Determination of Tin in Inorganic and Organic Compounds and Mixtures

---

The decomposition temperatures for all the rare earth metal oxalates are given in Table 111.

of the Davison Chemical Co. and the Lindsay Chemical Co. for the samples of thulium and lutetium oxides.

ACKNOWLEDGMENT

LITERATURE CITED

The author would like to thank James R. Slagle for the preparation of the methyl oxalate, and Richard ill.Mandle

(1) Blatt, A. H., “Organic Syntheses,” Coll. Vol. 2, p. 414, Wiley, New York,

1943, (2) Caro, P., Loriers, J., J . recherche5

centre natl. recherche sei. Labs. Bellevue j9,lo7 (1958). (3) Vickery, R. C., “Chemistry of Lanthanone,” pp. 250-1, Academic Press, Kew York, 1953. (4) Wendlandt, W. W., ANAL. CHEM. 30,56 (1958). (5) Zbid., P, 58.

RECEIVEDfor review April 28, 1958. Accepted October 7 , 1958.

Determination of Tin in inorganic and Organic Compounds and Mixtures MARIE FARNSWORTH and JOSEPH PEKOLA Research laboratory, Metal & Jhermit Corp., Rahway, N. J.

b The standardization of potassium iodate for the volumetric determination of tin and its determination in inorganic and organic compounds have been critically studied. Various rnethods for destroying organic matter, if present, are given. Inaccuracies inherent in certain methods are pointed out and a method is given for the determination of high tins colorimetrically after combustion in a Parr bomb. Copper interference is eliminated by using hypophosphorous acid as the reductant; in other cases iron powder and nickel are recommended.

I

volumetric determination of tin it is customary to titrate the reduced tin with a standard iodate solution with starch for the indicator. Potassium iodate is a primary standard and for most work it is satisfactory to use its theoretical equivalent, although this is not true for tin. Here it is customary to standardize the solution against a known amount of tin, and the tin equivalent thus found is higher than the theoretical value. This is attributed partially to the oxygen dissolved in the potassium iodate solution, and it would be difficult to remove this dissolved oxTgen completely. The experience gained in this laboratory in determining both stannous and total tin in very pure stannous salts should throw further light on the subject. While a number of metals and other reducing agents have been recommended for use in the volumetric determination of tin, the work reported here has been confined principally to nickel or to a combination of iron and nickel. Using nickel reduction alone in the analysis of very pure stannous compounds gave a value for stannous tin which was higher than the value for total tin. This is N THE

410 *

ANALYTICAL CHEMISTRY

clearly impossible without a n analytical error. Such results indicate that reduction with nickel alone is incomplete, although reproducible results are obtained. Incomplete reduction would give a tin equivalent for the iodate solution that is too high. When total tin is determined, the reduction is also incomplete to the same extent and correct values for tin are obtained. When stannous tin is determined and no reduction is involved, the high tin equivalent for the solution gives a value for stannous tin that is too high. To check this, a combination of iron and nickel was next used for the reduction. The following data show that reduction with nickel alone is incomplete, resulting in a higher tin equivalent for the potassium iodate solution: 0.006004 and 0.006054 gram per ml. for nickel Table I.

Total Tin in Inorganic Compounds

Total Tin, 7 0 SnEq. Sn Eq. from from hTi Fe Si Gravi- reducreducmetric tion tion

+

3 1 s SnSOd 54 74 931%’SnCl2’ 92W SnSOa 26V SnCzO4 57 41 Table II.

31N 93W 92W 26V

54.74 62.41 54 49 57.39

54 73 62 40 54 51 57.40

Stannous Tin in Inorganic Compounds

SnSO4 SnC12 SnSOl SnCaO4

Stannous Tin, % Sn Eq. SnEq. from from Fe Ni Xi reducreduction tion

+

Theoretical Sn++ Value

54.88 62.90 54.73 57.51

54.55 62.40 54.30 57.12

55.27 62.60 55.27 57.42

reduction and 0.005956 and 0.006010 gram per ml. for the corresponding iron and nickel reduction. Table I shows that nickel reduction is satisfactory when only total tin is sought. Table I1 shows that nickel reduction alone is not satisfactory for the standardization when stannous tin is being determined. It can be seen from Table I1 that the tin equivalent obtained by nickel reduction actually gives stannous values higher than theoretical for the very pure stannous chloride and stannous oxalate. Aside from the values for the tin equivalent thus obtained, other factors enter into the selection of a reducing agent. Kickel reduces only in a hot solution and it does not need to be removed when the solution is cooled to room temperature or loner for titration. Metallic iron must be entirely removed before the solution can be titrated, for it reduces in both the hot and the cold. Using up a reductant completely and not allowing the solution to reoxidize introduces some mechanical difficulties, when dealing with more than a few samples at one time. The combination of iron and nickel is to be preferred to iron alone. Stannous tin is very easily reouidized and must be protected from the atmosphere a t all times. Any water or acid used in the determination of stannous tin must be thoroughly boiled to remove all air. The number of samples involved sill, to some extent, determine the mechanical devices used. If a number of stannous tin samples are to be analyzed concurrently the hydrochloric acid (1 to 2) required should be prepared a t one time. DETERMINATION OF TOTAL AND STANNOUS TIN IN INORGANIC COMPOUNDS

Solutions. Standard potassium io-

d a t e solution (approximately 0.liV). For each liter of 0.1N solution required, dissolve 3.6 grams of potassium iodate in 200 ml. of water containing 1 gram of sodium hydroxide and 10 grams of potassium iodide. When the solution is complete, dilute to 1 liter. Starch solution (1%). Prepare fresh each day. Potassium iodide solution (10%). Boil 100 ml. of mater for 5 minutes, cool, add one pellet of sodium hyi droxide and 10 grams of potassium iodide, and stir to dissolve. PreDare fresh daily. Kickel Coil. Roll into a loose coil a 6 X 3 X 0.015 inch lsiece of nickel sheet. Before using for- the first time, clean it in petroleum ether and then place i t in hydrochloric acid (1 to 2) and boil for 5 minutes. Remove and rinse with water. Clean, each time it is used, with hydrochloric acid. Iron powder. Hydrogen reduced. Standardization of Potassium Iodate. Weigh and transfer three samples: of pure tin, each weighing 0.2500 to 0.2550 gram, into 500-ml. Erlenmeyer flasks and add 100 ml. of hydrochloric acid. Allow to stand a t room temperature until completely dissolled. When dissolved, add 180 ml. of kyater and 10 ml. of sulfuric acid, and swirl to mix. Add a nickel coil and 5 grams of iron powder, swirl to mix, and wash down the sides of the Erlenmeyer flask with water. Stopper the flask with a one-hole rubber stopper fitted IT ith a 'j4-inch glass tubing outlet (bent near the stopper a t an angle of approxinintely 45' and toward the end a t an angle less than 90") Lvhich is t o be immersed later into a beaker containing a saturated solution of sodium bicarbonate (dissolved in hot water in order to displace any air present in the water with carbon dioxide obtained by decomposition). Place the stoppered flask on a hot plate, bring to a boil, and boil gently for 20 minutes. Remove and ininierse the outlet tube in the sodium bicarbonate solution, taking care that no air is allowed to enter. Place the flask in a cooling bath and allow it to cool to room temperature or lower. Remove the stopper and quickly add a few pieces of dry ice, 5 ml. of starch solution, and 5 nil. of potassium iodide (1070). Titrate with the potassium iodate solution which is to be standardized. Procedure. TOTALTIN. Weigh a 0.4- t o 0.5-gram sample in an iodine cup or other suitable container. Place the container n-ith the sample in a 500-nil. Erlenmeyer flask. Add 100 ml. of hydrochloric acid and 100 ml. of water and swirl the solution to dissolve the sample. If the sample is not soluble in hydrochloric acid, use any suitable means to effect solution. Upon solution add an additional 100 ml. of water and 10 ml. of sulfuric acid. Reduce (nickel coil and iron powder) and titrate as described under standardization. STANNOUS TIN. Place 200 ml. of water and 100 ml. of hydrochloric acid in a 500-ml. Erlenmeyer flask. Add a few glass beads, place on hot plate, bring

to a boil, and boil gently for 10 minutes. Remove from the hot plate and attach immediately to a source of carbon dioxide which bubbles through the solution through a glass tube (bent a t a right angle) which is inserted through one hole of a two-hole rubber stopper. The rubber stopper fits tightly into the Erlenmeyer flask and the second hole allows the carbon dioxide to escape. This second hole should be sufficiently large to allow the carbon dioxide to escape when the buret tip is inserted in it. The gas should be passed through the acid for 5 minutes. Weigh a 0.4- to 0.5-gram sample in an iodine cup or other suitable container. Without shutting off the source of carbon dioxide, lift out the Iubber stopper just high enough to drop the sample into the solution, recap, swirl to dissolve, and cool in a cooling bath to room temperature or lower. Introduce a few milliliters of starch solution and 5 ml. of potassium iodide (lo'%), insert the tip of the buret through the second hole in the rubber stopper, and titrate with standard potassium iodate solution.

Table

111.

Determination of Presence of Copper

Tin

in

(Sn-Cu alloy) Tin Found, % Copper removed by Copper not electrolysis, removed, Sn reduced Sn reduced Ni with Hap03 Sample with Fe

+

1

57.91 -.

57.93

2

58.65 46.55

46.53

3 Table

58 67

IV.

Determination of Tin in Presence of Copper

(Bronze plating solution) Tin Found, Grams/Liter KCN Rochelle salt destroyed, copper 70 ml. HC1 removed by 50 ml. H20 Bronze electrolysis, added, Sn reduced SohSn reduced with Fe Xi with H3P03 tion"

+

+

+

1 2

3

(1

13 05 17.00 11 55

13.05 I7 00 11 55

5-ml. sample.

Elimination of Interference by Copper. One advantage of the volu-

metric method for the determination of tin is that it is subject to interference by relatively few elements. This is not true of the gravimetric determination. Copper is an element. often accompanying tin, which interferes in the volumetric method. Accurate results can be obtained by first separating the tin from the copper, but the methods available for this separation are often cumbersome and time-consuming. A method of reduction, first proposed by Evans (2) and later supplemented by Kinnunen and hlerikanto (9), Rapinvolves the port (Is),and Goldberg (6), use of hypophosphorous acid to reduce the tin and copper, mercuric chloride to catalyze the reaction, and ammonium thiocyanate, after reduction, to eliminate the interference from copper. The procedure has been tested exhaustively in the laboratory and found to be both accurate and rapid. The tin equivalent of 0.005891 for the potassium iodate was obtained by dissolving 200 mg. of pure tin in 70 ml. of hydrochloric acid, adding 50 ml. of water followed by reduction and titration as described below. This was repeated with the addition of 700 mg. of cupric chloride dihydrate. The tin equivalent was found to be 0.005889; hence there is no interference from copper. The equivalent found for the potassium iodate solution after reduction with hypophosphorous acid is only slightly higher than the equivalent found after the iron-nickel reduction. Both are interchangeable for routine work. For accurate work, especially where the tin content is high, the solution should be

standardized with tin reduced with hypophosphorous acid. Three copper-tin alloy samples were dissolved and the tin was determined in both the absence and presence of copper (Table 111). In addition, three bronze plating solutions containing tin, copper, potassium cyanide, potassium hydroxide, and Rochelle salt were analyzed. In one instance, the copper was removed and in the other the tin was determined in the presence of the copper (Table IV). PROCEDURE FOR TIN-COPPER ALLOYS

Weigh and transfer a 1.5-gram sample to a 250-ml. volumetric flask, add 25 ml. of hydrochloric acid, and immerse in an ice bath. After 2 minutes remove, add 5 ml. of hydrogen peroxide (30%), and swirl the solution for a few minutes. Cool to room temperature, add 5 ml. additional hydrogen peroxide (30%), and swirl as before, to dissolve completely. Repeat if necessary, dilute with water, cool, make up to volume, and mix well. Pipet 50 ml. of solution into a 500-ml. Erlenmever flask. and add 70 ml. of hvdrochlork acid, 7 ml. of hypophosphoro;s acid (50%), 5 ml. of mercuric chloride solution (6%), and a few boiling chips. Stopper the flask with a one-hole rubber stopper fitted with a '/,-inch glass tubing outlet (bent near the stopper at an angle of approximately 45" and toward the end a t an angle less than 90") which is to be immersed later in a beaker containing a saturated solution of sodium bicarbonate and place it on a hot plate. Bring solution to a boil and allow it to boil for 5 to 6 minutes. At the end of the boiling period, immerse the outlet tube in a saturated solution of sodium VOL. 31, NO. 3, MARCH 1959

411

bicarbonate, remove from the hot plate with the outlet tube always immersed in the sodium bicarbonate solution, and cool to a t least 20" C. When cool, remove stopper, and immediately add a few pieces of dry ice, 10 ml. of ammonium thiocyanate solution (50%), 125 ml. of cold, previously boiled slightIy acid water, 5 ml. of potassium iodide solution (lo%), and 5 ml. of starch solution (I%), and titrate with the potassium iodate solution. To prepare the acidified water, place 900 ml. of water in a 1-liter wash bottle, add 20 ml. of hydrochloric acid and a few glass beads, and place on a hot plate. Boil for 15 minutes, iemove the flask, and stopper with a one-hole rubber stopper fitted with an outlet tube. Immerse the outlet tube in a saturated solution of sodium bicarbonate and cool to a t least 20" C.

Tin in Organotin Compounds. Determination of tin in organotin compounds or in the presence of extraneous organic matter has been discussed to a limited extent in the chemical literature. In 1910 Pfeiffer (I,!?) and in 1928 Kocheshkov ( I O ) described gravimetric methods for the determination of tin in organotii compounds. Nitric and sulfuric acids were used in both methods and the tin was weighed as stannic oxide. Gilman (4, 6 ) later modified the method by using bromine in carbon tetrachloride to form the less volatile bromides. In the latter paper he recommends the use of sulfuric acid alone. Strafford (14) used hydrogen peroxide and sulfuric acid to decompose organic matter and determined tin by an indirect colorimetric procedure (molybdenum blue). Belcher, Gibbons, and Sykes (1) in a review article on the determination of metals in organic compounds, give only four references (4, 7, 8, 11) to tin; the reference to Hallett (7) consists of one sentence on the work of Holtje (8). Many of the methods reported are inconvenient and, in some cases, subject to considerable analytical error. Holtje (8) describes a titrimetric method for low tin content. Today, a satisfactory colorimetric method is available and is to be preferred (3). The other references advise gravimetric methods. It would be possible to carry out such gravimetric determinations in connection with a research program where only a few results are required, but in connection with a large research project or the production of organotin compounds, it would be extremely inconvenient and time-consuming. Part of the inconvenience could be removed by completing the analyses volumetrically by titration with iodate. This would avoid the evaporation of considerable quantities of sulfuric acid and eliminate errors arising from the presence of small amounts of nonvolatile impurities. In the \\-et oxidation of organotin compounds using nitric and sulfuric acids, a 412

ANALYTICAL CHEMISTRY

Table V. Determination of Tin in Organotin Compounds by Various Methods

+

Gravi- Volu- H&30a HNOa metric metric as Sample (4) ( 4 ) Recommended 1

2 3 4

36.39 36.07 37.80 37.86 38.28 34.08

37.25 36.10 37.84 37.29 36.76 37.97 47.60 47.48

37.16 37.28 38.59 38.45 39.05 39.09 47.75 47.81 ~~~

Table VI. Determination of Tin in Organic Compounds

Compound Dibutyl diphenyltin Dibutyltin diacetate Dibutyltin dichloride Dibutyltin dilaurate Dibutyltin di-2-ethyl hexoate Dibutyltin oxide Dimethyltin dichloride Tributyltin chloride Tetrabutyltin Tetralauryltin Tetraphenyldin Triphenyltin chloride RS-20 RS-31

Tin, % Assay Theory 30,82 33.88 39.12 18.81

30.66 33.82 39.07 18.80

22.69 47.68 53.93 36.25 34.10 14.68 27.80 30.75 16.92 18.47

22.87 47.69 54.04 36.47 34.19 14.91 27.79 30.79 16.90 18.60

considerable excess of nitric acid is necessary a t the beginnlng of the digestion, especially with organotin compounds containing chlorides. Many organotin compounds with or without chlorides are somewhat volatile and may be lost during evaporation with acids; however, a considerable excess of nitric acid prevents such loss. The use of 50 ml. of fuming acid is to be preferred, but is not necessary. Many analysts in the authors' laboratory use the ordinary acid, and the results are satisfactory. The solutions required and the standardization are the same as those indicated for inorganic compounds. PROCEDURE FOR TIN IN ORGANOTIN COMPOUNDS

Weigh a 0.4- to 0.5-gram sample in an iodine cup or other suitable container. Place the container with the sample in a 500-ml. Erlenmeyer flask, add 50 ml. of nitric acid, swirl to mix, and immediately add 20 ml. of sulfuric acid. Swirl to mix, add a few glass beads and 1 ml. of perchloric acid (70 to 72%), swirl t o mix, and place on an electric hot plate set a t high heat. Swirl dichloride or other solid compounds until completely removed from the iodine cup before placing on hot plate. Allow the solution to boil dovvn until heavy fumes of sulfur trioxide appear and fume for 10 minutes. Grip the flask with a pair of tongs and swirl over

the free flame of a Meker burner for 1 to 2 minutes. The solution should be colorless a t this point. If the solution turns brown or black, organic matter is still present. In the latter event add perchloric acid (70 to 72%) dropwise, until all the organic matter is destroyed. Allow the solution to cool, wash down the sides of the flask with 30 ml. of water, place it on a hot plate, and evaporate the solution until heavy fumes of sulfur trioxide are obtained. Cool, repeat the addition of water, and again evaporate until fumes appear. Cool, add 80 ml. of water and 100 ml. of hydrochloric acid, then 100 ml. of water. Reduce (with nickel coil and iron powder) and titrate as described under standardization. The method recommended by Gilman

(4) and that recommended above have

been compared for a few compounds (Table V). The first two samples are mixtures of dibutyltin dichloride, tributyltin chloride, and monobutyltin trichloride. The third sample is practically pure dichloride. The fourth is dibutyltin oxide and is free of chloride. The theoretical tin value for dibutyltin dichloride is 39.07%, that for dibutyltin oxide is 47.697,. In addition, the purity of the dibutyltin oxide was established by carbon-hydrogen determinations: carbon 38.49% and hydrogen 7.14%, both in good agreement with the theoretical values-38.60 and 7.29%, respectively. Column 1 follows Gilman (4). Column 2 uses Gilman's method for destroying organic matter, but the determination was completed volumetrically. Column 3 gives the results by the recommended method. This method has been used in the authors' laboratory for a number of years and has given satisfactory results on thousands of samples in the hands of both chemists and technicians. In these samples the tin has varied from less than 1 t o more than 50%. Table VI gives the assay value and the theoretical value for a number of nearly pure compounds. COLORIMETRIC DETERMINATION OF TIN IN ORGANOTIN COMPOUNDS

The difficulties encountered in attempting to determine tin by volumetric or gravimetric procedures in very volatile and in unsaturated organotin compounds by \vet oxidation with nitric and sulfuric acids led to a further investigation of the colorimetric determination of tin ( S ) , to check its applicability in the estimation of high tin percentages. Seven samples of very pure organotin compounds were used, consisting of four nonvolatile organotin compounds, t.wo unsaturated compounds, and one volatile compound. In addition to being analyzed colorimetrically, the four nonvolatile compounds were analyzed volumetrically for comparison.

The colorimetric method outlined below consists briefly of weighing the samples in a gelatin capsule, combusting them in a Parr bomb, leaching, acidifying, diluting to volume, adjusting the acid concentration, fuming, and developing the color. Some nickel is introduced from the fusion. No interference, however, was found from the nickel or from the presence of sodium salts. Table VI1 compares the results obtained colorimetrically, volumetrically, and the calculated theoretical tin. PREPARATION OF CALIBRATION CURVE FOR TIN

Standard tin solution (strong: 1 ml. 0.4 mg. Sn). Weigh and transfer 0.2000 gram of pure tin t o a 1000-ml. volumetric flask and add 250 ml. of hydrochloric acid. When solution of the tin is complete, dilute with water, cool, make up to volume, and mix well. Standard tin solution (\Teak: 1 ml. = 0.04 mg. Sn). Pipet 100 ml. of the strong standard tin solution into a 500-ml. volumetric flask, dilute to volume, and mix well. Sulfuric acid solution (3 to 7 ) . Thioglvcolic acid. Use as received (The Xlitheson Co.). Dithiol reagent (0.30%). Keigh 0.15 gram of dithiol (A. D. Rlackav, New york) into a 100:ml. beaker, add 8 drops of thioglycolic acid, and then add 50 ml. of sodium hydroxide solution (273. Stir well t o dissolve. If the solution is not clear, filter through a dry, fine-grained filter paper into a clean, dry, glass stoppered bottle, and store in a refrigerator. The solution is stable for not more than 3 days and then only if stored in a refrigerator. Sodium lauryl sulfate solution (2%). Weigh and transfer 10 grams of sodium lauryl sulfate to a 500-ml. volumetric flask. Add 400 ml. of warm mater (about 40' C.) and swirl gently to dissolve it. Cool, dilute to volume, and mix well. This solution may be slightly cloudy; solution will be complete when it is added to the acid solutions. If the sodium lauryl sulfate separates out, redissolve by warming. Calibration Solutions. Into four 50ml. volumetric flasks, pipet 10 ml. of sulfuric acid solution (3 to 7 ) . Pipet 2 , 5. 10, and 20 ml. of the standard tin solution (1 ml. = 0.04 mg. of Sn) into four 50-nil. volumetric flasks. Add 5 drops of thioglycolic acid to each, wash d o m the walls of the flasks with water, and swirl to mix well. Dilute to about 40 ml. with water and swirl to mix well. Proceed with color development. Blank Solution. Pipet 10 ml. of sulfuric acid solution (3 to 7) into a 50-ml. volumetric flask. Add 5 drops of thioglycolic acid, swirl to mix, and dilute to about 40 ml. with water. Color Development. Pipet into each flask 2 ml. of sodium lauryl sulfate solution (2%). Swirl gently for about 15 seconds to mix well (avoid excess foam=

Table VII. Determination of High Tin Values Colorimetrically

Volu- Colori- Theometric metric retical Bu2SnClp BusViSn Me2SnCl2 &n Bu2SnO Thermolite 31 VilSn MeaSn

39.08 39.00 37.50 53.94 53.83 27.76 27.80 47.60 47.58 17.63 17.66 52.42 66.23

39.07 37.43 54.03 27.80 47.70 52.32 66.37

ing). Pipet into each flask 1 ml. of dithiol reagent (0.30%), while keeping the solution in motion by swirling it gently. Cool, dilute to volume, and then mix well. Photometry. Transfer suitable portions of the solutions to absorption cells (1 cm.) and with the blank solution set a t 100% transmittancy, measure the transmittance of the solutions a t 530 mp. The color is stable for a t least one hour. Calibration Curve. Plot the photometric readings of the calibration solutions against milligrams of tin per 50 ml. of solution. Procedure. Weigh 1 gram of powdered sugar - and place in a nickel fusion cup. Into a weighed gelatin capsule weigh a sample of such size to give not less than 25 mg. of tin and not more than 40 mg., preferably, approximately 33 mg.; 3.3/per cent Sn expected will give the Iveight of sample to be taken for analysis. (If the compound is volatile, two tops or tn-o bottoms should be used rather than a complete capsule. The two parts are weighed, the sample is added to the bottom of one part and the other part is carefully inserted. This makes a tight seal and the sample is not lost during weighing.) Place the neighed sample in the nickel fusion cup containing the powdered sugar and add one level scoop of sodium peroxide. Seal the cup, mix the contents well, and combust the mixture in the usual manner. Carry a blank through the entire procedure. When cool, remove the cover and wash it with warm water, catching the washings in a 400-ml. beaker. Place the cup containing the fusion in the same beaker and add sufficient warm water to cover it. Cover the beaker with a flat borosilicate glass watch glass and when the reaction has subsided, place on a hot plate, insert a borosilicate glass stirring rod, bring to a boil, and allow it to boil for 1.5 minutes. Remove from heat, wash down the cover and sides of the beaker with hot water, remove, and wash the cup well Kith hot water. Set the fusion cup aside. Cover with a flat watch glass, slide cover over slightly, and introduce 35 ml. of hydrochloric acid, carefully, in small portions. Stir as much as possible without removing the match glass, especially near the end of the acid addition. Add 30 ml. additional hydrochloric acid, remove the cover, wash, stir, and transfer to a 500-ml. volumetric

flask. Place a few milliliters of water and a few milliliters of hydrochloric acid in the original fusion cup. Warm slightly, transfer the solution to the original beaker, and wash the cup with water. Wash down the sides of the beaker with 30 ml. of hydrochloric acid and transfer the solution to the 500-ml. volumetric flask. Wash down the beaker once more with 30 ml. of hydrochloric acid and finally tlvice more with water, transferring all washings to the 500-ml. volumetric flask. Swirl to mix, cool to room temDerature. dilute to volume. and mix \