March, 1925
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Causes of Errors in the Analysis of High-Grade Phosphatic Materials’ By R. J. Car0 and E. L. Larison ANACONDA COPPER MININGCo., ANACONDA, MONT.
URIKG the last few years there has been a notable in- siderable number of samples to be handled and which, with a crease in the manufacture of what are commonly known single precipitation, will give as nearly as possible the results as double superphosphates, which contain as high as that could be obtained by a double precipitation. In the preparation of the ammoniacal solution of the yellow 51 per cent total P&, 1 to 6 per cent citrate-insoluble P205, 40 to 50 per cent water-soluble PzOS,and 1 to 6 per cent mois- phosphomolybdate for the subsequent addition of the magture. Even among the most competent and experienced nesia mixture, the directions most generally given in the analysts great difficulty has been encountered in checking literature are “nearly neutralize” and “add HC1 until a analytical results on these products. This condition has led yellow precipitate forms which dissolves with difficulty.” D i f f e r e n t interpretatibns to a-great deal of investiga-and various means emtive work to determine why In the gravimetric method for the determination of ployed in procuring a soluthese discrepancies so fretotal P20, the exact condition of the solution before addition that will meet the requently occur. The purtion of the magnesia mixture has a decided effect on the quirements called for by pose of this paper is to sumresults. The relative importance of the contamination of these two expressions have marize the most important magnesium pyrophosphate by molybdenum is discussed. led to grave errors. It can points.which lead to grave Precautions are also necessary in the heating of magnesium readily be seen that the pererrors when analyzing these ammonium phosphate. sonal element enters to a types of products by the The volumetric method for the determination of total large extent unless these standard methods employed and citrate-soluble P2Oj can be used on high analysis proddirections are made very in the various fertilizer ucts with accuracy, but absolute standardization of the specific. laboratories throughout the procedure is necessary. It has been determined United States. It is hoped In the determination of citrate-insoluble PaOs it is after much experimentation that the points brought out important to remove all monocalcium phosphate on the that the exact condition of may contribute to a higher preliminary water wash. The formation of dicalcium the solution a t this pointdegree of accuracy in results phosphate on the water wash can be prevented by adding whether it is alkaline, neusecured by the use of the water on the water wash in the dilution ratio of at least tral, or acid-has a decided standard technical methods 200 to 1. A smaller sample than 2 grams is advisable for quite universally employed effect on the results obthis determination. in the fertilizer industry, as tained. As h a s been The proper dilution ratio of water to sample is also imwell as in the fertilizer conpointed out by Mr. Larison portant in the determination of free HaP04. Most diffiin his paper on this subject, trol work of the various culties by this method have been due largely to the lack of states, for the determinaa spread of over 1 per cent a standard method. tion of moisture, total, Pe05was found between a Care in handling the sample and the proper type of water-soluble, and citratesolution containing 1 cc. . . equipment are necessary for the accurate determination insoluble phosphoric acid. excess ammonium hydrox-. of moisture. Many of the points diside (specific gravity 0.90) cussed in this paper have and one containing 1 cc. already been rediewed by Mr. Larkon.* excess hydrochloric acid (specific gravity 1.18) over the neutral point, on a sample containing about 48 per cent total Gravimetric Method for Determination of Total P206 Pz06. Solutions meeting these conditions are quite within the range of the possibilities of ordinary technical manipulation. KOtrouble is encountered in the gravimetric procedure for I n order to bring out the points involved more clearly, the determination of total P205on these products t o the point where the ammoniacal solution of the yellow phosphomolyb- the writers took the Bureau of Standards Tennessee Phosphate date is neutralized for the addition of the magnesia mixture. Rock Sample No. 56 as a basis for their experiments. The This paper will be concerned only with a discussion of the ef- PZOScontent of this sample has been accepted as correct fects on total PzO, of the exact condition of the solution by the leading technical and research chemists of the United previous to adding the ammoniacal magnesia mixtures, States. Sample No. 56 was chosen in preference to a pure where a single precipitation of ammonium magnesium phos- salt, because it contains practically all the interfering elements phate is made. On account of the amount of time and extra found in the general run of phosphatic materials. Briefly stated, the procedure employed to bring out the labor involved a double precipitation is rarely made in this effects of the exact condition of the solution on the results obcountry where only the ordinary technical results are desired. All analysts of experience and ability are agreed that where tained was to take the ammoniacal solution of the yellow the greatest accuracy is desired a double precipitation is phosphomolybdate from aliquots representing 0.25 gram of necessary. The chemists of the fertilizer industry are only original sample. Four sets were used, made up of triplicate interested in procedures which will make it possible for a con- aliquots of 0.25 gram each. Each set was a t first neutralized in the following manner: To the cold ammoniacal solution 1 Presented by R . J. Caro under the title “Errors Encountered in the hydrochloric acid (specific gravity 1.18) was added slowly, Analysis of High-Grade Phosphatic Materials and Their Causes” before with constant stirring, until a yellow precipitate which the Division of Fertilizer Chemistry at the 68th Meeting of the American dissolved with difficulty formed. The solution was cooled a September 8 to 13,1924. Chemical Society, Ithaca, N. Y., J . Assoc. Oficral A g r . Chem., 7,394 (1924). second time, a piece of thin, well-stained litmus paper was
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dropped into the beaker and ammonium hydroxide (specific gravity 0.90) added slowly, a few drops a t a time, with vigorous stirring, until the paper just barely shaded toward red but still verged somewhat on the blue. The solution was then cooled for the third time. TO Set I of triplicates an ammoniacal magnesia mixture was added slowly, as ordinarily prescribed, followed by the addition of an excess of ammonium hydroxide after the precipitate had settled. To Set I1 of triplicates 1 cc. excess of ammonium hydroxide (specific gravity 0.90) was added, followed by ammoniacal magnesia mixture and ammonia as usual. To Set I11 of triplicates 1 cc. of hydrochloric acid (specific gravity 1.18) was added, a few drops a t a time, with vigorous stirring. No precipitate or color should be discernible at this point. Ammoniacal magnesia mixture and ammonia were then added as usual. Set IV of triplicates was prepared exactly as Set 111, with the exception that the final result was obtained by a double precipitation, the second precipitation being made in the same manner as described by the Bureau of Standards. The volume of each set after magnesia mixture and excess ammonia had been added was about 100 cc. Magnesium ammonium phosphate on all sets was allowed to settle overnight. The following morning each set was filtered on an ashless filter, washed with a 2.5 per cent ammonia solution until free of chlorides, ignited very gently, a t first, in a weighed platinum crucible and finally a t a heat of 1000"C. to constant weight. Samples were then cooled in a desiccator and weighed. The triplicates in each set checked very closely and the average of each was taken as the final result of that particular set. On the Bureau of Standards Tennessee Phosphate Rock Sample No. 56, the following results, dry basis, were obtained by these four different sets of experiments: Set I I1
111
IV
SOLUTION Neutral t o litmus paper Contained 1 cc. excess NHiOH (specific gravity 0.90) over neutral point Contained 1 cc. excess HCI (specific gravity 1.18) over neutral point Same as for Set I11 except that a double ppt. of magnesium ammdnium phosphate was made, second ppt. being made the same as described by Bureau of Standards
Per cent Pzo5 dry basis 31.11 30.92
31.44
31.38
The Bureau of Standards dry basis Pz05 value for thissample, by double precipitation, the first time in a slightly ammoniacal solution and the second time in an acid solution, is 31.33. The average of the bureau's twelve results by single precipitation made in an acid solution is 31.52 per cent PzOr, dry basis. From these figures it can be seen that a single precipitation of magnesium ammonium phosphate, either in an ammoniacal or neutral solution by the use of the ammoniacal magnesia mixture commonly employed, results in a low Pz05 content. A single precipitation of magnesium ammonium phosphate made in a solution containing an excess over neutral point of 1 cc. hydrochloric acid with this same magnesia mixture gives results nearer the true result obtained by the Bureau of Standards by double precipitation with a water solution of magnesium chloride slightly acidified with hydrochloric acid. It was thought that the presence of molybdenum in the magnesium pyrophosphate might be one of the reasons for the between the acid and the alkavariation of 0.52 per cent, P206 line solutions. Determinations of this element were therefore made on all the sets after combining triplicates into one sample, as follows: Set I I1 I11
IV
Per cent Mo01 0.023 Trace 0.058 0.015
The difference between the figure of 31.44 per cent PzO~,dry basis, obtained in Set I11 and the average of 31.52 per cent
Vol. 17, No. 3
PzOa, dry basis, obtained by the Bureau of Standards, both by single precipitation in an acid solution, as against 31.33, dry basis, by the Bureau of Standards by double precipitation, cannot be explained on the assumption that molybdic acid is the disturbing factor, unless one assumes that molybdic acid is not an occluded product but rather, as some have supposed, a compound of this acid with ammonium magnesium and phosphorus. Such a compound would necessarily have to contain but a small percentage of molybdenum and in addition would have to be one that would break up only at very high temperatures in order to account for the difference in results indicated. The amount of molybdenum in the final magnesium pyrophosphate seems to be increased by the presence of free hydrochloric acid, although it is by no means absent in the magnesium pyrophosphate obtained from neutral or alkaline solutions. This occurrence is of minor importance. It has been pointed out by G. E. F. Lundell, of the Bureau of Standards, that the higher results obtained by a single precipitation of magnesium ammonium phosphate are due, not to a contamination by molybdenum, but rather to an off composition of the precipitate. It does seem, both as a result of these experiments, and from deductions obtained from an abstract of the literature on this subject summarized in Mr. Larison's earlier paper, that in order to produce as nearly a true magnesium ammonium phosphate as is possible with a single precipitation, enough hydrochloric acid in excess over neutral point must be present to keep more than sufficient magnesium chloride in solution to precipitate all P&, so that when ammonia is added slowly as is the practice of the Bureau of Standards, the NHa enters into formation of the compound as added. The bureau uses an aqueous solution of magnesium chloride containing a slight amount of hydrochloric acid. I n the industry a strongly ammoniacal magnesium chloride mixture is universally used, the ammonia of which accomplishes the same purpose as does the addition of this compound by the bureau, with the exception that a much larger excess of magnesium chloride is usually present in technical practice. The use of the ammoniacal magnesia mixture necessitates a larger excess of hydrochloric acid over neutral point than is employed by the bureau in order that conditions may be correct for the formation of as nearly a true magnesium ammonium phosphate as is possible. Another source of error in the gravimetric determination of total PzO5 is in overheating. There seems to be no good reason for the heating of magnesium pyrophosphate above a maximum temperature of l l O O o C., which accomplishesall that is chemically necessary. Heating above this temperature for the purpose of driving off molybdic acid is both unnecessary and decidedly dangerous, owing to loss of pz.06 by volatilization and probably changes occurring in the asbestos fiber where a Gooch is used. Volumetric Method for Determination of Total and Citrate-Insoluble PZOS
For the determination of citrate-insoluble PZOSthe volumetric method has almost universal application. Its accuracy and reliability for the determination of total Pz05on any but products containing relatively small amounts of phosphorus have been seriously questioned, particularly by many of the older analysts. The experience of the writers is that with proper standardization in the hands of experienced analysts the volumetric method will yield as accurate and reliable results as the longer and more laborious gravimetric method. For several years in this laboratory both of these methods have been used alongside of each other, with equal success. Absolute standardization, both of the procedure and of the analyst himself, is extremely essential in order to secure
Illarch, 1925
263
INDUSTRIAL L X D ENGINEERIXG CHEMISTRY
WaterCitratethe desired accuracy. Details must be followed religiously, soluble insoluble Set Per cent Per cent Manipulations involving steps which can be classified as 5.15 I 34.30 "nearly or about" have been the cause of much of the adverse 4.84 I1 38.90 criticism directed towards this method. It has been the experience of the writers that in order to It is rare that the analyst encounters any difficulty with the use of the volumetric method for totalP205 determination in the remove all water-soluble P205 from a 2-gram sample of this preliminary steps involved in the decomposition of the sample , material it is imperative that small wash portions be applied except that it has been found that a much sharper end point with a churning effect and that each portion be allowed to run in the titration with standard allrali results when the precau- through thoroughly before adding a new one. A total wash tion is taken to destroy any organic material present. When very close to 260 cc. is required in order to accomplish the dehydrochloric and nitric acids are used for decomposing the sired effect, This is an operation into which the skill of the sample, it is very desirable, in order to insure destruction of analyst enters to a great extent in accomplishing the maxiall organic material, to add the latter acid first, then as many mum effect with a given volume of water. The question very naturally arises as to just why these types potassium chlorate crystals as are necessary to render the solution practically colorless, followed finally by hydrochloric of products are so gummy and sticky and consequently so acid. Presence of any color due to organics in solution tends extremely difficult to filter. The writers believe that the exto make the end point drag, with resultant low PZOS, particu- planation for this is that when water is added to monocalcium larly where the standard alkali's strength has been determined phosphate, where the dilution ratio is less than 200 to 1, a either against a standard acid, or, for illustration, a known precipitation of dicalcium phosphate takes place, according to the following equation: rock sample free of this interferent. The crucial point in the method is the condition under 2 CaH4PzOs = CadM"08 H6P~08 which the precipitation of the yellow phosphomolybdate is accomplished. Universal practice is to use one of three con- This is the white flocculent material so frequently noticeable ditions for this purpose-namely, in a water bath for 30 in the filtrate on this preliminary water wash. It is very minutes a t a temperature of 45" C.,in a water bath for 15 often seen adhering closely to the stem of funnels and near minutes at a temperature of 65" C., or a t room temperature the apex of filter papers used on the preliminary water wash. I n order to make a true determination for water-soluble either with shaking or stirring machine for 30 minutes. When the options of room temperature with shaking or stirring ma- P205on the filtrate from one of these washes, all the solid parchine for 30 minutes and 45" C. for 30 minutes are used, ticles adhering to different points on the apparatus used must accompanied by a solution of a standard sample having about be incorporated into the filtrate. A very interesting feature in the same percentage of P205 present in about the same volume, this connection is that in materials containing considerable with same concentration of molybdate solution and ammo- free phosphoric acid little or no decomposition of monocalnium nitrate, followed by as nearly the same manipulations as cium phosphate takes place. Thus the amount of dicalcium is possible, accurate and concordant results on these very high precipitate present in the filtrate serves as a rough indicator analysis products may be expected. The meager experience of the probable amount of free phosphoric acid present in the that the writers have had with the option of "15 minutes a t sample. It has been the experience of the writers that the easiest 65" C." seems to show that even with the most careful standand most accurate procedure to follow, when making a preardization results tend to be erratic. Very careful standardization of all flasks, pipets, and burets liminary water wash for citrate-insoluble and water-soluble is important on account of the small aliquots that must be used P205determinations is to drop the sample into a beaker containing enough water to make a dilution ratio of water to samin the determination of total PzOs. ple of a t least 200 to 1, stirring vigorously until all watersoluble P205 has gone into solution, and then to filter and deDetermination of Citrate-Insoluble P206 termine in the usual manner. Several analysts have suggested that a 1-gram sample inErrors encountered in this determination can rarely be traced directly to lack of neutrality or wrong specific gravity stead of the customary 2-gram sample be used for making citrate-insoluble Pzo5 determinations on these products, of the ammonium citrate solution. The most fertile source of error on these classes of products when following the general practice of washing on a filter is the failure on the preliminary water wash to remove all paper. A 1-gram sample could be more easily and effectively water-soluble phosphates. Double superphosphates usually washed in less time than a larger one. There is no danger of ~ water-soluble form. It over-washing in ordinary technical analysis, but there is contain from 40 to 45 per cent P z Oin is a tedious and long operation to remove all monocalcium grave danger of under-washing. These types of materials phosphate when employing the usual procedure of placing a usually contain a very small percentage of iron, so there is 2-gram sample onto a 9- or 11-em. filter paper and washing very little danger of affecting the citrate-insoluble result by with small wash portions up to a volume of 250 cc. allowing combination of iron and Pa05when conducting a very thoreach portion to run through thoroughly before a new wash is ough wash. added. A sticky, gummy residue forms, which works closely Determination of Free HaPO4 into the pores of the filter, making the filtration very slow, it not being uncommon for a successful extraction to require 3 I n these products free acid is usually determined as free to 4 hours. Water-soluble P205, if not completely removed, H3P04, since there is usually very little or no free HzS04or H F contaminates the neutral citrate solution and destroys its present. Great difficulty has been experienced by different maximum efficiency, giving high citrate-insoluble PzO5 analysts in checking results on this determination. This results. The magnitude of the error due to the incomplete is no doubt due to a great extent to the lack of a standard removal of this compound can be readily appreciated from the upon which work can be based. -411 the commonly used profollowing two sets of figures. Set I represents the effect cedures have their serious weaknesses. Particularly is this on the citrate-insoluble P205 result where only a partial re- true when the solution for water-soluble P205determination is moval of monocalcium phosphate was accomplished. Set used for this purpose. The equation given for the decompoI1 represents comparative effect following the complete re- sition of monocalcium phosphate produces free H3P04, as moval of all water-soluble p& shown previously. The experience of the writers has been
+
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that the quickest and most accurate way to make this determination in a water solution of the sample is to use 1 gram of the sample, drop into a beaker containing at least 200 cc. of distilled water, and stir frequently over a period of onehalf to three-quarters of an hour, then filter, This gives on these products a clear filtrate. This is diluted up to a volume of about 450 to 500 cc. Four or five drops of a 0.1 per cent solution of methyl orange indicator are then added and the solution is titrated with standard alkali. The neutral point is determined by comparison with a beaker containing the same volume of distilled water and methyl orange. Considerable work has been done to find a way to procure a standard of the general type of material analyzed, against which an alkali solution can be standardized. The value of such a standard can be readily appreciated. Of the several methods tried the one proposed by Wiley in his “Principles of Agriculture,” offers the most promise. When using this method as outlined, it has been found necessary to carry a sample of the calcium carbonate used through the same procedure, and in addition on these products the proper dilution factor must be used to prevent decomposition of monocalcium phosphate with consequent production of free HgPOa. Extraction of free HsP04 from the sample by means of absolute alcohol and subsequent determination of PzO, volumetrically gives considerably higher results on samples containing moisture than on dried portions of the same sample. This is probably due to a change in the composition of sample during drying operation.
Vol. 17, No. 3
A Double-Reacting Turmeric Paper’s2 By William Brinsmaid ILLINOIS DEPARTMENT OF AGRICULTURE, CHICAGO, ILL.
ERCOLATE 30 grams of freshly ground turmeric with a mixture of 180 cc. of 95 per cent alcohol and 120 cc. of water. Place the resulting liquid in a beaker and take to dryness on a steam bath. The residue will consist of a deep, red oily mass and a dry, brown scaly material. Add 10 cc. of clear alcohol and the red, oily mass will go into solution. If the first treatment with alcohol does not dissolve all the red, oily material, add small portions of alcohol with decantation until dissolved. The brown, scaly material will not dissolve in the alcohol. Filter through a paper, washing out the paper with clear alcohol. Add alcohol to make the liquid up to 100 cc. Prepare a paper pulp by disintegrating filter papers that have been washed with hydrochloric and hydrofluoric acids. Preparation of Test Paper a n d M e t h o d of Use
For each gram of dry paper pulp, place in a beaker 75 cc. of water and 15 drops of the turmeric solution. Add the paper pulp and stir thoroughly, breaking up any lumps. Place on a steam bath and allow to stand, .with occasional stirring, until the paper has absorbed all the color. Filter on a Buchner funnel with a fitted filter paper and suction, so that when the pulp has been sucked as dry as possible D e t e r m i n a t i o n of Moisture there will remain a layer of pulp about 3 mm. thick. Do . It is the opinion of the writers that improper exposure of not wash the pulp. Remove this on the bottom paper and the sample to the atmosphere before and during analysis is dry a t a gentle heat. The paper pulp will be colored a the chief source of error in this determination. These prod- clear lemon yellow. When dry, peel away the bottom ucts are very hygroscopic in nature. Great care must be paper and with shears cut the turmeric paper into small taken in handling the sample, and desiccators in which it is squares of about 7 or 8 sq. mm. Place in a tight container placed for the purpose of cooling must contain fresh, efficient, and keep in a dark place. dehydrating agents. Concordant results can be procured by For each test desired place one of the paper squares in a placing the sample (quite universally 2 grams in weight) in a depression in a spot plate. Moisten with a few drops of the round aluminium dish about 51 mm. (2 inches) in diameter solution to be tested. The paper should be thoroughly and 13 mm. (0.5 inch) deep, having a tight-fitting cover, placing moistened and a slight excess of solution does no harm. in a water oven at temperature of boiling water at the par- Place the spot plate and paper in a warm place and allow ticular altitude where determination is being made, and leav- to dry. The full heat of a steam bath is too high and may ing for 5 hours. The oven is not opened at any time while discolor the paper. It is essential that the paper be thorthe sample is drying, After 5 hours the sample is quickly oughly dry, but it must not be overheated. When dry the covered and put into an efficient desiccator to cool. paper will be a pink or cherry color if boric acid or its salts One very interesting point disclosed in connection with this were present. To confirm the test, place a drop or two of determination is the error encountered due to regrinding of 0.1 N sodium hydroxide solution on the pink paper. If the sample to a finer mesh than that universally specified for color is due to boric acid or its salts the paper will immediately phosphatic materials. As has been pointed out in a former turn a clear blue. paper by Mr. Larison, this leads to serious changes in the Notes constitution of the material. First, undue exposure to atFreshly ground turmeric is specified because ground turmeric mosphere, coupled with heat generated in grinding operation, that is old and has been exposed to light will bleach and lose changes moisture content. Second, these products invaria- some of its coloring power. bly contain free phosphoric acid, which when rubbed between Taking the percolation to dryness on the steam bath throws grinding surfaces adheres to these surfaces and becomes very out from the solution certah coloring matter in the turmeric hard to remove and reduce to a powder. Furthermore, which is insoluble in alcohol but is soluble in water. This is not responsive to the boric acid test, and it disguises adhering particles, owing to their great affinity for water, take coloring or inhibits the blue confirmatory reaction. Its removal is up a considerable percentage of water in a comparatively therefore necessary. short time. Several grades of filter paper were made into pulp and tested, New Proofing Process-A new and cheap process of proofing which it is claimed will make any textile fabric-whether wool, cotton, or silk-absolutely waterproof, is said to have been discovered by Leigh Murray, of England, who has been engaged on this problem for five years. The proofing is noninflammable, odorless, hygienic, and is claimed to be adaptable to the waterproofing of any material. It is further reported that a company is being formed under the title of the British Murcolene Proofing Company to develop the process.
but none were as satisfactory as pulp made from filter paper washed with hydrochloric and hydrofluoric acids. One cheap grade of filter paper gave a direct test for boric acid. Strips of filter paper dipped in the turmeric solution and dried will give the reactions, but the colors are not so clear and decided as on the squares, especially when the amount of boric acid present is small. Received December 18, 1924. For another method of making the turmeric solution and for mention of some substances interfering with the test see Chamot and Cole, THIS JOURNAL, 10, 48 (19181. 1 2
