 -0.3
Recovery,
Total presentb, mg: 148.2 151.7 65.8 69.3
Found, mg. 148.0 147.3 65.8 67.6
Nitrite Deviation, mg. -0.2 -4.4 0.0 -1.7
Recovery, 70 of total 99.9 97.1 100.0
97.5
Present, mg. 22.8 22.8 45.5 45.5
Found, mg. 22.3 21.9 44.0 43.6
C
hlillimoles c
~ ~ tion, mg. -0.5 -0.9 -1.5 -1.9 -3.6 -10.5
R ~ ~ i in~ Sample ~ ~ - ~ ery,
Tested
% ’
S,Oa--
97.6 96.0 96.7 95,9
1.3 1.4
0 6
0.6
NO,-
0.5 0.5 1.0 1.0
29.2 173.4 174.4 +I 0 100.6 23.4 19.8 86.5 1.5 0 5 23.4 12.9 58.4 202.6 +3.9 101.9 55.2 1 8 0.5 206.5 -6.0 46.8 40.8 89.1 29.2 101.3 98.6 -2., 97.3 0 9 1.0 46.8 35.9 -10.9 58.4 130.5 128.9 -1 6 98.8 78.4 1.2 1 0 169.6 -1.5 99.1 23.4 15.8 -7.6 D 26.4 171.1 67.5 1.5 0.5 11.5 -21.9 97.6 23.4 49.1 52.9 197.6 192.8 -4.8 1.8 0.5 26.4 98.8 94.7 -4.1 95.9 46.8 40.3 -6.5 861 0.9 1.0 52.9 124.6 +8.1 106.5 46.8 22.1 -24.7 132.7 47.2 1. I 1.0 a B = light yellow-colored commercial reagent ammonium sulfide, C = brown-colored commerciel reagent ammonium polysulfide, and D = very dark brown-colored commercial reagent ammonium polysulfide. b Sum of milligrams present in original sulfide aliquot and milligrams added. C Approximate values.
bubble oxygen-free nitrogen through the solution a t a moderately vigorous rate and gradually raise the temperature to 60” to 65’ C. Maintain the temperature in this range (thermometer in the solution) and continue the nitrogen treatment until no further hydrogen sulfide is evolved, as indicated by testing with lead acetate paper. Wash down the nitrogen delivery tube and thermometer with distilled water and remove them from the flask. Cool the flask to room temperature and filter its contents into a 600-ml. beaker through a Whatman KO.1 or KO.42 filter paper, choosing the No. 1 if the sulfur precipitate appears to be n-ell coagulated and the Xo. 42 otherwise. The filtrate must be completely clear. Wash the precipitate four or five times with distilled water. The filtrate is then analyzed for thiosulfate and nitrite by the procedure described in the first section of the paper, beginning a t the second sentence “Insert a stirring bar , . . etc.” As previously noted, air or carbon dioxide should not be substituted for the oxygen-free nitrogen specified in the procedure for the purging step. QUALITATIVE TEST FOR SULFITE IN PRESENCE OF THIOSULFATE AND IN PRESENCE O F BOTH THIOSULFATE AND NITRITE The qualitative tests for sulfite usually applied ( 4 ) fail in the presence of thiosulfate. A colorimetric test which detects a very amount Of in the presence Of a large amount of thiosulfate, is described below. The test is not so sensitive in the presence of ammonium acetate buffer as in its absence, but is still to be considered highly sensitive, provided care is taken with regard to the volumes of sample used and the volumes and concentrations of reagents added. Procedures are described as applied to buffered solution only; if the sample taken does not already contain buffer, the buffer is added so that no modifications will be required. This test is, therefore, applicable to samples from which sulfide has been removed by the nitrogen-purging and filtration steps of the second part of this paper. When nitrite is present an additional reagent is required, but with this addition the test can be made to function well for solutions from procedures described immediately above and in the first section of the paper. The test is first described for application in the simpler case, nitrite absent, and
then for the more complex case when nitrite is present, although it is likely that little need for the latter type of test will arise. Nitrite reacts with polysulfide and hence is not likely to be found in solutions from samples discussed in the third section of this paper, except in the case of very freshly prepared ammonium sulfide solutions, and as in this case the history of the sample would be known, the presence or absence of sulfite would not be The - likely to be unknown. solut’ions from operations discussed in the first and third sections of this paper with which the author worked were all known to be free of sulfite, but perhaps the qualitative test given here will prove useful in some cases when both t h i o s u l f a t e and nitrite are present‘ REAGENTS
Acetic acid, 50% solution. A m m o n i u m acetate, 25% buffer solution, adjusted to pH 7.5 to 7.7 by addition of ammonium hydroxide. Ammonium hydroxide, about 14% solution (dilute, 1 to 1). Bromothymol blue, indicator, 0.01 % solution in 0.002M sodium hydroxide. Hydrochloric acid, dilute, 1 to 1. Selenious acid, 20% solution. Sulfamic acid, C.P. crystals (required only when nitrite is present). When Nitrite Is Absent. Place 10 ml. of sample in a small beaker or flask and adjust the pH to 7.1 to 7.3 using acetic acid or ammonium hydroxide and bromothymol blue indicator (green color, neither blue nor yellow a t this range) or a suitable narrowrange pH paper. Avoid increasing the volume of solution any more than necessary during this pH adjustment step. Add 2 ml. of ammonium acetate buffer unless this material is already present in proper amount. Ten-milliliter samples from the nitrogen-purging step of other parts of the paper will contain the proper amount of buffer if the solution is made to a total volume of 100 ml. before sampling (20 ml. of buffer p-ere added a t an earlier stage). Then add the following reagents in the sequence given: 5 d.of selenious acid solution and 5 ml. of dilute hydrochloric acid. A red color indicates the presence of sulfite. Allow not less than 5 minutes nor more than 10 for the color development. Orange or reddish colorations will eventually develop if thiosulfate is present (because of sulfur dioxide formation) but not in the time specified and under the prescribed conditions. When Nitrite Is Present. Use a 10-ml. sample as in the previous procedure; adjust the pH and buffer content as before. Then add the following reagentsin the sequence given: 5 ml. of selenious acid solution, 2 grams of sulfamic acid, and 5 ml. of dilute hydrochloric acid.. Observe for red color formation within the time limits Of to lo minutes.
’
DISCUSSION
In the absence of nitrite the qualitative test outlined here will detect as little as 0.2 mg. of sulfite (SO$--) in the presence of as much as 100 mg. of thiosulfate. The procedure in the presence of nitrite will detect as little as 0.5 mg. sulfite in the presence of as much as 100 mg. of thiosulfate and 50 mg. of nitrite. These limits are believed very conservative; the threshold for sulfite could probably be made lower and the amounts of thiosulfate and nitrite tolerated made much higher. Excellent reproducibility on threshold amounts of sulfite and negative tests on blanks
320
ANALYTICAL CHEMISTRY
containing all reagents except the sulfite are obtained, when the directions given are adhered to closely. Larger amounts of more dilute solutions do not yield satisfactory results. It is suggested that in using the test both negative and positive control samples be run simultaneously with the unknowns. By negative control is meant a solution containing roughly the same amounts of thiosulfate, nitrite, etc., as the unknown, and by positive control is meant the same solution as the negative except that an amount of sulfite slightly above the threshold has been addedfor example, about 1 to 2 mg. of sulfite. By this means the distinction between solutions which contain sulfite and those which do not will be readily apparent. If the hydrochloric acid is added before the selenious acid to a solution containing thiosulfate but not sulfite, sulfite will be formed very rapidly from the thiosulfate and a false positive test will, therefore, be obtained. In the test as modified by the addition of sulfamic acid, the latter reagent effectively prevents the interference from nitrite. Without this reagent both positive and negative controls yield ungatisfactory results, the negative ones often rapidly yielding an orange color and the positive ones not showing appreciably more red color development than the negative. The qualitative tests for sulfite were developed to meet the specific need for such tests in connection with the analytical work on thiosulfate, nitrite, and sulfide solutions. They were based on the knowledge that: sel’enious acid reacts with sulfite to give a red color (selenium) and this reaction has been used in
the quantitative determination of sulfite (11) and that sulfamic acid is a reagent which reacts with nitrite (9). ACKNOWLEDGMENT
The author is grateful to E. St. Clair Ganta, Analytical Chemistry Branch head, for encouragement and suggestions in support of this work. This paper is published with the permission of F. W. Brown, technical director of the U. S. Naval Ordnance Test Station. LITERATURE CITED
(1) Bennett, H., and Harwood, H. F., Analyst, 60, 677 (1935). (2) Brasted, R. C., ANAL.CHEY.,24, 1111 (1952).
(3) Cool, R. D., and Coe, J. H., IND.ENG.CHEM.,SXAL. ED.,5 , 112 (1933). (4) Furman, N. H., ed., "Scott's Standard Methods of Analysis,” 5th ed., New York, D. Van Kostrand Co., 1939. (5) Griffin, R. C., ed., “Technical Methods of Analysis,” 2nd ed., New York, McGraw-Hill Book Co., 1927. (6) Khmelnitzkaya, I., and Verkhovskaya, A , Anilinokrasochnaya Prom., 4, 27 (1934). (7) Knowles, G., and Lowden, G. F., Analyst, 78, 159 (1953). (8) Merrow, R. T., Cristol, S. H., and Van Dolah, R. W., J . A m , Chem. SOC.,in press. (9) Pierson, R. H., AXAL.CHEM.,25, 1939 (1953). (10) Vinogrodov, A. V., and Dubova, A. O., Zavodskaya Lab., 11, 282 (1945). (11) Y o s t , D. bl., and Russell, H., “Systematic Inorganic Chemistry,” pp. 66, 332, S e w York, Prentice-Hall, Inc., 1946. RECEIVEDfor review January 31, 19.53. Accepted October 19, 1953. Presented in part before the Division of Analytical Chemistry a t the 123rd CHEMICAL SOCIETY, Los Angeles, Calif. Meeting of the AMERICAN
Accuracy of Determination of Hydrogen Peroxide by Cerate Oxidimetry EVERETT C. HURDIS and HENDRIK ROMEYN, JR. General Laboratories, United States Rubber Co., Parraic, N. J.
The accuracy of the cerate-hydrogen peroxide titration has been studied by comparison with a new absolute (gravimetric) method of hydrogen peroxide analysis. Results indicated a favorable comparison between the two methods. Additional information on the accuracy of the cerate-hydrogen peroxide titration was obtained by comparison between cerate and permanganate titrations. No significant difference in results by the cerate and permanganate methods was found. This further substantiates the accuracy of the cerate
T
HE determination of hydrogen peroxide concentration by
cerate titration has certain advantages of convenience over the usual permanganate titration method. However, although a number of authors (1, 2. 4, 6, 11) have reported successful use of the hydrogen peroxide-cerate titration, their work has not included any direct determination of the accuracy of the titration by comparison with an absolute method df analysis. The resulting doubt as to the complete reliability of the cerate method has tended to hinder its adoption for general use. It has, therefore, been the primary object of this work to investigate the accuracy of the cerate method by comparison with the results of an absolute method of hydrogen peroxide analysis. Huckaba and Keyes (7) have reported a careful study of the accuracy of the permanganate titration method for hydrogen peroxide determination. The work of these authors indicated a favorable comparison between the permanganate method and an absolute (gasometric) method. It, therefore, seemed desirable to include, aa a further object of the research reported here, an intercomparison between the permanganate and the cerate hydro-
titration, as previous investigators have reported good agreement between permanganate titrations of hydrogen peroxide and an absolute (gasometric) method of analysis. Intercomparison between hydrogen peroxide titrations by several different standard cerate solutions indicated that the accuracy was not affected by changing cerate concentration, by substituting sulfatocerate for nitratocerate, or by using reagent grade rather than primary reference standard grade cerate as oxidant.
gen peroxide titrations. An additional, although indirect, check on the accuracy of the cerate titration would thus be obtained. In addition to the above objects, it was decided to investigate the consistency of the cerate-hydrogen peroxide determinations by studying the comparative accuracy of titrations by several different cerate solutions. The variations to be investigated included changes in standard solution concentration, changes in cerate ion-e.g., use of sulfatocerate rather than nitratocerate solution-and change in the purity of the cerate salt used. For the purpose of investigating the accuracy of the ceratehydrogen peroxide titration, a new absolute method for the analysis of aqueous hydrogen peroxide solutions wya_q devised. This method (here termed the indirect absolute method) resembles the gasometric method of Huckaba and Keyes ( 7 ) in that it depends on decomposing the hydrogen peroxide according to the equation catalyst 0 2 2H202 -2H2O
+
However, while the gaeometric method requires an apparatus
