ANALYTICAL EDITION
SEPTElIBER 15. 1938
and equivalent materials that may be used for the purpose and that evert a large lowering effect on surface tension when used in v r y small quantities. These addition agents also assure complete and reproducible wetting of the plummet wire. The procedure employed in using these materials is to add a s m d l drop of a dilute solution (approximately 1 per cent) of sorliuin lauryl sulfate to the surface of the aqueous solution in the measuring cylinder, after immersion of the plummet of the 1)alance. I n this way, diffusion of the addition agent into the aqueous solution is minimized and its effect on the specific gravity of the solution will be negligible. -4s an example of the lieneficial effects of this procedure, the following measurements on the same distilled water sample as that previously cited are given. Ohserved Specific Gravity at 23.0' C. 0.9996 0.9995
Specific GraTjty a t 203/20 1 0001 1.0000
The d u e s obtained are independent of the direction of niotion of the plummet and are reproducible to the limits of
519
sensitivity of the balance. T h e behavior of the balance is very different after addition of the surface tension depressant. The short fast oscillations previously obtained with water are replaced by the long slow swings characteristic of a slightly damped analytical balance. The readings can be taken by balancing to zero in the usual manner, or can be obtained Ly the method of swings. As further proof that balancing difficulties are due to surface tension and wettability effects on the plummet nire, it is only necessary to examine the water surface by oblique illumination. With pure water, the meniscus a t the wire can be observed to move in the same direction as the plummet, as it is raised or lowered. After addition of the surface tension depressant, the plummet can be raised or lowered a t mill without visible alteration of the meniqcuj at the wire, thereby indicating its complete wettability. The operating procedure given above has been adopted as standard in the author's laboratory and is recommended R S being of material assistance in measurements of specific gravities of liquids of medium to high surface tension. RECEIVED J u l y 7 , 1938.
Determination of Ortho-, Pyro-, and Metaphosphoric Acids By Colorimetric pH Titrations ARTHUR B. GERBER
T
AND
FRiNCIS T . 3IILES, Rlonsanto Chemical Company, ;iriniston, Ala.
HE recent offering in conmercial quantities of anhydrous
phosphoric acids has occasioned interest in the coinpositions and hydration characteristics of such conipounds ( 7 ) . These acids, made by the partial hydration of phosphoric a n hydride, contain from 73 to 88 per cent of phosphorus pentoxide. They range from sirupy to thick viscous liquids; impurities are negligible. This paper describes a method for the determination of their compositions in terms of ortho-, pj-i-0-, and metaphosphoric acid by the use of colorimetric pH titrations. For the determination of the three phosphoric acids, Aoyama ( 2 ) neutralized with sodium hydroxide solution, added a measured excess of silver nitrate and enough alcohol to make the content 50 per cent, and determined the excess silver after removal of the precipitated silver phosphates by filtration. Dmorzak and Reich-Rohrwig ( 4 ) revised the procedure by introducing a two-step neutralization. The solution was first neutralized to phenolphthalein indicator, and then, after treating with silver nitrate and alcohol folloTved by filtration, it was again neutralized using 0.1 i\; sodium hydroxide until the newly formed silver precipitate showed a distinct gray tint due to the precipitation of silver oxide which is formed when all the phosphate is precipitated. After filtration, the excess silver was determined as by Aoyama. The precipitated silver phosphates were treated Ti-ith hydrogen sulfide (2) or hydrochloric acid (4) t o form the phosphoric acids which, after separation by filtration, were titrated with sodium hydroxide to both the methyl orange and phenolphthalein end points. From these two titrations and the silver determination the amounts of ortho-, pyro-, and metaphosphoric acids were calculated. These methods of determination may be greatly simplified,
especially for anhydrous acids as inentioned above, if the silver determination is replaced by measurement of the sodium hydroxide required to neutralize in the presence of an excess of silver, while the accuracy and precision of the acidimetric titrations may be greatly inrreased by making allowance for the differences between the equivalence points of the ortho- and pyrophosphates.
Outline of Proposed Method For titration purposes, meta-, pyro-, and orthophosphoric acids are considered, respectively, as mono-, cii-, and tribasic acids which may he quantitatively neutralized in three steps. For the determination a portion of the anhydrous acid, carefully diluted with ice n a t e r to avoid hydration, is titrated under stated conditions with standard sodium hydroxide solution to pH 4.4 (a provisional value) a t which point the reactions are :
+ + +
HPO, SaOH = Sap03 HzO H4Pz07 2SaOH = Sa2HzPz07 2Hz0 H3POI XaOH = XaH?P04 HzO
++
By continuing the titration to pH 8.8 (a provisional value) in the presence of a suitable quantity of added sodium nitrate to reduce hydrolysis, additional hydrogens of ortho- and pyrophosphoric acids are neutralized, giving:
+
+ +
SazHzP20, 2XaOH = NaaPzO;. 2H20 SaHZPO4+ NaOH = Iia:HP04 1120
If a t this point an excess of silver nitrate is added, normal silver phosphates are subsequently precipitated, and the remaining hydrogen of the orthophosphoric acid can be titrated with sodium hydroxide, using methyl red as indicator
INDUSTRIAL AND ENGINEERIKG CHEMISTRY
520
T h e provisional values of p H 4.4 and 8.8 used in the above explanation are subject to further definition depending upon the relative amounts of ortho- and pyrophosphoric acids present. Under the conditions of test, the equivalence point of monosodium orthophosphate is p H 4.6 while that of sodium acid pyrophosphate is 4.2. T h e equivalence point of disodium orthophosphate is likewise p H 8.5, while t h a t of tetrasodium pyrophosphate is 9.1. T h e end-point values for mixtures T d l lie between the equivalence points of the pure salts. If the relative proportions of ortho- and pyrophosphoric acids are roughly known, the desired end-point values may be obtained directly from a linear interpolation scale such as Figure 2 . If the proportions are unknown, buret readings should be taken a t p H 4.2, 4.4, and 4.6 in the bromocresol green titration, and a t p H 8.4, 8.6, 8.8, 9.0, and 9.2 in the thymol blue titration or a t least a t such p H values as will embrace the end point. The final titration volumes can subsequently be determined when the exact end points have been ascertained from a nomograph such as Figure 3. Since the methyl red titration may, on option, be performed after the thymol blue (or oleo red B) titration, two methyl red titration scales are shown in this figure. K h e n the polybasic acid present is chiefly pyrophosphoric, as foretold by a white, not yellow, precipitate upon the addition of silver nitrate, the substitution of Lahlotte oleo red B indicator for thymol blue is advantageous, as the color intervals at p H 8.8 t o 9.2 are then more distinct. I n this event, readings need not be obtained a t p H 8.4 and 8.6. Conversely, when the precipitate with silver nitrate is bright yellow, the titration need not be extended to include p H 9.0 and 9.2.
StLVER N I T R A T E M E T H Y L RED
END POINT
THYMOL BLUE (
OR OLEO RED 8 )
E N D POINT
pH 8.5 TO 9 .I WITH
SALT ADDED
BROMOC?CSOL GREEN
E\D p F 4 2
VOL. 10, KO. 9
POIYT TC 4 6
Apparatus and Solutions
FIGURE 1. RELATIONSOF TITRATION VALUESTO ORTHO-, PYRO-, AND M E T A - P ~ O CONTENTS ~ OF PHOSPHORIC ACIDS
I n each of the above titrations two moles of sodium hydroxide are equivalent to one mole of phosphorus pentoxide whether present as ortho, pyro, or meta. From the three titrations, the three components, expressed as ortho-, pyro-, and meta-Pz06, may be found b y two successive subtract,ions. Because of indicator conflicts, t h e above sequence of titrations is not followed in actual practice. Instead, tmo equal portions of the sample are titrated, one with bromocresol green indicator to p H 4.4, the other with thymol blue (or oleo red B) indicator to p H 8.8 in the presence of added sodium salt. At these equivalence points, silver nitrate may be added to either solution for the final titration with methyl red indicator. Since the methyl red end point is perhaps better seen in the solution containing bromocresol green, use of this solution is described in the procedure below. T h e methyl red titration equations are then:
+
+
NaP03 AgN03 = AgP03 SaN08 Na2H2P201 4rlgNOa 2NaOH = Ag4P20, 4SaNOs 2NaOH = AgSPO4 4- 3haN03 NaH2P04 3AgS03
++
++
+
++ 2H20 2H2O
Figure 1 shows the products present a t the different end points and the relations of titration values to the ortho-, pyro-, and meta-P2Os content.
Titrations are made in 250-cc. pH titrating flasks, Almquist type, marked to indicate the 100-cc. level. These flasks have side tubes of the same dimensions as LaMotte color standard tubes. The following freshly standardized LaMotte color standards are used: pH 4.2, 4.4, and 4.6, bromocresol green; pH 8.4, 8.6, 8.8, 9.0, and 9.2, thymol blue; andpH8.6, 8.8, 9.0, and 9.2, oleo red B. STANDARD SODIUM HYDROXIDE, 0.1408 N ; 1 cc. = 0.01 gram of P20a. Prepare a carbonate-free solution from centrifuged sodium hydroxide liquor (1). Standardize against pure benzoic acid. SILVER NITRATE SOLUTIOS: 0.85 N ; 144 grams of pure RATIO crystals per liter. Ten cubic ORTHO : PYRO centimeters are equivalent to 0.2 gram of ortho-P205, 0.3 gram of pyro-P20s, or 0.6 gram of meta-PzOj. Use in such quantities that all phosphate (and chloride if present) will be precipitated and at least 5 cc. in excess be pres4.4 8'8 ent. W 40: 60 BROMOCRESOL CREES Isw (13 DICATOR, 0.4 per cent soiutioii. Dissolve 0.4 gram of the dry m indicator in 4.1 cc. of 0.1408 N sodium hydroxide and a few cubic centimeters of alcohol. Dilute with water to FIGURE2 . PH END 100 cc. POINTS FOR VARYING THYXOLBLUE ISDICATOR, ORTHO:PYRO P206RATIOS 0.4 per cent solution. Dissolve 0.4 gram of the dry indicator in 6.1 cc. of 0.1408 W sodium hydroxide and 5 to 10 cc. of water. Dilute with water to 100 cc. METHYLRED ISDICATOR, 0.2 per cent solution. Dissolve 0.2 gram of the dry indicator in 60 cc. of alcohol. Dilute with water t o 100 cc. OLEORED B INDICATOR, 10 times normal strength. Mix 20 cc. of 60 per cent alcohol with 40 cc. of LaMotte oleo red B strong solution,-lj time. normal qtrength.
2
2
ANALYTICAL EDITION
SEPTEMBER 1.5, 1938
521
Procedure for Anhydrous Phosphoric Acids
60
PREPARATIOX OF SOLUTIOX.Set up a 400-cc. beaker containing about 200 cc. of ice water I OO I 50 stirred with a motor stirrer. Slon-ly pour the $ample, about 10 grams weighed by difference, into the beaker in such a manner as to avoid local overheating, When well mixed, transfer to 40 a 500-cc. volumetric flask, add water to mark, and remix. Transfer a 25-cc. aliquot (containing 0.3 t o 0.4 gram of phosphorus pentoxide) to each of tv-o pH titrating flasks. Make the titrations belovi without delay. BROMOCRESOL GREEN TITRATION. To one aliquot add 0.5 cc. of 0.4 per cent bromocresol green indicator. Titrate with standard sodium 40 hydroxide solution until pH 4.2 is approached, then dilute to 100-cc. volume. Continue the titration, recording the volumes of sodium hydroxide required to bring the pH to 4.2, 4.4, 0 / and 4.6 when compared against color standards. METHYLRED TITRATION. To the solution above at 4.6 pH, add 25 cc. of 0.85 Ar silver nitrate solution and shake briskly to coagulate k0 the precipitate. Add 0.5 cc. of 0.2 per cent methyl red indicator. Titrate with standard sodium hydroxide until the pink color of the liquid is just discharged. When near the end 70 point, shake briskly after each sodium hydroxide addition, then allow the precipitate to subside momentarily in order that the exact point of color change may be clearly seen. Shaking is important, as othervise the color of the solu80 tion may be muddy. THYMOL BLUETITRATIOS.To the other aliquot add 0.5 cc. of 0.4 per cent thymol blue indicator (or 0.5 cc. of oleo red B indicator, 10 times normal strength, when pyrophosphoric acid predominates) and 20 grams ( * 1 gram) of neutral sodium nitrate (or 14 grams of sodium IO0 chloride). Titrate with standard sodium hvdroxide 'until pH 8.4 is approached, then dilute FIGURE 3. PH NOMOGRAPH FOR FISDING ESD POINTS FROM TITRATIOSS to 100 cc. Continue the titration, recording the volumes of sodium hydroxide required t o reach pH 8.4, 8.6, 8.8, 9.0, and 9.2 as shownbycolor standards. Subtract from each volume a titration blank (usually Using these corrected volumes: 0.1 cc.) found by titrating to pH 8.8 a solution of 20 grams of sodium nitrate crystals in sufficientRater to make 100-cc. volume. 37.20 X 0.01 = 0.3720 gram of total PzOs ( 0 P 4-m )
gO1
CALCULATIONS. From the pH nomograph (Figure 3) determine the exact end-point values for the composition of the acid under test. Using these values, determine by interpolation the volumes of sodium hydroxide required to reach the two end points. Correct the methyl red titration volume for any shortage due to starting the titration a t pH 4.6. From the three corrected volumes designated below as BCG, TB, and JfR,calculate the composition :
+ ++
BCG X 0.01 = grams of total P205(0 p m) ( T B - BCG) X 0.01 = grams of P205(0 p ) [ M E - ( T B - BCG)] X 0.01 = grams of P205(0) Ex.niPm. A 9.7596-gram sample of acid (0.4880 gram in each aliquot) gave the following titrations: Bromocresol green
3 7 . 0 2 cc, t o p H 4 . 2 3 7 . 2 0 cc. t o p H 4 . 4 3 7 . 3 8 cc. t o p H 4 . 6
Methyl red (from p H 4.6)
53.41 cc. t o M R end point
Thymol blue (less blank)
7 2 . 3 6 cc. 7 2 . 7 0 cc. 7 3 . 0 0 00. 7 3 . 3 5 cc. 7 3 . 7 0 cc.
to to to to to
pH pH pH pH pH
8.4 8.6 8.8 9.0 9.2
Lay a straight edge on the pH nomograph connecting 53.41 on the left-hand scale (since the methyl red titration was run on the bromocresol green solution) with 35.80, 73.00 - 37.20 on the right-hand scale. This cuts the pH scale at 4.4 and 8.8. Using these end points, the results become: B C G titration M R titration (from p H 4.4) T B titration (less blank)
3 7 . 2 0 cc. t o p H 4 . 4 53.41 (37.38 - 37.20) 7 3 . 0 0 cc. t o p H 8 . 8
+
-
53.59 cc.
(73.00 [53.59
+
- 37.20) X 0.01 = 0.3580 gram of PzOs ('3 4-P ) - (73.00 - 37.20)j X 0.01 .= 0.1779 gram of PZOS(0)
By subtraction: iLleta-P205 = 0.0140 gram and pyro-P205 = 0.1801 gram. Converting these P20rvalues to acids n-e have 3.2 per cent of meta-, 46.3 per cent of pyro-, and 50.3 per cent of orthophosphoric acid.
Advantages of the Method Although neutralization with sodium hydroxide after treatment with silver was employed by Dworaak and ReichRohrwig (4),no use was made of the amount, of sodium hydroxide so required. I n the present method, the end point of the neutralization is determined by methyl red indicator which is satisfactory even in the presence of the other specified indicators, instead of by the less easily detected appearance of silver oxide. The sodium hydroxide required is then used in obtaining the orthophospl~oricacid content. The addition of alcohol is unnecessary, since the solubility of silver metaphosphate in aqueous solution is then of no concern. In those methods ( 8 , l O ) for the analysis of mixtures of orthoand pyrophosphates where calcium chloride has been used for precipitation, use has been made of the barium hydroxide or sodium hydroxide required after precipitation and heating to neutralize to phenolphthalein. The control of conditions to ensure a calcium phosphate of theoretical composition is, however, difficult. Titrations with methyl orange and Phenolphthalein indicators as previously performed are subject to inaccuracy be-
INDUSTRIAL AND ENGINEERING CHEhlISTRl-
522
cause of the relatively slovi p H changes in the neutralization of the phosphoric acids. The specifying of the indicators alone is insufficient; accuracy can be obtained only by careful p H measurements at the equivalence points (6). Furthermore, allowance should be made for the fact that tlie equivalence points of the ortho- and pyrophosphates do not coincide. I n the present method, the equivalence points a t a stated concentration are defined in terms of colorimetric pH, these equivalence points are a t the sensitive section of the p H range of the specified indicators, and the addition of salt to reduce hydrolysis is prescribed in such a concentration that the equivalence points of tetrasodium pyrophosphate and of disodium orthophosphate are brought into definite proximity as sliown in Figure 4. Moreover, a sliding scale of end points has been constructed for varying proportions of ortho- and pyrophosphoric acids. These features, in conjunction with the use of p H color standards, greatly increase the precision and accuracy of the acidimetric titrations. Since in the present method these titrations are done before the addition of silver nitrate, the filtration and treatment with hydrogen sulfide or hydrochloric acid to regenerate the free acids are unnecessary. No color standard is used in determining the methyl red end point on account of the precipitate present in the solution. However, since the polybasic phosphoric acids have been precipitated by the silver, the titration is that of a strong acid, nitric, nit11 a strong base giving a sharp break in p H a t the end point. The break is so sharp that by using a low-resistance glass electrode the end point may be satisfactorily determined by potentiometric means 7%ith a n electron beam sectrometer (9) which indicates only changes of e. m . f . on addition of titrating qolution and does not determine p H values. The use of this instrument permits a more rapid determination, as no paiiic3 during titration are required to view the indicator coloi, but i i not a necessity in obtaining good titration value
