Mar., 1914
T H E J O r R - V A L O F 1 , V D C S T R I A L A N D E Y G I N E E RI N G C H E MI: T R Y
centration of t h e soil moisture would also be temporarily increased, due t o increase in solubility with heat. During t h e course of t h e evaporation t h e concentration of t h e soil moisture would increase t o t h e saturation point, after which t h e mineral matternrould bedeposited on t h e surface of t h e film as evaporation went on.' Also t h e materials held in solution in t h e interior of t h e permeable particles would be partially deposited on t h e surface a s t h e water evaporated. Upon adding water t o t h e soil after having been dried, it is probable t h e materials deposited from previous evaporation would be more soluble t h a n t h e other mineral constituents. In addition a certain a m o u n t of oxidation a n d other chemical changes in t h e organic m a t t e r might reasonably be expected t o t a k e place, which would have some effect on t h e solubility of t h e mineral bases t h a t t e n d t o combine with t h e organic matter. Upon shaking with water a soil previously dried, t h e solution t h e n obtained should be of a greater concent r a t i o n t h a n t h a t prepared from t h e air-dried soil. With t h e absence of soil films a n d a more or less altered condition of t h e colloids present t h e solvent mould have more ready access t o t h e soil particles during a short period in addition t o coming into immediate contact with salts deposited on t h e surface of t h e particles. I n t h e light of these views t h e n t h e solubility of soils before a n d after drying becomes more intelligible. W h y several of t h e mineral constituents of t h e soil should be so markedly more soluble when heated t o 250' C. t h a n at t h e other temperatures is a question n o t easily answered. T h e difference i n physical effects were quite noticeable in t h a t there was a greater aggregation of particles. Again there was a more complete destruction of organic m a t t e r effected at this temperat u r e a n d also i t is not entirely impossible t h a t drying at 100' C. for eight hours does n o t effect a complete elimination of t h e soil moisture a n d especially t h e water of chemical combination. It seems reasonable t h e n t h a t t h e effects of heating t o 100' C. a r e simply magnified when heated t o 250' C. added t o this being a more complete destruction of organic m a t t e r , t h e results both physical a n d chemical being of t h e same general n a t u r e b u t more complete a t t h e higher temperature. T h e destruction of organic constituents being more complete would necessarily increase t h e solubility of t h e mineral m a t t e r held in combination, a s i t is generally conceded t h a t t h e organic constituents of t h e soil in i t s n a t u r a l s t a t e are quite insoluble in water a n d acids, more especially t h e former. There is also evidence of t h e existence of f a t t y or resinous organic m a t t e r which would materially affect t h e properties of t h e soil film. F o r t h e decomposition of such bodies i t would be necessary t o heat t h e soils considerably above 100' C. I n addition t o t h e above-mentioned effects of h e a t t h e relation between solid a n d solvent would naturally be affected b y other factors. Among these, would be absorption or "fixing power" of t h e soil.2 It is reason-
I n 1871, Fresenius, Neubauer a n d Luck2 published a method for t h e determination of reverted phosphoric acid in phosphates which involves t h e use of a solution of neutral ammonium citrate, specific gravity 1.09. This method, with a change in t h e temperature of t h e solvent, has been in constant use since t h a t t i m e 3 with n o a t t e m p t b y a n y one t o give a n explanation of t h e chemical reaction involved. It has been quite generally believed t h a t t h e neutral ammonium citrate solution possesses a selective power which enables i t t o separate dicalcic-phosphate from tricalcic-phosphate. This is not true, for it has been found i n this laboratory t h a t IOO cc. of t h e Official ammonium citr a t e solution2 is capable of dissolving 1.3, grams of precipitated tricalcic-phosphate i n one-half hour a t a temperature of 65' C. This dissolving of t h e tricalcic-phosphate is accompanied b y a precipitation of calcium citrate.
King (loc. c i f . ) in discussing t h e relative solubilities of fresh and dried soils advanced this idea. Richter (Landw. Vers. Stat., 41 (1896), p. 269) found t h a t heating t h e soil increased t h e absorptive power of t h e soil for water
1 Read before t h e Association of Official Agricultural Chemists, Washington, D. C., Nov. 17, 1913. * 2. anal. Chem., 10, 133. 8 U. S. Dept. Agr., Bur. of C h e m , Bull. 101 (revised).
able t o expect soils with widely varying physical a n d chemical properties, such as t h e samples used in t h i s series, t o vary widely in their absorptive power. Hence i t is n o t at all unlikely t h a t t h e lack of consistency i n some of t h e results obtained is due primarily t o this factor. Not only is there lack of uniformity in t h e absorptive power of soils b u t t h e y also show considerable selective power in t h e absorption of mineral constituents. Soils high i n h u m u s should t e n d t o have a high fixing power, d u e t o t h e properties of this constituent of chemically combining with minerals as well as i t s power of absorption, a n d , therefore, t h e effect of heat upon highly organic soils should t e n d t o increase t h e solubility of t h e minerals. This was found t o be t r u e in t h e highly organic soils of t h e series. Another factor is t h a t of precipitation following extraction, being t h e more marked in t h e acid extract due t o a more complete extraction. I n passing from 2 5 0 ° C. t o ignition t h e effects a r e apparently of a specific rather t h a n general nature, which have been previously discussed. Among these are t h e volatilization of certain sulfur compounds, conversion of bicarbonates into normal carbonates, dehydration of silicates, etc., replacing of potash b y lime a n d other chemical transformations. I n addition there is produced a greater aggregation of t h e soil particles, resulting i n a decrease in surface area exposed t o t h e solvent a n d a n accompanying change in t h e fixing a n d absorbing powers of t h e soil. It is possible, b y application of these conceptions, t o explain t h e majority of changes, both increases a n d decreases in solubility resulting from ignition. Acknowledgments are hereby extended t o Dr. W. P. Kelly, chemist a t this Station, for valuable suggestions a n d assistance otherwise rendered. HAWAIIEXPEXIMEKT STATION HOXOLULU
THE USE OF S O D I U M CITRATE F O R THE DETERMINAT I O N OF REVERTED P H O S P H O R I C ACID' By ALFREDW. BOSWORTH
T H E ' J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
228
This separation of calcium citrate led t o t h e belief t h a t t h e solvent action of t h e citrate solution was t h e result of a double decomposition started by t h e free phosphoric acid always present in a n aqueous solution which is in contact with a solid phase composed of a phosphate.' This double decomposition might be indicated by t h e following: CaHP04 2C6&06("4)3+ (NH4)2HP04+ [CeH506("4)2]2Ca CasPzOs 6CsH50e(NH4)3+ z(NH4)3P04 3 [C6H606("4)2]2Ca If appreciable amounts of calcium are taken into solution, calcium citrate will separate out. 3 [ C ~ H ~ O ~ ( N H ~ ) Z I 4CsHs06(N ZC~+ H4)3 (Cdho&Ca3 A great deal of work has been done upon methods of making neutral ammonium citrate solutions a n d several such methods have been published. T h e fact t h a t neutral ammonium citrate is very unstable a n d easily loses ammonia has not been sufficiently considered in this connection, however. Why should extreme care be taken t o secure a n absolutely neutral solution, if this solution is t o lose ammonia when heated a few degrees above t h e room temperature? Most chemists who have used t h e neutral ammonium citrate solution know t h a t ammonia is constantly given off during t h e half hour allowed for t h e solvent action t o t a k e place. T h e final result then, is not t h e action of neutral citrate b u t rather t h e action of a n acid citrate. There seemed t o be no theoretical reason why a solution of sodium citrate should not be just a s effective a solvent a n d i t possesses t w o distinct advantages. I t is a more stable salt a n d as t h e base in it is not volatile t h e solution would remain neutral throughout t h e whole operation. All trouble in securing a neutral solution would be eliminated, for a solution of citric acid could be neutralized with sodium hydroxide, using phenolphthalein as a n indicator, or t h e neutral crystals of sodium citrate could be dissolved in water, and t h e solution made up t o t h e required volume. I n order t o learn what t h e action of a solution of sodium citrate might be, one was made which was of t h e same molecular concentration as t h e Official2 ammonium citrate solution, i. e . , 3 1 4 grams crystallized sodium citrate, ( C ~ H ~ O & ~ ~ ) ~ . Iper I Hliter. ~ O , This solution was used t o determine t h e amounts of insoluble a n d reverted phosphoric acid in several fertilizers, Thomas slag, ground bone, ground rock phosphate, dicalcicphosphate, CaHP04, a n d tricalcic-phosphate, Ca3P20s. T h e results, together with those obtained by t h e use of t h e Official citrate solution, are given in the table. I n connection with these figures, it is noticeable t h a t t h e differences between t h e figures obtained with t h e two solutions are, in most cases, of t h e same magnit u d e as the variations in the figures obtained by differe n t chemists working upon t h e same sample.3 It is also interesting t o know t h a t Samples j, I O a n d I T , which show t h e largest differences, all contain bone. T h e duplicate determinations, in all cases, showed
+
+
+
+
I
Cameron a n d Hurst, Jour. Amer. Chem. S O C . , 26, 905. U. S . D e p t . Agr., Bur. Chem., Bull. 107 (revised). THIS JOURNAL, 3, I 1 8 and 5, 957 T h e differences between t h e
extremes in these two cases are 1.23 per cent and 0.90 per cent, respectively.
Vol. 6, No. 3
closer agreement with sodium citrate solution t h a n with t h e Official citrate solution. T h e Official method directs t h a t t h e flask in which t h e reaction takes place should be loosely stoppered, during t h e time it is being maintained a t 6 j " C., in order t o prevent evaporation. The use of stoppers often results in t h e loss of .a determination through t h e breaking of a flask. It is suggested t h a t t h e flask be closed with a one-hole rubber stopper carrying a n e m p t y calcium chloride t u b e , 300 mm. in length, which will serve as a condenser. T h e use of such a condenser will not interfere with t h e shaking a n d it furnishes a vent which prevents t h e breaking of t h e flask. The last column of t h e table shows t h e amounts of ammonia given off during t h e half hour of t r e a t m e n t with ammonium citrate solution prescribed by t h e Official method. This ammonia was caught in standard acid by means of a n air current which was passed through t h e Erlenmeyer flask in which t h e solvent action was taking place. These figures seem t o bear some relation t o t h e difference given in t h e preceding column. By noticing t h e large amounts of ammonia given off b y t h e Thomas slag, rock phosphate a n d ground bone when treated with ammonium citrate a t 65" C. for one-half hour a n indication as t o the reason for t h e liberation of t h e ammonia may be found. T h e fertilizing materials, after being extracted with water, leave a residue which, in most cases, contains alkaline material, alkaline phosphates, carbonates of calcium a n d magnesium a n d oxides of other elements. These all tend t o drive off ammonia from t h e citrate solution. COMPARISON FOR
OF T H E USE O F AMMONIUMC I T R A T E A S D S O D I U M C I T R A T E DETERMIXATION OF REVERTED P H O S P H O R I C ACID
THE
BY
sodium citrate
6.18 1.75 2 . 7 6 10.63 8.73 3.76 1.42 3.55 9.58 6.50 0.76 2.32 12.33 11.90 0.02 0 . 4 1 14.59 1.21 4.01 9.37 10.92 3.76 0.58 6 . 5 8 11.18 8.73 0.34 2.11 9.61 4.24 1.62 3.75 7.31 0.95 2 . 5 9 3.77 8.79 0.00 5.22 3.57 19.91 1.84 6 . 3 4 11.73 13.07 8.42 0 . 2 2 4.43 11.69 4.33 3.68 3.68 Bone 20.95 0 . 0 0 13.36 7.59 Slag 17.57 0.00 9.40 8.17 Rock phosphate 29.72 0 . 1 9 27.57 1.96 CaHPOd 1 gram) , . . 0.00 , , , CdaP208 taken f .., 0.00 . . , Ammonium citrate heated t o 65' C. Ammonium citrate heated t o 75' C. 1
2 3 4 5 6 7 8 9 10 11 12 13
{
2 . 6 1 1.84 0.91 1.89 3 . 0 8 0.47 1 . 1 1 1.97 0.35 0.00 0.43 0.02 9.07 4 . 3 1 5.06 1 . 1 1 6.05 0.53 0 . 6 6 1.79 0 . 3 2 2.80 2.57 1.18 4.58 1.78 1.99 7.78 1.01 2 . 5 6 14.89 3 . 1 8 8.55 0.77 3 . 8 8 0.55 4.15 3.21 0.47 15.82 5.13 2 . 4 6 15.691.88 6.29 28.20 1.33 0.63 0.00 , , 0.00 0.00
,,
0.00
14.9 12.7 10.9 6.5 12.5 13.5 16.0 14.2 10.5 29.0 23.0 13.5 16.9 14.0 65.0 20.5 3.5 14.0 2.6 36.0
It is realized t h a t t h e small amount of evidence presented in this paper does not settle t h e question as t o t h e desirability of substituting sodium citrate for ammonium citrate in t h e determination of reverted phosphoric acid. The subject is simply brought forward a t this time in order t h a t those chemists who are interested may give it some thought. CHEMICAL LABORATORY N. Y. AGRICULTURALEXPERIMENT STATION GENEVA
