sample in a proper solvent which is a time-consuming and not always possible procedure. The accuracy of this method compares favorably with the one of existing methods for the quantitative analysis of polymers-for example, infrared methods. By observing the scattering of points on the working curve the propylene content of copolymers can be estimated to within about *2%* The reproducibility of analytical data is
satisfactory; cis an example the value of the ratio R (mean from three simultaneous pyrolysis) for a control sample was found to be always from 0.500 to 0.510 in 10 determinations made over a 3-mbnth period. LITERATURE CITED
(1) BentleY~F.
F.1 RapPaporti G*,ANAL. CHEM.26, 1980 (1954). ( 2 ) De Angelis, G., Ippoliti, P., Spina, N., Rzcerca xi. 28, 1444 (1958).
(3) Harms, D. L., ANAL. CHEM, 25, (1953). ( 4 ) Kruse, P. F., Nallace, W. B., Ibzd.,
25,1156 (1953). (5) Natta, G., lfazzanti, G VaIvassorl, A., Pajaro, G., Chim. e 2nd. (IZIilan) 39,733 (1957). (6) Reding, F. P., Lovell, C lT., J . Polgmer Sci. 21, 157 (1950). (7) Zemany, P. D., ASAL. CmLi 24, 1709 (1952). ~
RECEIVEDfor review July 22, 1959. Accepted September 8, 1959.
Determination of Chlorine in Silicate Rocks LEE C. PECK and EDWIN J. TOMASI U. S. Geological Survey, Denver, Colo.
lamps and serve as the beam sources, are fastened t o the back of the well block. Two l/c-inch holes, drilled through the back of the block, centered on and 1 inch above the bottom of the well, admit light from the lamps to the solutions.
b In a rapid accurate method for the determination of chlorine in silicate rocks, the rock powder is sintered with a sodium carbonate flux containing zinc oxide and magnesium carbonate. The sinter cake is leached with water, the resulting solution i s filtered, and the filtrate is acidified with nitric acid. Chlorine is determined by titrating this solution with mercuric nitrate solution using sodium nitroprusside as the indicator. The titration is made in the dark with a beam of light shining through the solution. The end point of the titration is found by visually comparing the intensity of this beam of light with that of a similar beam of light in a reference solution.
REAGENTS
C
is usually determined in rocks by a silver chloride precipitation method, one modification of which is described by Kolthoff and Sandell (1). This method is too timeconsuming if a large number of determinations must be made. Because of the increase in requests for chlorine determinations it became necessary to develop a more rapid method. Such a method must cover the range of chlorine concentrations normally found in silicate analysis. The chlorine content of a silicate rock will seldom exceed 0.2% and will probably never exceed 1%. T o fit the needs of the geologist, the analyst must be able to determine 0.01% with certainty and to differentiate between hundredths of a per cent, a t least in the range between 0.00 and 0.107,. Many methods for the determination of chlorine have been cited but most of them are unsuitable for silicate analyses. Kuroda and Sandell’s (2) colorimetric method based on the formation of a colloidal precipitate of silver sulfide works well for micro amounts of chloHLORINE
2024
0
ANALYTICAL CHEMISTRY
Figure 1 .
Titration apparatus
rine, but is not accurate enough for larger amounts. The method described below has been thoroughly tested for two years a t the U. S. Geological Survey laboratory a t Denver. Its greatest advantage is its speed. Where a large number of determinations are to be made, 12 analyses can be completed each day with ease as compared to four analyses per day by the silver chloride method. APPARATUS
TITRATION EQUIPMENT. An aluminum block has beaker wells just large enough to admit 150-ml. beakers. Two light housings, which contain 15-watt
0.1N MERCURICSITRATE SOLUTION. Dissolve 5.415 grams of mercuric oxide in 10 ml. of hot 1 to 1 nitric acid. Dilute the solution to 50 ml. with water and filter it through a small fine-textured paper into a 500-ml. volumetric flask. Wash the beaker and paper with water. Dilute the filtrate to the mark and mix. 0.01N MERCURIC NITRATESOLUTION. Pipet 50 ml. of the above solution into a 500-ml. volumetric flask. dilute the solution to the mark, and mix. STANDARD TURBID SOLUTION. Dissolve 0.1 gram of silver nitrate in 100 ml. of water and transfer the solution to a 500-ml. reagent bottle. Add 2 ml. of a sodium chloride solution containing 0.1 mg. of chloride per ml. to the silver nitrate solution. Mix the solution and let it stand overnight. Dissolve 0.1 gram of gelatin in 100 ml. of hot water. Filter this solution through a fine-textured paper into the bottle containing the silver nitrate solution. Dilute the solution to 500 ml. and mix. Store the solution in a dark place. FLUX.Pass 700 grams of sodium carbonate, 200 grams of zinc oxide, and 100 grams of magnesium carbonate through a 20-mesh screen to remove or break up lumps. AIix the combined ingredients using any available powder mixer. PROCEDURE
Place 0.500 gram of the rock powder in a 25-ml. platinum crucible containing 5.0 grams of flux. Mix the sample and the upper part of the flus with a small glass rod and then stir the mixed portion into the rest of the flux. Be careful not to overmix, as the rock powder may be-
come segregated at the bottom of the crucible. Preheat an electric furnace to 800" C. and heat the crucible and its contents in the furnace for 30 minutes. Never heat a zinc-bearing flux in platinum over a burner. Cool the crucible, add a little water to the contents, and crush the cake with a flat-ended rod. Transfer the cake to a 50-ml. beaker with a stream of water from a wash bottle. The volume of the solution in the beaker should be from 20 to 30 ml. Let the solution stand overnight to leach the soluble material from the cake. Filter the solution through a finetextured 7-cm. paper into a 150-ml. beaker. Break up the cake with the stirring rod and stir the solution before each portion is added to the filter. Wash the residue twice by decantation with small amounts of a 1% solution of sodium carbonate; then sluice the residue into the paper with the same wash solution. TF'ash the residue on the paper 10 times with the sodium carbonate solution and discard the residue. Dilute the filtrate to 90 ml., add 3 drops of 301, hydrogen peroxide, and stir. Cautiously add 10 ml. of nitric acid through the lip of the beaker. Stir the solution to expel most of the carbon dioxide, and wash the cover glass and the inside of the beaker with water. Add 1 ml. of a freshly prepared 10% solution of sodium nitroprusside to the solution and place the beaker in the left-hand well of the titration apparatus. Add 10 ml. of the standard turbid solution to a 150-ml. beaker and dilute to 100 ml. Stir, remove the stirring rod from the beaker, and place the beaker in the right-hand well of the titration apparatus. Turn on the beam lights; then begin titrating the sample solution in the dark with O.OlhJ mercuric nitrate solution from a 10-ml. buret. When the beam of light produced by the addition of one drop of the titrant fades slowly during a 15-second stirring period, the end point is near. Continue adding the titrant dropwise. Stir the solution after the addition of each drop and wait until the beam in the sample solu-
tion fades to the intensity of the beam in the reference solution before adding another drop. When the addition of one drop of mercuric nitrate solution makes the beam unmistakably brighter than that in the reference solution and the intensity of the beam does not decrease after a l-minute stirring period, the end point has been reached. Make a blank determination for chlorine introduced by the flux. Carry four 5-gram portions of the flux through all the steps of the procedure and average the four values that are obtained in the titration of the final solutions. This blank determination need be made only once for each lot of flux.
acidified by reducing manganese to the bivalent state. If a nitric acid solution containing sodium nitroprusside and a chloride is titrated with a mercuric nitrate solution, all the chloride must be complexed by mercury before a precipitate of mercuric nitroprusside forms. This colloidal precipitate coagulates slowly, and, therefore, the titration is usually carried past the end point before the presence of the precipitate can be recognized. Noponen (3) made the titration in the dark, with a beam of light shining transversely through the solution. If 0.1N mercuric nitrate solution is used as the titrant, a brilliant opalescence appears a t the equivalent point. Where small amounts of chlorine are to be determined, this method is not sufficiently sensitive, and, if 0.01N mercuric nitrate solution is used to increase the sensitivity, the end point of the titration is uncertain. When a few drops of 0.01N mercuric nitrate solution are added to the chloride solution, the local excess of the titrant precipitates colloidal mercuric nitroprusside and the precipitate so formed scatters the light, thus making the beam visible. Excess chloride dissolves this precipitate and the light beam fades. 4 s the equivalent point is approached, the rate of fading becomes so slow that it is difficiilt to determine when solution
DISCUSSION
The rock powder must be heated with an alkaline flux to destroy the structure of rock minerals and to convert chlorine to soluble chlorides. Magnesium carbonate makes the mixture of flux and rock powder refractory so that it will sinter without fusing. The sinter cake is easy to remove from the crucible and it disintegrates readily in water. Zinc oxide retains most of the silica in the residue when the solution of the sinter cake is filtered (4). Because the filtrate contains little silica, no precipitate forms when it is acidified. A few drops of hydrogen peroxide added to the filtrate prevents the precipitation of hydrated manganese dioxide when the filtrate is Table 1.
Reproducibility of Titrations C1 Added, Rlg. 0.00 0.50 1.00 1.50 2.00
Determination 1 2 3 4
Average Less blank M1. 0.01N Hg(T\;Oa)?required to titrate added C1
Table
II.
3.50
5 00
Ml. of 0.01.V Hg(NO,)? Required 15.03 15.05 15.04 15.06 15.05 0.33
0.32 0.32 0.36 0.32 0.33 0.33
1.88 1.86 1.90 1.89 1.88 0.33
3.41 3.35 3.40 3.36 3.38 0.33
4.83 4.80 4.83 4.84 4.83 0.33
6.35 6.35 6.32 6.33 6.34 0.33
10.72 10.76 10.75 10.75 10.75 0.33
0.00
1.55
3.05
4.50
6.01
10.42 14.72
Analysis of Rock Samples
Tl-pe of Rock Altered quartz latite Silver chloride method, impalpable powder Mercuric nitrate method, 80-mesh powder Mercuric nitrate method, 80-mesh powder
Mercuric nitrate method, 80-mesh powder Recovery from cake on samples from preceding line
Total, t w o preceding lines Mercuric nitrate method, impalpable powder
Spherulite glass
Andesite obsidian
Garnet Phonolite ijolite Tinguaite yo C1 Based on 0.5-Gram Sample
Tinguaite breccia
White tuff
____
0.01
0.10
0.23
0.33
0.41
0.52
0.71
0.88
0.002
0.095
0.22
0.32
0.43
0.47
0.67
0.86
0.003
0.097
0.22
0.33
0.42
0.47
0.67
0.86
0.003
0.097
0.22
0.32
0.40
0.48
0.65
0.87
0.00 0.003
0.00 0.097
0.00 0.22
0.01 0.33
0.01 0.41
0.05 0.53
0.04 0.69
0.00 0.87
...
...
...
...
0.41
0.52
0.70
0.87
VOL. 31, NO. 12, DECEMBER 1959
2025
of the precipitate is complete. If the formation of a permanent light beam wcre to be used as the end point, the titration would be almost interminable. At least 15 minutes are required for the heam to fade completely after the addition of one drop of the titrant when the addition is made near the equivalent point. However, if the titration is made against a reference beam in another solution, and the beam in the sample solution is allowed to fade only to the intensity of the beam in the reference solution before another increment of the titrant is added, the titration procceds rapidly. The addition of one drop of the titrant a t the equivalent point niakes the beam in the sample solution unmistakably brighter than the reference bean1 and its intensity will persist after the solution has been stirred for several minutes. In original experiments the reference solution was prepared by adding 0.05 ml. of 0.01N mercuric nitrate solution to 100 ml. of 1 t o 19 nitric acid solution containing sodium nitroprusside. Tho turbidity in this reference solution could not bp reproduced; furthermore, the instability of the disperse system made it unsuitable as a reference a here a series of titrations was to be made T o solve this problem, a standard turbid solution was prepared which consisted of a colloidal precipitate of silver chloride in a gelatin solution. This turbid solution, which does not deteriorate in storage, is diluted as needed to make the reference solution. The new method is singularly free from interference by other elements. Bromine and iodine interfere but it is unlikely that either d l be present in silicate rocks in more than trace amounts.
EXPERIMENTAL
A set of experiments was made to determine the reproducibility of the mercuric nitrate titrations. Fusions were made on 5-gram portions of the flux, the cakes were taken up with water, and the solutions were filtered. Chlorine was added to these solutions as sodium chloride. Four cach of a series of solutions were prepared to which 0.00, 0.50, 1.00, 1.50, 2.00, 3.50, and 5.00 mg., respectively, of chloride were added. The results of the titrations on these solutions are shown in Table I. The maximum difference in any set of four determinations was 0.06 ml. By subtracting the average of the blank determinations from the average of the other sets of determinations, the amounts of mercuric nitrate necessary to titrate the added chlorine were found. The amounts were not proportional to the amounts of chloride taken. However, a curve may be drawn using this data which will give the correct amount of chlorine for any given amount of titrant. Eight rock samples containing widely varying amounts of chlorine that had been analyzed previously by the silver chloride method were analyzed in triplicate by the new method using SO-mesh powder. Some of the results were lower than those obtained by the siiver chloride method (Table XI). The residues from the filtrations on one set of samples Tore hcated on the water bath for an hour with dilute solutions of sodium carbonate and the solutions were filtered. No chlorine was found in these filtrates. The residues from the filtrations on the last set of samples were dried and re-fused with sodium carbonate. The melts that were obtained were carried throughout the entire pro-
cedure. When the amounts of chlorine obtained from these residues were added to those originally obtained, the results were in good agreement Lvith those obtained by the silver chloride method. Samples that showed low results when a n 80-mesh powder was used were reanalyzed using a finely ground powder. The results (Table XI) agreed with those obtained by the silver chloride method. Because fine grinding is time-consuming, much of the advantage of this method would be lost if all samples were to be ground to impalpable powders. The data in Table I1 show that satisfactory results are obtained with 80mesh powder if the chlorine content of the rocks is below 0 2%. Fortunately, a t least 90% of all rocks submitted for analysis will contain less then 0.2% chlorine. The authors’ practice is to analyze the samples using the 80-mesh powder normally used for the other constituents. The occasional sample containing more than 0.2% chlorine is reanalyzed using a finely ground powder ACKNOWLEDGMENT
Tables I and I1 are the result of analyses made by Sertie C. Smith of the U. S. Geological Survey, Denver, Colo. LITERATURE CITED
(1) Iiolthoff, I. hl.; Sandell, E. B., “Texb
book,, of Quantitative Inorganic Analysis, 3rd ed., p. 721, hlacmillan, New York, 1952. (2) Kuroda, P. K., Sandell, E. B., ANAL.
CHEM.22, 1144 (1950).
(3) Noponen, G. E., Minnesot,a hlinin and Mfg. Co., St. Paul, hlirin., o r 3
communication, 1941. (4) Shell, H. R., Craig, R. L., .4NAL CHEM.26, 996 (1954).
RECEIVEDfor review March 12, 1959. Accepted September 21, 1959.
Accurate Particle Size Distribution with Electroformed Sieves R. R. IRAN1 and C. F. CALLIS Research Department, Inorganic Chemicals Division, Monsanto Chemical Co., St. louis 66,
b A method is described for calibrating sieves. Particle size distributions in the range of 10 to 100 microns can b e determined with calibrated electroformed sieves with accuracies a t the 95% confidence limits of k9.4 and &9.2% for the geometric mean diameter and geometric standard deviation, respectively. The corresponding precision is A2.6 and f6%, respectively. 2026
ANALYTICAL CHEMISTRY
P
size distribution analyses with woven sieves can be erroneous when about 35% or more of the sample passes through a 325-mesh screen (44 microns) (1). However, sieve analysis is very popular because of its simplicity and applicability in routine control laboratories. Johnson and Newman (6) suggested a way for calibrating and checking relatively coarse sieves. A recent ARTICLE
Mo.
novel approach in sieve analysis is the development of electroformed micromesh sieves that have been shown (2) to be precise in the size range 20 t o 100 microns. The authors calibrated the sieves by examining microscopically a number of openings in each sieve. However, because a particle can pass through a hole whose diameter equals its smallest dimension only if the orientation of the particle is favorable,
