INDUSTRIAL CHEM ISTRY and ENG I NEE RING ANALYTICAL EDITION
21,000 Copies of This Issue Printed
Issued July 15, 1939
Vol. 31, Consccutivc No. 27
Harrison E. Howe, Editor V o l. 11, N o . 7
De t e r m in a t io n o f To t a l Dis s o l v e d So l id s in Wa t e r b y El e c t r ic a l Co n d u c t iv it y...
... H. Gustafson and A. S. Behrman 355
Rig id it y o f St a r c h Pa s t e s...
... Bernadine Brimliall and R. M. Hixon 358
C o l o r i m e t r i c D e t e r m i n a t i o n o f F l u o r i n e w it h
Fer r o n...Joseph J. Fahey 302
D e t e r m i n a t i o n o f C ad m iu m in S i l i c a t e R o c k s . . . ...E . B. Sandell 364
Co l o r im e t r ic Det f.r m in a t io n o f Nic k e l a s Nic k e l- Am m o n ia Co m p l e x Io n ...
...Gilbert II. Ayres and Francene Smith 365
Su l f a m ic Ac id in Se p a r a t io n o f Ra r e Ea r t h s . . . .
. . . J. Kleinberg, W . A. Taebel, and L. F. Audrieth 368
Co n t in u o u s Ex t r a c t io n Dif f u s io n De v i c e...
...Martin Meyer 369
T in - P h o s p h o r u s P r e c i p i t a t e in B r o n z e A n a ly s is . .
...Owen Gates and Louis Silverman 370
Ph o t o m e t r ic Me t h o d f o r De t e r m in a t io n o f Ca r b o n
Di o x i d e...
. . . . Richard J. Winzler and J. Percy Baumberger 371
V o l u m e t r ic O x i d a t i o n o f I o d id e t o I o d a t e b y S o d iu m C h l o r i t e . . . . L. F . Yntema and Thomas Fleming 375
D e t e r m i n a t i o n o f F l u o r i n e in W in e . H. G. Rempel 378
Ef f ic ie n t De f o a m in g Ag e n t; ... .
...Phileas A. Racicot and Carl S. Ferguson 380
D e t e r m i n a t i o n o f U r i c A c id i n M ix e d E x c r e m e n t s o f B ir d s . Ray L. Shirley and A . H . VanLandingham 381
D e t e r m i n a t i o n o f E l e c t r o m e t r i c E q u i v a l e n c e P o i n t s ...John R. Gay 383
P h y s i c a l a n d C h e m ic a l P r o p e r t i e s o f P e t r o l e u m F r a c t i o n s . . . Harry T. Rail and Harold M . Smith 387
D e t e r m i n in g A m y l A l c o h o l C o n t e n t o f D i s t i l l e d S p i r i t s ...S . T. Schicktanz and A . D . Etienne 390
Kn ia s e f f Fa t Te s t ...
...Horace II. Selby and Thelma A. Selby 393
El e c t r o d ia l y z e r f o r St a r c h ...
...R. M. Hixon and Vera Dawson Martin 395
Gr e a s e l e s s Hig h-Va c u u m Va l v e...
...R. H. Crist and F. B. Brown 396
Im p r o v e m e n t f o r Me n is c u s Re a d e r...
...Edgar J. Bogan 396
S o le n o id S t i r r i n g D e v ic e f o r U s e in C o n f i n e d S p a c e s . . . H. I I. Rowley and Robert B. Anderson 397
Im p r o v e d Th r e e-Wa y St o p c o c k...
... Marvin A. Smith and Frank L. Hayes 397
E l e c t r o n T u b e D i r e c t C u r r e n t V o l t m e t e r o f N e w D e s i g n ...R . L. Garman and M . E . Droz 398
C o n t i n u o u s S u p p ly o f H o t D i s t i l l e d W a t e r . . . .
...George G. Marvin 399
Mic r o c h e m is t r y :
De t e r m in a t io n o f Le a d b y Dit h iz o n e
Karl Bambach 400
Re f r a c t iv e In d e x Me a s u r e m e n t s in Qu a l it a t iv e
Or g a n ic Mic r o a n a l y s i s...
... Paul L. Kirk and C. S. Gibson 403
N e e d l e V a l v e f o r M ic r o - D u m a s D e t e r m i n a t i o n o f N i t r o g e n . E . B . Hershberg and Lyon Southworth 404
H a n d li n g o f H y g r o s c o p ic S u b s t a n c e s in M ic r o c h e m ic a l D e t e r m i n a t i o n o f C a r b o n a n d H y d r o g e n ...Clement J. Rodden 405
Im p ro v e d M e r c u r y - S e a l e d M ic r o A b s o r p tio n T u b e
... Rex J. Robinson and Donald .T. Doan 400
C o l o r i m e t r i c M i c r o d e t e r m i n a t i o n o f B o r o n . . .
...James A . Naftel 407
M i c r o v i s c o m e t e r ...John R. Bowman 409
Qu a n t it a t iv e Or g a n ic El e m e n t a r y Mic r o a n a l y s is
w i t h o u t M i c r o b a l a n c e ...J. B. Niederl, V. Niederl, R. H. Nagel, and A. A. Benedetti-Pichler 412
The American Chem ical Society assum es no responsibility for the statem ents and opinions advanced by contributors to its publications.
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Determine Titanium and Zirconium with Newest MALLINCKRODT Analytical Reagent ACID P A R A - H Y D R O X Y - P H E N Y L A R S O N I C
Simpson and Chandlee* describe a new method for effectively separating titanium and zirconium from other commonly occurring ions by means of a single precipitation. The reagent used was Mal
linckrodt Acid Para-Hydroxy-Phenylarsonic, A.R.
This newest addition to the Mallinckrodt Analytical Reagent family is designed according to specifica
tions for this analytical procedure, and is ready for use as received.
4 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 11, NO. 7
Send for descriptive literature on this new chemical and for the Mallinckrodt Catalog of Analytical Reagents and Laboratory Chemicals, which shows the predetermined maximum limits of impurities for nearly 500 chemicals and reagents.
* Sim pson, C. T . and Chandlee, G. C., Ind.. and Eng. C hem .’Anal. Ed., 10:642, N ov. 15, 1938.
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6 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 11, NO. 7
• Do yo u test f o r la r g e amounts of M an g an ese in an ore, or small amounts in an alloy?
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Potassium Periodate, Reagent
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INDUSTRIAL and ENGINEERING CHEMISTRY
A M LYTIC A
1EDITION ♦ llu rrison E. H ow e, Editor
Determination of Total Dissolved Solids in Water by Electrical Conductivity
II. GUSTAFSON AND A. S. BEHRMAN, International Filter Co., Chicago, 111.
T
HE determination of total dissolved solids in natural waters may at times serve a number of different purposes, but the principal value of the measurement in a fairly complete mineral water analysis is to check the sum of the constituents as individually determined or calculated. Lack of reasonably good agreement between the total solids de
termined as such and the total solids as calculated from the analytical data is usually evidence either of an error in analy
sis or computation, or of the presence in important quantity of some substance that has been overlooked in the analysis.
For example, the usual practice in water analysis does not determine the alkali metals directly, but calculates and reports in terms of sodium the stoichio
metric alkali metal equivalent of the excess of anions remaining after combination or compari
son with the determined cal
cium and magnesium. Since nitrates are not usually deter
mined in the ordinary mineral -water analysis, it would thus be possible to overlook com
p letely a large q u a n tity of sodium nitrate unless some method is provided for bringing the omission to light. The de
termination of total dissolved solids accomplishes this purpose.
If it is to serve its purpose well, the total solids determina
tion must be dependable when applied to all types and con
centrations of waters and must have an accuracy comparable with that of other analytical methods employed. For practi
cal reasons, the procedure should require a minimum of time and labor.
The usual method of deter
mining total solids by evaporat
ing a measured volume of water and drying the residue at a definite temperature is not en
tirely satisfactory. Complete dehydration of the residue can
not be secured in all cases without serious loss of certain mineral constituents, particularly chlorides and nitrates (1).
The presence of organic matter and fine suspended material often adds to the difficulty of the determination.
As an alternative to the measurement of total solids L weight, electrical conductance has been employed for a nurr ber of years in certain special applications, as in detection <
condenser leakage, examination of boiler and irrigation watei {2, 4, 5), and as a rapid means for following fluctuations in i particular water. The conductivity method has consistent!}
been described as approximate; several investigators have shown that a simple factor cannot be applied for all types of
waters or for different concen
trations of the same type of water. R ecently, how ever, Kitto (S) has described a con
ductivity procedure wherein electrical conductance and ana
lytical data are employed in the estimation of total solids; while the method appears rather com
plicated and cumbersome, it does constitute a direct effort to take into consideration the nature of the chemical compounds present when interpreting conductance measurements.
The method of determining total solids by conductance devised by the present authors is based on a recognition of the quantitative influence of the nature of the compound on the conductance. One of the first steps in devising the method, therefore, was a series of meas
urements of conductance values for the various electrolytes commonly found in natural waters, and for the low concen
trations normally encountered.
These concentrations range in general from a few to several thou sand p a rts per m illion (roughly 0.0001 Ar to 0.1 N). In order to avoid complicating The principal utility of the determ ination of total
solids in a reasonably com plete m ineral analysis of water is to provide a check on the other analytical data. The standard m ethod of determ ining total solids is to evaporate a m easured volum e of the water and dry the residue at a specified tem pera
ture to constant w eight. This usually requires several hours and an appreciable am ount of labor;
furtherm ore, the procedure is n ot always a satis
factory one, since its accuracy m ust depend on heating the residue to a tem perature high enough to ensure com plete dehydration w ithout causing any chem ical decom position. '
Electrical conductance has been utilized for m any years as a rough em pirical index o f the total solids content of a water, and has been pu t to prac
tical use in such applications as the detection of condenser leakage. Efforts to use conductance as a quantitatively accurate m easure of total solids, however, have generally been unsuccessful, because o f the m aterially different conductance values of the various com pounds present in nearly all waters.
The present au thors have determ ined the conduct
ance values in dilute solutions of the com pounds involved, and have devised a m ethod whereby a single, quickly m ade conductivity m easure
m ent serves as an accurate check of the total solids calculated from the chem ical analysis. Consistent accuracy has been observed in using the m ethod in the analysis of well over a thousand water sam ples;
and the determ ination is now em ployed in place of the evaporation m ethod as a routine procedure.
355
356 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 11, NO. 7
T a b l e I. S p e c if ic C o n d u c t i v it y o f 0.001 N S o l u t i o n s Concentration SpeciSc C onductivity at
25° C. (X 10*)
Salt N orm ality P. p. m. as
CaCOa Determ ined Calculated Mg(HCOa)j
MgSO< 0.00127 6 3 .5 11.55
0.00127 6 3 .5 14.7 14.4
MgCls 0.00127 6 3 .5 15.4 15.35
M g(N O j), 0.00127 6 3 .5 14.95 14.7
Ca(H C O i)i 0.001005 5 0 .3 9.77
CaSO< 0.001005 5 0 .3 12.2 12.0
CaCli 0.001005 50.3 12.8 12.8
Ca(NOa)! 0.001005 50 .3 12.75 12.3
NaliCO a 0.00099 4 9 .5 9 .0 8
Na:SO< 0.00099 4 9 .5 12.3 12.1
NaCl 0.00099 4 9 .5 12.3 12.3
NaNOa 0.00099 4 9 .5 11.7 11.8
To prepare the approximately 0.001 N solutions required, the proper volumes of the strong solutions were transferred by pipets to 1-liter volumetric flasks, diluted with distilled water, aerated to remove free carbon dioxide, and finally diluted to tho mark with aerated water at a temperature of 25° C. Sulfate, chloride, and nitrate solutions of the same equivalent strengths were made by placing the same quantities of the strong bicarbonate solutions in 1-liter flasks, adding the proper volume of 0.1 N acid solution, aerating to remove carbon dioxide, and diluting to the mark with aerated distilled water at 25° C. A dip-type conductivity cell with cell constant 0.0963 was used. (Cell constant was determined with a 0.001 N solution of sodium chloride.)
From the determined values given in Table I were calcu
lated the conductivities per part per million at the concen
tration stated (Table II).
T a b l e II. C o n d u c t i v it y p e r P a r t p e k M i l l i o n
Salt N orm ality Specific C onductivity per
P. p. m. at 25° C. (X 10s) Mg(HCOa)j Av.
Oa(HCOa)î NaHCOa
0.00127 0.001005 0.00099
0.1 82 0.194
0.1 83 0.1 86
MgSOi CaSO<
NaiSOi
0.00127 0.001005 0.00099
0.231 0.242
0.2 4 8 0.2 40
M gCli CaChNaCl
0.00127 0.001005 0.00099
0.2 43 0.254
0.249 0.2 49
Mg(NOa)j Ca(NOa)i NaNOa
0.00127 0.001005 0.00099
0 .2 3 5 0 .2 53
0 .2 36 0.241
Conductivity D eterm ination of Total Solids
It seemed possible that the data of Tables I and II might serve as a basis for calculating the conductivity of 0.001 N solutions of known composition. Comparison of this calculated value with the measured conductivity, also at 0.001 normality, would serve to check the sum of the total solids computed from the other analytical data. With this thought in mind, various waters of accurately known composition and approximately 0.001 N were prepared and conductivities measured. Table III shows the analysis of these waters, the
T a b l e III. S p e c if ic C o n d u c t i v it y o f S y n t h e t i c W a t e r s
-C om position- C onductivity at 25° C. (X 10»)
irater Ca M g N a IiCOa SO* Cl NOa Determ ined Calculated
P arts per M illion as Calcium Carbonate
1 25.1 0 24 .7 25.1 2 4 .7 0 0 10.7 1 0 .6
2 0 3 1 .7 24 .7 24 .7 3 1 .7 0 0 12.0 12.2
3 0 3 1 .7 24 .7 3 1 .7 24 .7 0 0 11.8 11.8
4 25.1 3 1 .7 0 3 1 .7 25.1 0 0 11.9 11.9
5 12.6 12.7 2 7 .2 18.9 2 1 .2 12.4 0 11.8 11.7
6 5 .0 0 4 4 .5 2 9 .7 14.8 5 .0 0 10.7 10.3
7 17.6 9 .5 2 4 .8 2 .5 3 4 .5 9 .9 5 .0 12.4 12.4
8 2 7 .7 12.7 12.4 10.1 5 .0 3 7 .7 0 12.7 12.5
9 3 .5 1.9 4 4 .6 4 5 .0 2 .5 2 .5 0 9 .8 9 .6
10 3 5 .3 12 .7 5 .0 4 8 .0 2 .5 2 .5 0 10.4 10.2
factors existing in the more concentrated solutions, it appeared desirable to determine conductivities at a concentration as low as practicable. To eliminate the necessity for correction for the diluting medium (ordinary distilled water aerated to remove free carbon dioxide), the dilution must be limited to such a point that the conductance of the diluting medium is unimportant compared with the conductance of the solution.
The specific conductance of the aerated distilled water used by the authors is about 2 X 10-6 reciprocal ohm which is about 1.5 per cent of the conductance at 0.001 N, the con
centration selected by the authors for the modification of the method finally adopted, and which is reported here.
The ratio — for 0.001 N solutions is about 0.98 for uni- Ao univalent electrolytes—e. g., sodium' chloride—0.94 for uni- bivalent electrolytes—e. g., sodium sulfate—and 0.86 for bi-bivalent electrolytes—e. g., calcium sulfate. Specific conductivities calculated from published values for Ao using the above-mentioned conductance ratios are compared in Table I with values obtained by the authors at concentrations of approximately 0.001 N.
Preparation of Standard Solutions
Solutions of sodium bicarbonate (from c. p. NaHCOj), calcium bicarbonate (by carbonating a slurry of c. p. CaCOj), and of magnesium bicarbonate (by carbonating c. p. MgCOj) were prepared in concentrations ranging from about 0.02 to 0.1 N.
determined specific conductivity, and the calculated con
ductivity. Calculated conductivity equals the sum of HCO, X 0.186, SO* X 0.240, Cl X 0.249, and NO* X 0.241.
Most of the synthetic waters listed in Table III are similar in composition to natural waters. It will be noted that the maximum deviation of the calculated from the observed values is 4 per cent. In all cases, agreement is sufficiently good for the intended application of the procedure.
For applying the method to natural waters, several some
what different procedures were tested. The one finally adopted, and now employed as a routine procedure in the
T a b l e IV. D i l u t i o n T a b l e
T otal Solids from To D ilute D ilution Factors,
A nalytical D ata to 250 Cc. M ultiply by:
P. p. in. as CaCOs Cc.
0 to 55 250 1.00
55 to 75 200 1.25
75 to 100 140 1.79
100 to 150 100 2.50
150 to 200 70 3.5 7
200 to 300 50 5.0 0
300 to 400 35 7 .1 5
400 to 560 25 10.0
560 to 700 20 12.5
700 to 1000 15 16.7
1000 to 1500 10 2 5 .0
1500 to 2000 7 35.7
2000 to 3000 5 5 0 .0
Ta b l e V . Co n d u c t iv it y o p Na t u r a l Wa t e r s
W ater Ca M g N a HCOa S04 CL NOa Determ ined Calcuiat
P arts per M illion as Calcium Carbonate
1 375 200 18 270 313 10 0 130 128
2 160 120 939 172 17 1030 0 298 293
3 8 7 28 2 31 10 0 8 .8 10.3
4 282 66 166 210 157 147 0 116 113
5 97 42 32 80 83 8 0 3 7 .6 3 6 .8
6 2 6 485 334 104 55 0 104 101
7 5 4 17 5 1 4 16 5 .3 6 .0
8 16 6 9 17 10 4 0 6 .5 6 .5
9 205 18 24 169 31 23 24 5 3 .3 5 0 .3
10 270 82 831 350 3 830 0 280 272
11 12 20 827 396 349 114 0 183 186
12 92 112 449 76 259 318 0 154 155
13 687 400 117 244 954 6 0 278 276
14 32 26 9 53 10 4 0 13.4 13.2
T a b l e VI. D e t e r m i n a t i o n o p N o n c a r b o n a t e S o lid s N oncarbonate Solida
C arbonate C onductivity B y
W ater Solids m ethod analysis
1 270 320 323
2 172 1064 1047
3 2 34 41
4 210 308 304
5 80 91 91
6 334 168 159
7 5 17 21
8 17 13 14
9 169 88 78
10 350 860 833
11 396 438 463
12 76 560 577
13 244 930 960
14 53 14 14
laboratories of the writers’ organization is essentially as fol
lows:
The water sample is diluted in accordance with the amount of dissolved total solids (exclusive of silica) computed from the chemical analysis to provide a concentration of 40 to 60 p. p. m.
as calcium carbonate. (Table IV was prepared to facilitate the diluting procedure.) Air is then bubbled through the diluted sample to remove free carbon dioxide. If necessary, the tem
perature is adjusted to 25° ± 3° C. The conductivity is now determined in any suitable manner. The authors employ the familiar combination of (1) the slide wire of a Leeds & Northrup potentiometer, (2) a resistance box, (3) a microphone hummer as a source of high-frequency current, and (4) a dip-type conduc
tivity cell. The measured conductivity is corrected to 25° C. by use of the usual correction of 2 per cent per degree rise in tempera
ture.
The natural waters listed in Table V show the agreement obtained with highly different types of mineralization. De
termined conductivity equals conductivity at 40 to 60 p. p. m. concentration multiplied by the dilution factor.
Calculated conductivity equals the sum of the values ob
tained by multiplying the bicarbonate, the sulfate, the chlo
ride, and the nitrate (all as calcium carbonate) by 0.186, 0.240, 0.249, and 0.241, respectively. Inspection of a large group of samples indicates that the agreement between de
termined and calculated conductivities in Table V is perhaps a little poorer than average. The per cent difference be
tween total solids computed from the analysis and total solids by conductivity is readily obtainable by comparison of corresponding calculated and determined conductance values. Actual total solids by conductivity are secured by multiplying the sum of either the cations or the anions by the ratio of determined to calculated conductivity.
Examination of the data in Table V will show that the per cent differences between determined and calculated values are less than 4 per cent (the maximum difference in Table IV) in 11 of the 14 samples. Deviations exceed 4 per cent in samples 3, 7, and 9. In terms of parts per mil
lion, the differences amount to 6, 3, and 15 parts per million,
respectively. Consistent accuracy of the same order has been observed in the analyses of well over a thousand samples of water in which the conductometric determination of total dissolved solids has been employed instead of the evaporation method.
Rapid D eterm ination of Noncarbonate Solids •
The conductometric procedure for total solids presents an opportunity for a rapid, partial water analysis of considerable usefulness in connection with two rather recent developments in water treatment: hydrogen zeolite and anion ex
changers. The two determinations, alkalinity and conduc
tivity, when used in the formula below give carbonate and noncarbonate solids, the essential values for engineering calculations pertaining to hydrogen zeolite or to hj'drogen zeolite followed by anion removal. The sample should be diluted to a specific conductivity lying between 9 X 10 ~5 and 13 X 10-5 reciprocal ohm. The value obtained is then multiplied by the dilution factor.
Conductivity (X 10s) - (alkalinity X 0.186) _ noncarbonate
0-25 solids as
calcium carbonate The determinations and calculation require only a few minutes and results are amply accurate for the purpose served.
The usual procedure, involving analysis for sulfate, chloride, and nitrate as well as alkalinity, requires several hours’
time. Calculations have been made from the data of Table V to illustrate the usefulness of this simple procedure (Table VI).
Sum m ary
Determination of conductivity as described serves as a reliable rapid means of verifying the sum of the total solids computed from the other analytical data. The procedure is applicable to all different types of waters ordinarily encoun
tered and accuracy is consistent over the entire range of con
centration of natural waters. In addition, the conductivity procedure provides means for a rapid estimation of noncar
bonate solids, a determination ordinarily obtainable only at the expenditure of considerable time and effort.
Literature Cited
(1) Howard, C. S., Ind. Eng. Chem., Anal. Ed., 5, 4 (1933).
(2; Kerr, H. J., U. S. Patent 1,773,735 (Aug. 20, 1930).
(3) Kitto, W. H-, Analyst, 63, 162 (1938).
(4) Scofield, C. S., TJ. S. Dept. Agr., Circ. 232 (1931).
(5) Scofield, C. S., and Wilcox. L. V., U. S. Dept. Agr., Tech. Bull.
264 (1931).
Presented before the D ivision of W ater, Sewage, and Sanitation C hem istry at the 97th M eeting of the Am erican Chem ical Society, Baltim ore, M d.
The Rigidity of Starch Pastes
BERNADINE BRIMHALL AND R. ¡VI. HIXON Iowa Agricultural Experim ent Station, Am es, Iowa
M
AXWELL (18) in 1868 proposed an equation relating the rigidity of a body to its viscosity by means of the relaxation time. Twenty years later, Schwedoff (21) devised an apparatus for measuring the rigidity and relaxation time of sols. He stated that the viscosity of a liquid is not necessarily an index of its rigidity. A number of investiga
tors (6, 17, 22) have since worked out formulations for struc
tural viscosity on the assumption that it is due to the presence of elasticity. [Rigidity is the reciprocal of elasticity. For definitions, see (3).]
Michaud (14, 15), by an ingenious method of his own, in
vestigated the effect of acids, bases, salts, and some organic compounds on the rigidity of gelatin and agar gels. Philip- poff (18) described a dynamic method for determining elas
ticity of cellulose solutions, while Neale (16) measured the _ elasticity of air-dried starch films.
Porst and Moskowitz (19) summarized Bingham’s concept of rigidity as applied to starch. Their attempts to measure the “yield shear value” by extrapolation of flow-shear curves gave indefinite results because of the gradual slope of the curves obtained. Farrow, Lowe, and Neale (7), using both capillary and Couette-type viscometers, observed flow in starch pastes at rates of shear below Bingham’s theoretical
“yield value.”
McDowell and Usher (12) advanced a simple mechanical explanation, supported by striking experimental evidence, for the phenomena of rigidity and anomalous viscosity in sus
pensions and gels: “If rigid particles suspended in a liquid in which they are insoluble are not prevented from cohering—
whether by an electric charge or by an envelope of a soluble substance—they will in time form aggregates, the presence of which will always cause the viscosity to be a function of the rate of shear; and which, if completely interlinked, will im
part rigidity to the suspension as a whole.” They mention that variable viscosity is shown sometimes when rigidity is absent, and always when it is present.
Hess and Rabinowitsch (9), after heating starch grains just above their gelatinization temperature, punctured the granule membrane with a microneedle and photographed a liquid oozing out at the point of puncture. They believe that swollen starch grains have a certain amount of inner structure which, like the membrane, possesses elasticity.
Badenhuizen (2) described experimental evidence which would seem to contradict this concept of granule structure.
Woodruff and MacMasters (23) made measurements of relative viscosity and gel strength on starches from different varieties of corn and on starches from the same variety of corn treated in different ways. They found that viscosity differences were very small as compared with differences in gel strength and that the two properties frequently did not fluctuate in the same direction. They give this as further evidence that viscosity and gel strength seem to measure two different sets of properties in the starch.
Most of the methods now in use for the determination of gel strength in starch pastes involve actual disruption of their structure—i. e., measure the degree to which they can be stretched before breaking. This value fluctuates with the rate at which the force is applied, so that a high degree of accuracy is not obtained.
The method developed by Schwedoff (21) measures the resistance offered by the paste to being stretched. The results are more accurate, since the gel is not deformed beyond its
elastic limit, and the values obtained are independent of ap
paratus constants. By this means, rigidity has been dem
onstrated in a variety of gels (1, 8, 10, 11, 15, 18, 20, 21), but in only two cases has mention been made of starch:
McDowell and Usher (11) measured the rigidity of non- aqueous suspensions of raw starch, and Arcay and Etienne (1) included starch in the list of substances whose gels they tested for the presence of rigidity.
Caesar (4, 5) has designed a consistometer for characteriz
ing starch pastes by their relative resistance to violent me
chanical agitation at high concentrations (10 to 30 per cent).
Consistency, as measured by this method, is partly condi
tioned by rigidity, as well as by viscosity, plasticity, and thixotropy.
The results obtained from the application to starch pastes of the Schwedoff technique for measuring rigidity are reported in this paper.
Experim ental
D e m o n s t r a t i o n o f R i g i d i t y i n S t a r c h P a s t e s . Prelimi
nary experiments were made using a MacMichael viscome
ter. The paste, after cooling, was placed in the viscome
ter cup and a definite twist given to the wire. With pastes of low concentration, the disk eventually swung back to its original position, but if the concentration was high 358
359 enough, it came to rest before reaching the zero point, thus
demonstrating the presence of elasticity.
However, any disturbance of the paste—even the passage of the disk attached to the wire during measurements—
resulted in a breaking up of the gel structure and subsequent lowering of elasticity values. Since this factor made it difficult to obtain accurate and reproducible results, the de
sign of the MacMichael viscometer was modified into an ap
paratus resembling Schwedoif’s (21) for the quantitative determination of rigidity.
D e s c r i p t i o n o f R ig id o m e te r . The design is illustrated in Figure 1. A MacMichael viscometer wire, W, encased in a metal shaft, hangs from the bottom of a dial, D, graduated in degrees. By means of a piece of rubber tubing, the shaft around the wire is firmly attached to a small glass cylinder, C, so that the latter may hang freely inside a larger cylinder, H. A mirror on the shaft reflects a beam of light from its source, L, onto a gradu
ated scale in front of the apparatus.If water or other ideal liquid is placed in the larger cylinder, the angle through which the wire is turned by the dial will be identical with the angle through which the cylinder turns in the liquid (as read by the position of the light beam on the scale).
If a paste showing rigidity is placed in the larger cylinder, the inner cylinder will be deflected (when a torque is applied to the wire) by an amount depending on the elasticity of the paste, providing the force applied through the wire is not great enough to cause shear of the paste structure.most of the experimental work reported in this paper, the outer glass cylinder, H, and the calibrated dial, D, were mounted independently in a rigid brass frame inside a constant-temperature air bath. When the inner cylinder had been set in place and secured by means of screws through a brass collar, the paste was poured in through a hole in the side of the outer tube and the inner cylinder released by turning back the screws. A mirror was Two modifications of this apparatus have been used. For
also attached to the dial to provide for extra precision in setting and reading.The second modification which is shown in Figure 1 was de
veloped later to allow for greater convenience and speed of handling. Hydrometer cylinders (250-cc. capacity) were used for the outer tubes. The paste was poured in through the top, the calibrated dial put in place, and a center piece, P, bearing the wire and inner cylinder lowered into it. The whole unit was then set in a constant-temperature water bath until equilibrium was reached and finally moved over to the light source for meas
uring.
The first method is the more precise, while the second method is simpler in operation. In the first-described ap
paratus, the paste may be poured in after it has been cooled and requires only about 3 hours to reach equilibrium. The unit is not moved from the time the paste is put into it.
In the second modification, the paste must be poured in while hot, so that it will be fluid enough for the inner cylinder
to center itself. A paste prepared in this manner and placed in a 25° C. water bath requires at least 10 hours to reach equilibrium. No skin is formed on the surface of the paste, however, since the top piece prevents appreciable evapora
tion.
S t a n d a r d i z a t i o n o f P r o c e d u r e . Experiments were first carried out to determine the effect of time and tempera
ture on rigidity. If, as McDowell and Usher (12) imply, anomalous viscosity is correlated with rigidity, then a 5 per cent cornstarch paste which shows abnormal viscosity at 90° C. should also show rigidity at that temperature. Such was found to be the case. The rigidity increases with time of standing in an irregular manner during the first 3 hours at 90°. Measurements taken after 3 hours at this temperature are subject to error because of evaporation of water from the paste. In order to eliminate this difficulty, the pastes were first heated to 90° and then cooled to 25° before being allowed to set. The resulting data showed that rigidity increases rather rapidly with time during the first 3 hours and then be
comes nearly constant. (This holds for the unmodified starches under the conditions of experiments reported.
It is likely that the time required varies with the concentra
tion and previous treatment of the starch.) Lampitt and Money (10) have obtained similar results with pectin gels.
During measurements of elasticity, it is possible, by im
parting sufficient twist to the u'ire, to strain the paste beyond its elastic limit and shear it. At this point the gel ruptures and the inner cylinder drifts along in the direction of twist on the wire. With pastes of fairly low elasticity this point is evident as a well-defined break, but with pastes of high elas
ticity it cannot be accurately detected. Tendency to shear, expressed in reciprocal form as shear value, must be consid
ered because of its important effect on rigidity values. (Shear value is the torsional moment of the wire, N, multiplied by the number of degrees through which the wire may be twisted before shearing the paste.) Shearing tends to break up gel structure, so that once a paste has been sheared it is useless to make further rigidity measurements on it. If a certain paste shears so easily that its rigidity is difficult to determine, the shear value may be increased by using a higher concentra
tion of paste, or by preparing the paste at a higher tempera
ture, providing the granules have not been ruptured. The use of a lighter wire (one having a lower torsional moment) facilitates the measurement of pastes with low shear values.
P r o c e d u r e . The studies on the effect of time, tempera
ture, and shear led to the adoption of the following method for determinations of rigidity:
The weighed starch sample, suspended in 200 cc.
of distilled water, is heated at the desired tempera
ture in a water bath until it has come to equi
librium. This requires approximately 60 minutes of heating at 70° C., 50 minutes at 80°, 40 minutes at 90°, and 30 minutes at 99°, as determined by the change in volume of centrifuged granules at regular time intervals. The paste is then cooled to 25° by shaking in a stream of running water and poured into the tubes of the rigidometer where, after stand
ing undisturbed for 3 hours, it is measured by placing varying degrees of twist, 8, on the wire and noting the corresponding deflection, u, of the cylinder.
These two values, 5 and a (converted to angular degrees), when plotted against one another, make a straight line. Then the slope of this line, S/a>, may be substituted in the equation mentioned by Hatschek and Jane (S):
f = J L ( J4tth KRo1 R S j WL ) i
F i g u r e 2. R i g i d i t y - C o n c e n t r a t i o n C u r v e s f o r C o r n s t a r c h P a s t e s P r e p a r e d a t D i f f e r e n t T e m p e r a t u r e s
where E is the modulus of rigidity in dynes per square centimeter; N, the torsional moment of the wire used; h, the height of the paste on the inner cylinder; and Ro and R\, the radii of the
360 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 11, NO. 7
Characterization of Starches by M eans of Rigidity
Differentiation of Cornstarches
inner and outer cylinders, respectively. (The wires were standardized by measuring the oscillation time of a suspended disk of known mass. Then N = 4tt2MR2/2T2, where T is the period of oscillation and M and R are the mass and radius of the disk used.) Wires with torsional moments of 0.0256, 0.0665, and 0.1460 erg were used interchangeably with entirely consistent results, showing that wire size may be suited to the strength of the paste being measured without introducing error in this re
spect.
very similar in shape, differing only in the temperature re
quired to produce rigidity. Waxy maize has nearly the same temperature of maximum rigidity as tapioca, although a much higher concentration of the former is needed to pro
duce the same amount of rigidity.
Having tried out rigidity measurements on different kinds of starch, an attem pt was next made to apply them to dif
ferent varieties of the same starch. Rigidity-concentration curves (Figure 5) were obtained on seven different corn
starches prepared in the small-scale milling plant in this labo
ratory and on two commercial cornstarches. In all cases the pastes were made lip at 99° C. and measured at 25°.
Samples of these starches in 2.7 per cent concentration were then prepared at 99°, cooled to 25°, and viscosities deter
mined by the capillary method. Table I is characteristic of the results obtained.
Since viscosity in pastes of this concentration varies with pressure, the viscosity values are given at three different pressures: at 5 cm. of water, where the decrease in viscosity with increasing pressure is extremely rapid; at 15 cm., where the bend in the viscosity-pressure curve is most pronounced;
and at 30 cm., where the curve has leveled off and viscosity remains practically constant with further increase in pressure.
The lack of correlation between rigidity and viscosity values indicates that they measure different properties of the pastes.
R elation of Granule M em brane to Rigidity
The downward trend of the rigidity-temperature curves after reaching their maximum (Figure 4) has been ascribed above to a change in the condition of the granules. In order to get a better insight as to the character of this change, microscopic technique was employed. A glass cell mounted in a hot-plate substage provided the means for observing the progressive swelling of granules in water suspension inside the cell. It was noted with tapioca and waxy maize starch that after the temperature of maximum rigidity had been reached, A comparison was made of three different starches showing
extreme variation in physical properties: a commercial corn
starch, tapioca starch, and waxy maize starch, which gives a reddish brown color with iodine.
Figure 2 shows rigidity-concentration curves for the com
mercial cornstarch. At 70°, 80°, and 90° C. the curves are nearly identical in shape, the rigidity changing very slowly up to a certain critical concentration, above which it increases enormously for each slight increase in the percentage of starch. If the lower arm of the curve at 90° is extrapolated, it intersects the X-axis at approximately the same concentra
tion at which structural viscosity sets in, 2.7 per cent. The same thing appears to be true of the curves at 70° and 80°, although they cannot be accurately extrapolated because the high tendency to shear makes rigidity values uncertain at lower concentrations. Above 90° a change in the condition of the granules begins to set in, and at 99°, instead of bending sharply and shooting upward almost vertically, the curve slopes upward only gradually. Finally, a paste heated in the autoclave at 120° shows comparatively little rigidity.
Fig u r e 3. Rig id it y-Co n c e n t r a t io n Cu r v e s f o r
Ta p io c a a n d Wa x y Ma iz e St a r c h Pa s t e s Pr e p a r e d a t Dif f e r e n t Te m p e r a t u r e s
With tapioca (Figure 3), it is evident that even at 70°
changes have begun to take place in this starch, and the slope of the rigidity-concentration curve decreases progressively with increasing temperature of preparation. Rigidity in waxy maize, even at 70°, is very low and at 75° is no longer evident. Rigidity increases in strictly linear relationship to concentration after the initial bend in the curve has been passed, the bend being more or less sharp depending upon the degree to which the paste has been altered at that tem
perature.
Another method of treating these data is illustrated in Figure 4, where rigidities at a given concentration of paste are plotted against temperature. This type of curve shows the temperature at which each starch exhibits its maximum rigidity. Tapioca and commercial cornstarch give curves
It is interesting to compare these curves with the consist- ency-temperature curves obtained by Caesar (4, 5). Al
though very similar in shape, they are quite different quanti
tatively because of the difference in properties measured and methods employed.
Fig u r e 4.
115- TEMPERATURE r c j
Ef f e c t o f Te m p e r a t u r e o f Pr e p a r a t io n o n
Rig id it y o f Th r e e St a r c h e s
