Volumetric Determination of S o h ble Silicates in Detergents A. M. LAWSON, L. A. JONES, and 0. T. AEPLI Pennsylvania Salt Manufacturing Co., Wyandotte, M i c h .
i rapid volumetric method has been developed for the determination of soluble silica in alkaline detergents containing various compositions of the soluble silicates. Sodium oxide can also be determined on the same portion of the sample. RIodifications necessary for the accurate volumetric determination of the soluble silica in the presence of carbonates, phosphates, and wetting agents are described and discussed. The results obtained are in agreement with the gravimetric values to within &0.05% of silicon dioxide.
T
H1.; gravimetric methods for the determination of soluble silica consist of evaporating the solutions containing the silicates with either hydrochloric, sulfuric, or perchloric acid in order to dehydrate the silica. The silica is filtered off, washed, dried, and suhsecjuently ignited and weighed. For very accurate results it is also necessary to treat the final weighed silica with hydrofluoric acid and to subtract the residue from the total weight t,o ohtain the actual silica present. These procedures are tedious and time-consuming. Since this laboratory is interested in the rapid determination of silica in various alkaline silicates and detergents for plant control during manufacture, a rapid procedure for t,he determination of silica which can be used successfully by nontwhniral laboratory personnel is desirable. Titi1 (3)mentions that soluble silica can be determined in alkalinr silicates hy first tit.rating a suitable aliquot with 2;L' hydrochloric acid t,o a methyl red end point and then completing the titration after the addition of sodium fluoride. Si02
+ 6F- + 6 H + +. 2 H + + SiF,-- + 2H20
T h e ithove reaction is not instantaneous, making it very difficult. to obtain the proper end point. €l:tlft,er ( 1 , 8)determined silicn in alkaline silicates bj- adding to :i xnrasurctl volume of 0.33-1- sodium fluoride or potassium fluoride solution, 0 . 1 S hydrochloric acid to a pH of 4.9. This solution was t,hen mixed with a measured quantity of an alkaline silirate solution containing no ration ot,her than alkali. The solution n'as again titrated h r k to a pI1 of 1.9with 0.LV hydrochlo!ir acid in the presence of bromocresol green or purple as inclicaat,ors. The calculation is hascd on the equation: 6SaF
+ SiOz + 4HC1 = XaZSiFg + 2H20 + f S a C l
This procedure is also slo~r-and does not give the quick reliahlr, results necessary for plant control. Consequently t'he following rapid volumetric met,hod was developed. K i t h the follouing modifications, accurate and quick determinat,ions of silica can be made on composit,ions conhining carbonates, phosphattxs, and surfact,ants. The sodium oxide may also be determ i n d on the same portion of t,he sample.
PROCEDURE AKD RESULTS
Analysis of Sodium Silicates. A 20-gram sample of sodium silicate is dissolved in distilled water, transferred to a 1000 ml. volumetric flask, cooled, and diluted to the mark. A 50-nil. aliquot, is t'ransferred to a 250-ml. beaker, and approximately 0.5 ml. of methyl red indicator is added. The solution is titrated with I'V h:-drochloric acid t o the first color change and the volume of I N hydrochloric acid at this point, is noted. Approximately 5 grams of sodium fluoride are added to the titrated solution and as much as possible is dissolved by agitation. ;\iter the addition of 25 ml. of ethyl alcohol, the titration with 1 S hydrochloric acid is continued until the color of the solution ip definitely red, indicating a n excess of hydrochloric arid. An excess of about 2 ml. of LV hydrochloric acid is sufficient. hbont 0.5 ml. of the methyl red-xylene cyanol FF indicator is added a t this point and the solution is titrated back with I S sodium hydroxide until the end point is reached. The end point color is an intermediate gray or the color immediately after the disappearance of the pink and before the appearance of the green. Calculation. a = ml. of 1 S hydrochloric acid to first methyl red elid point b = ml. of LY hydrochloric arid after the addition of sodium fluoride and ethyl alcohol c = nil. of 1.V sodium hydroxide used for back-tit,ration
% SiO,
% S a 2 0 = (a)(0.03100) (100) The authors analyzed samples of Ort,hosil (Pennsylvania Salt Manufacturing Co.) and other soluble sodium silicates by the volumetric method and compared the resu1t.s (Table I ) Tr-ith the gravimetric method, using sulfuric acid to deh>-tllat'e tlic soluble silicic acid to insoluble silica. Analysis of Sodium Silicates in Presence of Carbonates. The sample of the detergent to be analyzed is prepared as above and a 50-ml. aliquot is t'ransferred to a 250-ml. beaker. Approsimately 0.5 ml. of methyl orange is added and the solution i::
Table I.
Per Cent of Silica in Sodium Silicates Graiimetric Method 1 2 .iv.
Sample
Volumetiic Method 1 2 A T .
Orthosil"
29.92 29.89 2 9 . 9 1 2 9 . 9 1 29.03 29 00 2 9 . 0 2 29.04 2 8 . 7 0 2 8 . 7 5 2 8 . 7 3 28.73 29.80 2 9 . 8 3 29.82 29 85 29.00 29.10 2 9 . 0 5 29.04
Sodiumsilicate
27.68 4.87
27.66 4.87
27.66 4 87
2 7 . 6 3 27.63 4.86 4.86
Liquid sodium silicate Metsoh
25.39 28.44
25.37 28.53
25.38 28.49
25.35 28 45
Sodium disilicate (GD)b
55.03
55.17
55,lO 55.20
a b
REAGENTS
Sodium fluoride, reagent grade, Merck & Co., 11ic. H~-ilrochloric acid, standardized, 1 s . Sotlium hydroxide, standardized, 1 s . Ale:h?-1 red indicator (1 gram dissolved in 600 ml. of 95% ethyl :tlcohol diluted with 400 ml. of water). XIethyl red-xylene cyanol FF indiczt>or,0.8 gram of methyl red and 0.2 gram of xylene cyitriol FF dissolved in 1000 ml. of 95y0ethyl alcohol. hleth:.l orange, 0.1% in water. Ethyl alcohol, 95%. Foamex, Glj-co Products C o , . 26 Coirnt St., Brooklyn 2, N. Y.
= (b-a-c) (0.01502) (100)
7 -0.02 t o 02 - 0 01 -0.01 -0 03
27.63 4.80
- 0 03 -0.01
25.35 25.35 28,50 28.48
-0 03 -0.01
53.10
fO.03
55.1.:
Pennsylvania Salt Ilanufacturinq Co. Philadelphia Quartz Co.
Table II.
99.00 95.00 90 00 50.00
1810
29.94 2 9 . Q 3 29.04 2 9 . 0 1 28.70 28.72 29.80 2 9 . 8 3 29 00 29.02
~~~~~-
Per Cent of Silica in Orthosil-Sodium Carbonate Compositions
1.00 5.00 10.00 50 00
2 9 . 0 3 29.00 29.02 27.47 27.38 2 7 . 4 3 2 6 . 0 5 25.84 23.95 1 4 . 4 0 14 42 1 4 . 4 1
29.04 29.04 29 04 2 7 . 3 7 27.11 27.39 25.85 2 3 . 8 5 2 5 . 8 5 14.44 1 1 . 4 4 14 44
+O.0? -0 04 -0.10 +O 03
V O L U M E 27, NO. 11, N O V E M B E R 1 9 5 5
1811
T a b l e 111. Per C e n t of Silica i n Orthosil-Sodium Carbonate-Surfactant Compositions Composition, % KaKOd Surfactant, 2 % Pionic . . Kreelon Santomerse ... Nonic 1.OO 8.00 Sonic 8.00 lireelon
Orthosil 58 98 98 57 90 90
T a b l e I\-. Orthosil 66,67 60.00
Gravimetric Method 1 2 hv. 28.55 28 50 28.68 28 52 28.52 28.52 28.50 28.55 28.53 28.55 28.53 28.50 25.71 25.69 25.66 25.63 25.68 25.07
Composition. “0 SaQCOj Phosphate 21.07 11.66as Quadrafos 15.00assodium 23.00 t ripolyphosp hat e
% Sa?O =
% -0.05 -0 04 +0.01 -0.05 -0.04 +0.01
Volumetric Method 1 2 Ax,. 19.07 19.07 19.07
+0.03
18.10
18.10
-0
(h-a-c) (0.01502) (100) ( a ) (0.03100)
Difference,
Gravimetric Method 1 2 .-i 1.. 19.03 19.04 111.04
Calculations. CI = nil. of I S hydroch101,ic acid to first methyl orange end point 7, = nil. of I-\- liycli~ochloricacid after addit,ion of S a F and CzHbOH c = nil. of I S sodium hydroxide used for back-titration =
Difference,
Per C e n t of Silica in Orthosil-Sodium Carbonate-Phosphate C o m p o s i t i o n s
i i t r:ited t o the end point with lA\-iiydrorhloric acid. The carbon dioxide is removed from the solution b y air-lancing with carbon dioxide-free air for 5 minutes. The incomplete removal of c:irhon dioxide will give low results. dpproximately 5 grams of sodium fluoride are added to the solution and as much as possible is dissolved by agitation. hbout 0.5 nil. of methyl red-xylene c>xnol FF indicator is added and after the addition of 25 ml. of Cathy1 alcohol the titration is continued until an excess, or about 2 nil. of hydrochloric acid, are present. The excess hydrochloric :icid is tiack-titrated with l.\- sodium h>.droxide t o the end point.
% SiOr
Volumetric RIethod 1 2 Av. 28.40 28.50 28.48 28.48 28.46 28.60 28.52 28.56 28.54 28.48 28.46 28.60 25.63 25.65 25.67 25.68 25.70 25.65
(100)
lristures of OrthoPil a n d sodium carbonat,e were prepared in the laboratory in order to shon. the effect of the presence of carbonates. T h e results and coinposit,ions of the mixtures are listed in Table 11. Analysis of Sodium Silicates in Presence of Carbonates and Surfactants. The pro-edures described above are followed, exccl)t that it is necessary to add 2 or 3 drops of Foamex during the air-lancing to prevent exress foaming. The presence of surfactants-such as Sonic. (Sharples Division, Pennsylvania Salt llanufacturing Co. ), Kreelon (Kyandotte Chemical Co. ), and Santomerse (llonsanto Chemical Co.) does not interfere in the titration for solihle si1ic.a. The detergent compositions analyzed \$-(’reprepared in tlie laboratory. The results are given in Table 111. Analysis of Sodium Silicates in the Presence of Carbonates, Phosphates, and Surfactants. The reagents required are identical to those above, except that methyl orange indicator is used instead of methyl red. A suitable aliquot is transferred t o a 230-1111. beaker and titrated to thc methyl orange end point XTith I .I- hydrochloric acid. The titrated solution is air-lanced for :ihoiit 5 minutes with carbon dioxide-free air. Five grams of sodium fluoride are dissolved in the titrated solution and the resulting solution is titrated with I S hydrochloric acid after the aridition of 25 nil. of ethyl alcohol and 0.5 ml. of xylene cyanol FF indicator. -4n excess of about 2 mi. of the 1-I‘ hydrochloric :ic.id is added and back-titmteti with lA\- sodium h?-droxide to the rnd point. Calculations. a = nil. of lA\-hydrochloric acid t,o methyl orange end point h = nil. of 1-V hydrochloric acid after addition of sodium fluoride and rthyl alcohol
18.12
18.11
c =
18.10
18.10
”% 01
ml. of 1-V sodium hydroxide used for hack-titration
% Si02 = (b-a-e) (0.01502) (100) % Sapo= (a)(0.03100) (100) The analyses of mixtures containing Orthosil, sodium cart)onate, and phosphates are listed in Table Is’. DISCUSSlOS
The reactions
+ 2HC1+ 2SaCl + HZSi03 + H20 + 6 S a F + H20 SaaSiF6 + 4NaOH S a O H + HC1- S a C l + H2O
SazSiOl,H20 H,SiOl
+
involved in this analytical procedure show that 4 moles of sodium hydroxide are liberated for each mole of sodium silicate contained in the det,ergent,. The resulting liberat,ed sodium hydroxide can t.hen he tit,rated with the standardized acid. I n preliminary xork the authors attempted t,o titrate the liberated sodium hydroxide direct,ly but found that, the reaction was slow and the end point obscure. S e a r the end point, it was necessary to add the standardizrd acid in small increments and wait until the reaction was completed before continuing to the end point. Incorrect results could tie obtained if t,he time factor n-as not considered. ’The authors found t,hat h y adding an excess of standard acid and ethyl alcohol the reaction rate \\-as increased, so that no n-aiting period n-as necessary to obt,ain a sharp and accurate end point. The use of xylene cyanol FF-methyl red indicator increases thv accuracy of the end point so that it can he readily ohserved k)y nonteclinicul personnel. ACKh-OWLEDGMEST
The authors wish to express their appreciation to t,he I’enrisylvania Salt llanufacfuring Co. for permission to puhliph this article. REFERENCES (1) H a l f t e r , G e o r g e , A?igew. Ciiem.. 61, 413 (1949). (2) H a l f t e r . G e c r e e , Z . anal. Chem.. 128, 266-9 (1948). (3) \-ail, J. G . , “Soluble Silicates. T h e i r P r o p e r t i e s arid Uses.” r o l . 1, pp. 40-1, R e i n h o l d , N e w York, 1952. (4) Willard, H. W., a n d Diehl, H., “ A d v a n c e d Quantitative .-lnaivs i s , ” p. 186, S’an S o s t r a n d Co., S e w York. 1943.
RECEIVLDfor review December 29, 1’354, Accepted July 19.55. P ~ P sented before the Analytical Division a t the 16th 11idwert Regional \Ier.tinz of tlie AMERICASC a ~ ~ r r c SOCIETY, .4~ Omaha. Xeh., 1
