Phosphate Solubility in Relation to Cations and pH: Magnesium1

ALFRED T.

E effect of pH and cation on phosphate solubili-J. ty and availability is of great importance. Many

publications have appeared on this general subjectdealing with it either directly or indirectly. The ef-fect of magnesium on phosphate solubility and avail-ability has been studied for many years and recentlythe role of magnesium has been studied intensely. Arecent issue of Soil Science3 has been devoted entirelyto magnesium and soils.

The present paper includes data showing the effectof some cations commonly found in soils on phos-phate solubility in the presence of kaolin. The workwas outlined to give information on the role magnesi-um plays in phosphate solubility with the purpose ofhelping evaluate the use of dolomitic limestone inplace of calcitic limestone on acid soils deficient inphosphate, and on the role that magnesium mightplay in rendering the phosphate of rock phosphateavailable. Among the many publications dealing withphosphate solubility may be listed that of Gaarder(3)4 who showed that phosphate in the presence ofiron or aluminum is almost completely insoluble atan acidity of pH 3.0, but that as the solutions be-come more basic the solubility of the phosphate in-creases. This and similar data have led to the generalstatement on precipitation of phosphate that iron andaluminum are active.below pH 5.5, that calcium, isdominant at pH 6.5, and that magnesium enters thepicture at pH 7.5 (2).

• METHODSPhosphate solubility and precipitation was determined by

placing 0.0075 gram of ortho phosphoric acid in a go-micentrifuge tube with 5 grams of kaolin, ground to pass a 100-mesh screen, and the indicated amounts of calcium, magnesi-um, and ferric chloride. The pH of the suspension was ad-justed by hydrochloric acid or sodium hydroxide and about25 ml of water. The mixture was shaken over night in an end-over-end shaker running at 6 RPM m a constant-temperatureroom at 25° C. After shaking, the tubes were centrifuged toclarify the supernatant liquid, which was removed for pHdeterminations and phosphate analysis. The amount of phos- 'phate precipitated or fixed was determined by difference andreported along with pH values in Tables I, 2, 3, and 4.

The pH measurements were made with a glass electrodeand the phosphate in solution was determined by the coeruleo-

molybdate method of Deninges as improved by Atkins (I).A Coleman spectrophotometer was used in the phosphatemeasurements.

DISCUSSION OF RESULTSEFFECT OF CATIONIC CONCENTRATION

The results obtained are presented in Tables i, 2, 3,and 4. Reference to the tables shows that, within thelimits of the cation phosphate ratios studied, regard-less of pH, an increase in cation phosphate ratioincreases the percentage of phosphate precipitated.This increased precipitation holds for calcium, mag-nesium, and iron and for mixtures of these cations.The only noteworthy exception reported is where wecompare phosphate equivalencies of one-third cal-cium plus one-third iron with the corresponding two-thirds and three-thirds equivalencies of these samemixed cations, as shown in Table 4 between pHvalues 3.0 and 6.0. Other exceptions are irregular,minor in nature, and approximately within experi- 'mental error.

EFFECT OF pH

The effect of reaction and single cations on phos-phate solubility is shown in Tables I, 2, and 3. Cal-cium, except when added in minimum amounts equi-' valent to one-third of the phosphate, shows a generalincrease in phosphate fixation from pH 2.5 throughpH 9.5. The phosphate precipitation curve withequivalencies of one-third calcium is different thanthe curves for greater concentrations of calcium.

The effect of magnesium on phosphate fixation issomewhat different than is that of calcium. Magnesi-um acts much like calcium from pH 2.5 to pH 4.5,with fixation increasing as the pH rises. However,from this pH reaction to pH 9.5 phosphate precipi-tation gradually decreases in the presence of mag-nesium, while in the case of calcium there is a con-stant increase. From these data it would appear thatphosphate precipitation by magnesium at pH valuesin the agricultural range is of little consequence.

Iron( ferric) acts much like magnesium in pre-cipitating phosphate, the maximum precipitation oc-curring at reactions of pH 2.5 and steadily decreas-

TABLE.I.—Effect of calcium on phosphate solubility.

Calcium equivalencyof phosphate

i /3=CaH 4 (P0 4 ) 2 . . . . . . ' . . . . . . . .2/3=CaHPO4. . . . . . . . . . . . . . . . .3 / 3 = C a s ( P O 4 ) 2 . . . . . . . . . . . . . . . .4 /3=Ca, (PO 4 ) 2 +Ca. . . . . . . . . . . .6/3=Ca a(PO4) !+3Ca.. . . . . . . . . .

Percentage phosphate precipitated at pH of

2-5

• oo

263233

3-o

323i556266

3-5

4041637881

4.0

3946658085

4-5

3542648088

5-o

354i628087

5-5

3237597986

6.0

3237598087

6-5

3040638387

7.0

2240678692

7-5

13S'798693

8.p

1056868697

8-5

857898898

9.0

757929098

9-5

4579i9499

'Contribution No. 343 Department of Chemistry, Kansas Agricultural Experiment Station, Manhattan, Kan.2Soil Chemist and Professor of Chemistry.3Vol. 63:1-78. 1947.'Figures in parenthesis refer to "Literature Cited", p. 187. " ' ' '

185

i86 SOIL SCIENCE SOCIETY PROCEEDINGS 1947

TABLE 2 .—Ef fec t of magnesium on phosphate solubility.

Magnesium equivalency 'of phosphate

i / 3=MgH 4 (P0 4 ) 2 . . . . . . . . . . . . . .2/3 = M g H P 0 4 . . . . . . . . . . . . . . . .3 / 3 = M g 3 ( P 0 4 ) 2 . . . . . . . . . . . . . . .4/3 = Mg 3 (P0 4 ) 2 +Mg. . . . . . . . . .6/3 = Mg3(P04)2+3Mg... . . . . . .

Percentage phosphate precipitated at pH of

2-5

5129

2323

3-o

1622386263

3-5

2938467377

4.0

3638488084

4-5

3736507980

5-o

3534

• 41

7676

5-5

3030327372

6.0

2625287071

6-5

1732296869

7-o

133033.6565

7-5

1325275762

8.0

12

19234460

8.5

96

203563

9.0

44

ii2350

9-5ooo

1231

TABLE 3.—Effect of iron (ferric) on phosphate solubility.

Iron (ferric) equivalency Percentage phosphate precipitated at pH of

i/3=FeH6(P04)3...............2/3 Fe2H3(P04)8.......... ......3/3FeP04....................4/3 (FeP04)3+Fe. .............6/3 FeP04+Fe. ...............

2-5«o387791

IOO

3-0

476991

IOO

3-5

536989

IOO

4.0

5266Q —87

IOO

4-5

516280

IOO

5.0AT.

567698

5-5325i62

98

6.0

29609996

6.52640589995

7.0

253547949i

7-5

408380

8.0

306067

8.5

8183750

9.0

48

32

9-5

0

9

TABLE 4.'—Effect of mixed cations on phosphate solubility.

Cation equivalency of Per cent phosphate precipitated at pH of

i/3 + 1/3+o..................1/3+0 + 1/3.. ................0 + 1/3 + 1/3..................I/3 + I/3 + J/3... '.............2/3+2/3+0. .................2/3+0+2/3.. ................0+2/3+2/3. .................2/3+2/3+2/3................

3/3+3/3+0. .......'..........3/3+0+3/3- .................0+3/3+3/3- .................3/3+3/3+3/3- ...............

2-5

86541

37

797068

H868488

3-0

795757

35687268

30908289

3-5

33816161

3575767i

4iQ39088

4.0

3083SQ5835617268

AT.

8890

4-5

26845757

35696863

4487QC

87

5-o

35835556

35666458

2°,849484

5-5

4i8052u

38646058

37849083

6.0

29745950

406361CQ

39879084

6-5

25614855

44636163AC898288

7-o

4055T.Q

61

60635566

69867494

7-5

d34929SO

s->83634-4-7393847396

8.0

4632165784634i81

97917i97

8.529161652'

8163T.A

86

98925999

9.0

25171842

80631386

999278IOO

9-5

25

27

7863584

7992

IOO

ing through a pH value of 9.5. The fact that mag-nesium acts more like iron than calcium in precipita-ting phosphate between pH values of 4.5 and 9.5 isnoteworthy as magnesium and calcium both occur inthe second group of the periodic table while ironoccurs in the eighth group.

When we compare the effect of pH on solutionscontaining two or three cations, the general trend ofeach cation can be seen. Mixtures containing phos-phate equivalencies of one-third calcium plus one-third magnesium show variable results in precipitat-ing phosphate. The effect of both ions is shown.Mixtures of two-thirds calcium plus two-thirds mag-nesium and three-thirds calcium plus three-thirdsmagnesium equivalencies give phosphate fixationcurves that show the influence of the magnesiumbetween pH 2.5 and 7.0 in lowering phosphate pre-cipitation and the influence of calcium from pH 7.5through 9.5 in almost duplicating the calcium curve.Mixtures of calcium and iron, each the equivalent toone-third of the phosphate, show maximum phos-

phate precipitation at, a reaction of about pH 4.5, in-dicating the influence of iron. Mixtures containingequivalencies of two-thirds calcium plus two-thirdsiron and three-thirds calcium plus three-thirds ironshow little variation in phosphate precipitation be-tween pH values of 2.5 and 9.5, showing the influ-ence of both ions.

Mixtures of magnesium and iron, whether in equi-valencies of one-third plus one-third or two-thirdsplus two-thirds or three-thirds plus three-thirds,show a maximum precipitation at reactions in theregion of pH 4.0. Both ions show this characteristicwhen acting individually.

Mixtures of the three cations in phosphate equi-valencies of one-third for each ion show a maximumphosphate precipitation at a reaction of approximate-ly pH 3.5, showing the influence of the iron andmagnesium. The mixtures of two-thirds calcium plustwo-thirds magnesium plus two-thirds iron andthree-thirds calcium plus three-thirds magnesiumplus three-thirds iron show the influence of all three

PERKINS: PHOSPHATE SOLUBILITY IN RELATION TO MAGNESIUM I87

cations in the acid range and definitely the effect ofthe calcium at the basic reactions as phosphate pre-cipitation increases with pH value through pH 9.5.

EFFECT OF MIXED CATIONS WITHCONSTANT CATIONIC CONCENTRATIONS

When phosphate, precipitation caused by mixturesof cations is compared with precipitation caused by asingle cation, important data are obtained. In general,the more complex the mixture, the less the phosphateprecipitation. This is especially true in the agricul-tural range of soil reactions and between pH values3.5 and 7.0. Tables i, 2, and 3 show the phosphateprecipitation due to single cations, while Table 4shows the amount precipitated by mixed cations. Weshall first compare solutions having the same totalcationic concentrations, that is, we shall comparephosphate solubility in solutions containing one-thirdcalcium. equivalency plus one-third magnesium equi-valency with solutions containing two-third calciumequivalency, etc. For the present we shall compareonly solutions with the same total cationic equivalen-cies of calcium, magnesium, and iron. Comparison ofsolutions containing one-third calcium plus one-thirdmagnesuim equivalencies with solutions containingtwo-thirds calcium equivalency shows that the mixedcations precipitate less phosphate than the calciumalone, with an exception at pH 5.5. A comparisonwith the solution containing one-third calcium plusone-third magnesium with that containing two-thirdsmagnesium equivalencies shows that the mixtureprecipitates less phosphate at the acid end of the pHscale and a greater amount at the basic end than themagnesium mixtures. Comparison of solutions con-taining equivalencies of two-thirds calcium plus two-thirds magnesium with solutions containing four-thirds calcium and with solutions containing four-thirds magnesium shows that the mixture precipitatesless phosphate at any pH value than does calciumalone. The mixture precipitates less phosphate frompH 2.5 through pH 7.0 than does the magnesiumalone, but at pH values of 7.5 and higher the mixtureprecipitates more phosphate than does the magnesiumalone.

Comparison of equivalencies of three-thirds calci-um plus three-thirds magnesium with six-thirds cal-cium and six-thirds magnesium shows that at pH val-ues of and more acid than 7.0 and 6.5 the mixture

precipitates less phosphate than either ion by itself,but that at the more basic reactions the mixture pre-cipitates more phosphate than either ion alone.

Similar comparisons of mixtures containing equi-valencies of one-third calcium plus one-third ironand one-third magnesium plus one-third iron withtwo-thirds calcium, two-thirds magnesium, or two-thirds iron, as well as other comparisons, are of greatinterest and can well be studied from the data pre-sented in the tables.

SUMMARY AND CONCLUSIONThe effect of various mixtures of cations with

phosphate on phosphate precipitation has been stud-ied through a range of pH values from 2.5 to 9.5.The results show:

1. Increasing cationic concentration increases phos-phate precipitation whether single or mixed cationsare used.

2. As pH increases from 2.5 to 9.5, phosphatefixation by calcium steadily increases except for asmall dip around pH 5.5, by magnesium increases toa maximum around pH 4.0 and then steadily de-creases, and by iron steadily decreases.

3. At acid reactions calcium precipitates slightlymore phosphate than magnesium, but at basic re-action much more.

4. In general, the more complex the cation solutionwith total cation concentration remaining a con-stant, the less is the phosphate precipitation. This isespecially true at reactions on the acid side of pH7.0 and in the more concentrated mixtures.

5. In the higher cationic concentrations at constantcationic concentration, a mixture of calcium and mag-nesium precipitates less phosphate than calcium aloneat any pH value in the agricultural range and in manycases less than the magnesium alone.