Determination of Soluble Silica in Water A Photometric Method H. LEWIS KAHLER W. H. & L. D. Betz, Philadelphia, Penna.
than the pyrocatechol type and therefore gives higher silica equivalence. This 1%-orkshowed conclusively that the Dienert-Kandenbulke test could not be used for accurate work in x-aters having tannins present. Isaacs (4) reported that reduction of the silicomolybdate complex by sodium sulfite to molybdenum blue of composition Mo808.H20( 8 ) in low acidity is not accompanied by color from phosphomolybdates. Bertrand (1) criticized the method, but Foulger (3) showed that phosphate, free from silica, gives no color with the reagent a t the acidity recommended by Isaacs. The latter procedure was not found suitable for rapid photometric work, as it employs acetic acid a t relatively high pH, which gives color progression too great for accurate work. I n the present study, the use of hydrochloric acid for development of the silicomolybdate complex a t p H 2.4 to 2.7, followed by reduction by sodium sulfite, was found to be very satisfactory, giving adequate silica sensitivity and no practical interference from phosphate and tannins. The effect of pH on the formation of phosphomolybdate before reduction and the subsequent silica equivalence after reduction is given in Figure 1. As the acidity decreases above pH 2.7, the effect of phosphate becomes negligible, but the color development when silica is present requires more time and is accompanied by considerable color progression. The p H range of 2.4 to 2.7 before reduction is considered optimum and is the basis for this present method.
This method measures the molybdenum blue color developed by reducing the yellow silicomolybdate complex with sodium sulfite at suitable pH. It requires only 4 minutes for a test, a 10-ml. sample, and gives an accuracy on par with gravimetric analysis. Phosphate, alkalinity, tannins, iron, and other ions present in natural and boiler waters in addition to color, temperature, and color progression, offer no practical interference. The work was completed with a Klett-Summerson photoelectric photometer, but the method can be employed with any similar instrument.
D
I E K E R T and IVandenbulke (2) have shown that small concentrations of soluble silica can be determined colorimetrically by measurement of the yellow color that develops on the interaction of soluble silica and ammonium molybdate in acidic solution. The yellow color is stated to be a complex of composition H&i(hIo0,)cH20 (2, 6). Since the appearance of this method, Thayer (12) has shown that it is subject to interference from phosphate and iron, due to formation of phosphomolybdate and iron silicomolybdate. Xumerous other investigators have used essentially this same method for the determination of silica, most of them using Nessler or Hehner tubes for comparison or different types of colorimeters (6,7,9) such as Duboscq (IO). An exhaustive survey of the literature has failed to reveal any mention that tannins in natural and boiler waters interfere with this method. This interference was noted in this laboratory while attempting to apply the Dienert-Wandenbulke method using the KlettSummerson photoelectric photometer. Satural and boiler maters gave higher results than gravimetric analysis. I n order to investigate this further, tannins of two different classes, pyrocatechol and pyrogallol, were investigated for their reaction with molybdate by the Dienert-Wandenbulke procedure applied to the Klett-Summerson photometer and the Taylor visual comparator. The results recorded in Table I show that both types of tannins interfere with this method, giving high silica equivalence. The pyrogallol type per unit weight forms a more intense yellow complex with molybdate
EFFECT OFACIDITY ON REDUCT ION OF
PHO SPHOMO LY BDATE BY S U L F I T E I N ABSENCE OF S I L I C A FICURE 1
/
/
I
/
30 2.5 2.0 PH OFSOLUTION BEFORE REDUCTION
Reagents Hydrocliloric acid reagent, 0.248 N , 20 mi. of 38 per cent grade (1.19 specific gravity) acid per liter, TABLE I. INTERFERENCE OF TANNINS WITH THE DIENERT-WANDENBULKE Ammonium molybdate reagent, 102 grams of amMETHODIN THE ABSENCEOF SILICA monium molybdate (grade 81 per cent Mo03) per --Pyrocatechol Class Tannina,--Pyrogallol Class Tanninoliter. Sodium sulfite reagent, 170 rams of sodium Si02 Si02 sulfite (grade 97 per cent, anhyjrous) per liter. Si02 equivalence, Si02 equivalence equivalence, Taylor equivalence, Taylor Sodium silicate solution, 102.0 mg. as silica per Tannin photometer Tannin comparator Tannin photometer Tannin comparator liter. Pp.m. P.p.m. P.p.ni. P.p,m. P.p.ni. P.p.m. P.p.711. P.p.nz.
Procedure and Conditions of Test The procedure uses 10 ml. of sample for analysis,
5 ml. of hydrochloric acid reagent, 5 ml. of ammonium molybdate reagent, and 10 ml. of sodium
* Commercial grades of silica-free tannins.
sulfite reagent. 536
The sample is treated with the
August 15, 1941
ANALYTICAL EDITION
TABLE11. EFFECTOF PHOSPHATE Disodium Phosphate Present P . p . TIL. PO, 20 40 100 200 300
7 -
Present,
9.2 p. p. m. P. p , Wk.
9.3 9.4 9.4 9 .6 0.0
Silica Found b y Photometer Present, Present, 42.0 p. p. m. 24.7 p. p. m. P . p . ni. P . p . n1. 24.6 42.3 24.6
42.2 42.2
24.6 23.0
24.8
41.5 40 8
TABLE111. EFFECTOF TANNINS~ Tannin Present Pyrocatechol Pyrogallol Pure tannic type type acid 1’. 11. 711. P . 11. 111. I’. p . l i i .
7 -
Si02 Present
Si02 Found
P.p,m.
P.p.in.
Si02 Difference P.p.m.
18.5
18 6 19.7 21.2 21.6 ‘2.5
+0 1 +1.2 f2.7 +3.1 +4 0
1X.i
+0.2 +0.8 +2.8 +3.6
400 BOO
+4.5 +5.2
800 1000
lS.5
19.3 21.4 22.1 23.0 23.7
io0 400 600 1000
.. ..
200
..
.. ..
100 600 1000 lS..5
20.5 +2.0 200 23.0 +4.5 600 25.0 +6.5 .. 1000 0 All products were SiO.-free. Pyrocatechol a n d pyrogallol types are i o % tannins; pure tannic acid is 100%.
acid andmolybdatereagentsand allowed to stand for approximately 1 minute. This gives the silicomolybdate complex time to develop completely, after which it is stable for 5 minutes. During this time interval, another sample is treated with the acid and sulfite reagents in addition to distilled water equivalent to the molybdate reagent, and is used to adjust the photometer to zero reference. When this operation is complete, the test sample is reduced by the sulfite reagent and tested a t the end of 1 minute. The Klett-Summerson photometer has two photocells of the blocking type, a standard source of illumination, a sensitive galvanometer, color filters of average bands, a 21-cm. (14-inch) logarithmic scale, and operates with 110 volts alternating or direct current. Summerson (11) gives a detailed discussion of the photometer. The work u-as completed with a 13-mm. test tube to facilitate rapid routine analysis. The instrument also accommodates sample cell depths of 2.5, 10, 20, and 40 mm. The blue reduction color of the final silica test was found to absorb best in the spectral region 600 to 700 mp. In this work, a 620 mp filter (590 to 680) was employed. With this procedure a standard curve n-as established from a series of points of known silica content. The curve is linear throughout the concentration range of 0 to 50 p. p. m. of silica used in this study. The curve was checked by other standard solutions prepared from pure silica fused in sodium carbonate, and hydrolyzed silicon tetrachloride. Hard-rubber bottles were used throughout this study to prevent contamination from glass bottles.
Effects of Ions and Conditions on the Method Orthophosphate, a common constituent in boiler waters, does not interfere with the method, as shown in Table 11. A comparison of results given in Figure 1 and Table I1 shows t h a t phosphate in the absence of silica gives some silica equivalence, while in the presence of silica this effect is observed only with low silica content. I n Figure 1, 300 p. p. m.
537
of disodium phosphate as PO4exhibit 0.8 to 2 p. p. m. of silica between p H 2.7 and 2.4. I n Table 11, 300 p. p. m. of POr in the presence of silica give as high as 0.2 p. p. m. of silica in low silica content and give low silica results on the order of 1 p. p. m. in high silica content. The reason for this phenomenon is not clear. It cannot be explained on the basis that the phosphate buffers the solution away from the correct p H range, as the bufferingeffect of this concentration of phosphate was found to be only 0.1 pH. It must in some way interfere with either the complete formation of the silicomolybdate complex or the reduction of t’his complex to molybdenum blue. Tannin of two commercial varieties and pure tannic acid offer no practical interference, as Table I11 reveals. The effect of the tannins is different in the presence and absence of silica. The yellow tannin molybdate complex is not reduced by sulfite to molybdenum blue, but its color gives a n additive effect to the silica present. Even in the most highly colored boiler maters, the tannin concentration is rarely over 200 p. p. m., which offers no practical interference to the method. Snell (IO) states that iron interferes with the DienertKandenbulke met’hod. The effects of both ferrous and ferric ions on the suggested method are reported in Table IV. Ferric iroii gives no interference throughout the concentration investigated, and the effect of the ferrous iron is negligible in low silica content and not serious in high silica concentration. Average natural and boiler n-aters contain far less concentration of these ions than is required to give any d i c a interference. The temperature of sample and reagents is a factor in this method and should be maintained within * 5” F. of the solution temperature at time of standardization. Temperatures a few degrees helom or above this limit give no serious error. High temperatures promote color development with attendant high silica results, while lower temperatures have the opposite effect. TABLE IV. EFFECTOF FERROUS AXD FERRIC IONS Present 1’. p .
1,i.
--Silica as Si()?Found 1’. p.
---
7,l.
6.3 6 4 6.3 6.3 6,; 6.3 6.3
6 4
2.5.5
-0
Present P . p . m.
1
O’i
0 0
+0.1 -0.1 +0.1
1. 3 2.7 6.7
ti:7 13.4 26.8
..
1 -0.1 -0 1 +I 2 -0
i.6
FeTPresent I’. p .
!,
