ADSORPTION AND T H E PERMEABILITY O F MEMBRAKES
11. Copper Ferricyanide as a Semipermeable Membrane BT HARRY B. WEISER
In an earlier paper' it, was deduced that a distinctly porous membrane such as copper ferrocyanide will be semipermeable under two conditions: first, if the membrane exhibits such marked negative adsorption for the solute that the pore walls are covered with a film of pure solvent which completely fills the pores; and second, if the positive adsorption of the solute is sufficiently great that the chains of oriented molecules, extending into the solvent from a monomolecular layer on the surface of the pore wall, saturate the solvent in the pores within the range that the adsorption is practically irreversible. Between the two extreme conditions which produce semipermeability, there are an indefinite number of gradations in degree of permeability of a membrane by a dissolved solute. The conclusion that semipermeability results from negative adsorption was predicted by Mathieu2 and by N a t h a n ~ o h nthe , ~ conditions were formulated by Bancroft4 and the theory was given experimental support by Tinker5 who observed that cane sugar which ordinarily does not diffuse through a copper ferrocyanide membrane is adsorbed negatively from solution by precipitated and dried copper ferrocyanide gels. The conclusion that semipermeability to a dissolved solute may result from practically irreversible positive adsorption of the solute was arrived a t from a study of the behavior of copper ferrocyanide toward alkali ferrocyanides. The precipitated gel adsorbs the alkali salts practically irreversibly through an appreciable range and a properly prepared membrane is almost completely impermeable to the salts in concentrations below that which would cause coagulation of the colloidal film and so open up cracks at weak points in the septum. To account for this behavior, it was assumed that the fixed walls of the pores hold chains of oriented molecules extending from a monomolecular film on the surface into the pore water. If this practically irreversibly adsorbed ferrocyanide is sufficient to saturate the pore water no more can enter from the side of the membrane in contact with the ferrocyanide solution and since none can be washed out, none will pass into water or sugar solution on the opposite side. On the other hand, potassium sulfate and copper chloride which are adsorbed by the membrane pass through the pores readily because the adsorption is much weaker and is not irreversible a t any concentration. 'Colloid Symposium Annual. 7. r i j (1930); J. Phys. Chem., 34, 335 (1930) Physik, (4) 9, 340 (1902I . a Jahresber. wiss. Botan., 40, 431 (rsoQ. J. Phys. Chem., 21, 441 (1917 ) . 6 Proc. Roy. Soc., 92A, 357 11916); 93A, 266 f1917).
* Ann.
ADSORPTIOX A S D PERMEABILITY O F MEMBRANES
1827
If one objects to the statement that the impermeability of the copper ferrocyanide membrane to alkali ferrocyanide is due to saturation of the pore mater with the practically irreversibly adsorbed ferrocyanides, an alternative statement of the case is that the pores of the membrane become filled with a network of oriented chains of adsorbed ferrocyanide molecules to the point where no more can enter, within the range that the adsorption is practically irreversible. There are certain consequences of the adsorption theory of the semipermeable membrane which call for further experimental test. Thus Hartung‘ objects to Tinker’s conclusion that the impermeability of copper ferrocyanide to sugar is due to negative adsorption, on three counts: First, the adsorbent used by Tinker was thoroughly dried copper ferrocyanide powder and not the hydrous gel which constitutes the osmotic membrane. Second, Tinker showed merely that water was adsorbed by the powdered salt more strongly than sugar; but he did not show that sugar was not adsorbed at all. Finally, Hartung himself showed that potassium sulfate is adsorbed by copper ferrocyanide more strongly than potassium chloride and the former passes through the membrane much less readily than the latter. What Hartung says concerning sulfate is that, “even the thinnest membranes are impermeable to it.” This statement is unquestionably incorrect. The explanation proposed to account for the impermeability of the copper ferrocyanide membrane to alkali ferrocyanide suggests that a gelatinous ferrocyanide or ferricyanide membrane will be permeable to salts of the corresponding anions if the adsorption is partly reversible throughout the entire concentration range. Moreover, it would appear that, other things being equal, a salt which is strongly adsorbed by a membrane would diffuse through the membrane more slowly than one that is relatively weakly adsorbed. These several questions will be dealt with experimentally in the next section. Experimental The experiments herein reported were carried out chiefly with copper ferricyanide since preliminary observations disclosed that this salt like the corresponding ferrocyanide, gives a highly gelatinous membrane that is impermeable to sugar and since Ki1liams2 reports that the copper ferricyanide gel thrown down from alkali ferricyanide solution and thoroughly washed, is not greatly contaminated with alkali ferricyanide. h few observations were made with cadmium ferricyanide also. Adsorption Experiments Adsorption 01 H20 by Cz~3(FeC,.1;6)2. Since a copper ferricyanide membrane is impermeable to cane sugar, it would follow from analogy with the corresponding ferrocyanide that sugar would be negatively adsorbed by the ‘Trans. Faraday Soc., 15, (3) 160 (1920). “The Chemistry of Cyanogen Compounds” (191j).
1828
HARRY B. WEISER
salt from aqueous solution. To test this, copper ferricyanide was precipitated from sugar solution and the change in concentration of the sugar was determined before and after the precipitation. The procedure was as follows: The calculated amount of CuC12.2Hz0 to give j grams of Cu3(FeC6N6)2was dissolved in 50 cc of water in one container of a mixing apparatus previously described’ and an equivalent amount of K4Fe(CX), together with a definite weight of sugar was dissolved in IOO cc of water in a second container. After thorough mixing, followed by centrifuging, the concentration of sugar in the supernatant solution was determined in a Reichert Soleil-T’entzke saccharimeter. For comparison, a determination was made of the concentration of a sugar solution containing the same weight of sugar in the same volume of water as above, together with KC1 equivalent to 5 grams of C U ~ ( F ~ C & ~ ) ~ . The results of two experiments with different concentrations of sugar are given in Table I. The saccharimeter reading in Ventzke degrees is an average of 20 readings made under carefully controlled conditions. It will be seen that the concentration of sugar is greater in the solution from which the gel separated.
TABLE I Adsorption of H 2 0 from Cane Sugar by C U ~ ( F ~ C & ~ ) Z Substances mixed in grams CuCl2.zH20 K8Fe(CN)s KCl 4.1624
5.3570
0.0
0.0
0.0
3.6395
4.1624
5.3570
0.0
0.0
0.0
Cn3(FeC~N!)Z precipitating g.
Cane Sugar
HzO
8.0 8.0
150.0
5.0
150.88*
0.0
13.0 3.6395 13.0
* 4.1624 g. CuClz.2H20contains 0.88
150.0
5.0
1jo.88*
0.0
Saccharimeter reading
HzO adsorbed by CudFeC~ll’dZ
Ventzke
perg.
mol per mol
19.75 19.30
0.666
22.6
31.48 31.05
0.476
16.2
oo
g.
g. H20.
From the increase in concentration of sugar, the adsorption of water given in the last column of the table was calculated. The results of the above experiment show conclusively that water is adsorbed strongly relatively to sugar by precipitated copper ferricyanide, the extent of the adsorption being greater the more dilute the solution. Since this behavior is similar to that observed by Tinker with dry copper ferrocyanide in sugar solutions, it would seem that Hartung’s criticism of the experimental procedure employed by Tinker is not valid. Although the above results furnish strong evidence in support of the view that the impermeability of copper ferricyanide to sugar is the result of marked negative adsorption, the case would be even stronger if it could be shown that no sugar a t all is adsorbed by the precipitated gel. Unfortunately it is difficult to determine the presence of a trace of sugar in the presence of an excess of copper ferricyanide but the following experiment indicates that little or J. Phys. Chem., 34, 340 (1930).
1829
ADSORPTION AND PERMEABILITY O F MEMBRAXES
none is adsorbed: A gram of the ferricyanide gel precipitated in the presence of sugar x-as washed repeatedly by the aid of the centrifuge until the wash water was free from sugar, using 0.5 ?; KC1 solution which prevented peptization of the gel. The suspended gel was then digested for a day in dilute HCl, a procedure which should invert any cane sugar that might be present. h sample was then subjected to the Fehlings test by dissolving the gel in alkaline sodium tartrate solution and heating. The results were negative. Adsorption of K S e ( C S ) , by C U ~ ( F ~ C , Sgel. , ) ~ Solutions of &Fe(CN), and of CuCl*.zH?Owere prepared such that jo cc of the former was exactly equivalent to I O O cc of the latter and on mixing these amounts theoretically I gram of C U J ( F ~ C ~ ? was ~ ~formed. )P The adsorption isotherm was obtained by taking 100cc of the copper solution and mixing with it increasing amounts of &Fe(Cx), solution above j o cc, in a total volume of 2 5 0 cc. To prevent peptization of the gel IO cc of I.j S KC'1 solution was always included. The mixing was carried out in the mixing apparatus previously referred to. After standing 48 hours, samples of the supernatant solution were pipetted off and analyzed for &Fe(C1\)6 by the iodometric method of Xiller and Diefenthaler.' This analytical procedure was employed also in standardizing the ferricyanide solution employed. The copper content of the C.P. laboratory reagent was determined by converting the CuClz to CuSOn and analyzing for copper electrolytically. The results of the adsorption experiments are given in Table I1 and shown graphically in Fig. I . The upper curve shows the adsorption plotted against the equilibrium concentration and the lower curve the logarithm of the adsorption against the logarithm of the equilibrium concentration.
TABLE I1 Adsorption of K3Fe(C?j)6by Cu3(FeC6K6)? Solutions mixed (Total volume 250 cc)
Equilibrium concentration of K3FeiC?J'6 millimols/l
K3Fe(CN)8adsorbed
CuCI?
K,Fe(CS),
IO0
jo.0
0.000
0.0000
0.0000
IO0
52.5
0.077
0.04j2
IO0
55.0
0.0606
IO0
60.0
0,535 1.737
IO0
jO.0
4.2;s
IO0
80.0
6.841 9.396 1 1 ,956
0.0840
0.1092 0.1380 0.16j3 0.1767 0.1835 0 "94.3
0.08j3
0.2018
IO0
IO0
90.0 100.0
gram per gram
0.OjIs
o,oj64 o .0;98
mol per mol
I40 . o 22.50j 0.I 0 1 2 0.2339 The above results show that the adsorption is quite marked; but the form of the curve indicates that the adsorption is not completely irreversible even at very low concentrations although at such concentrations the curve approaches quite close to the x-axis. IO0
* Z. anorg. Chem., 67, 418 (1910).
1830
HARRY B. WEISER
4
h 2 s5
/yy-j
62
k
d
24
5
iz'Z
2;
27
F
Ps
2
2
4o io
0
io
M
0
3.3
'10
20
0.0,
9 s
%u
o.Db
d
E L
dom ul)
Y
r"
+
i5 B d0i P
b
i
LL
0
.?I
LY
1.6
18
1831
ADSORPTION AND PERMEABILITY OF MEMBRANES
TABLE I11 Adsorption of K?SO, by Cu3(FeC&J2 K?SOI added I j g.,
cc
1.
BaSO, in I O 0 cc g.
0
0 , 6 7 3 7 (in I50
Equilibrium concentration 0f,K?SO4 millimols/l
gram per gram
mol per mol
19.24
0.0126
0.0444
KzSO4 adsorbed
cc)
25
0.6488
27.80
0 0150
o.oj30
50
0.8495
36.39
0 . O I j4
0.0543
75
I
0.0677
I
44,9I 53.96
0.0192
IO0
,0483 ,2482
0.0213
0.0751
FIG.3 .4dsorption of KC1 by C U ~ ( F ~ C ~ N ~ ) ~
termining the adsorption of sulfate. The analysis of the supernatant solution for chloride was made by the rolumetric method of 11ohr. The silver nitrate used in the titration was standardized against a carefully prepared solution of pure IiCl and all titrations were made to the same end point. The results are given in Table IV and plotted in Fig. 3.
TABLE IT' .Idsorption of KC1 by CIu3(Fe(*bSL)2 Solutions mixed #Totalvnlume z j o cc! CuC1K?Fe!CS15 KCI IO p l . cc CC cr 50 0 I O 0 IO0
. i 0
25
IO0
50 50 50
50
IO0
IO0
-/ -3 IO0
Equilibrium coricrnt rat ion KC1 adsorbed of KCI millimols.'I. gram per gram mol per mol 39.04
o ,0003
.45 6j.86 79.28 92.67
0.0004
52
0.0004
0.0026 0.0033 0.0033
0.0003
0,0026
0.0007
o.ooj8
Fig. 4 is a composite diagram showing the adsorption isotherms for E;3Fe(CS!i;.Ii?SO; and IiCl by Cu3(FeCbSb)2 gel. It will be seen that the ferricyanide is adsorbed much more strongly than the other salts especially at lorn equilibrium concentrations. At an equilibrium concentration of 2 j
1832
HARRY B . WEISER
millimols per liter of the respective salts the adsorptions in mols of salt per mol of copper ferricyanide gel is K3Fe(CS), = 0.24; Ii&14 = o . o j ; and IiCl = 0.003. Incidentally, this is a striking example of the influence of the valence of an ion on its adsorbability. :rhCion- MlUkrnolJ K,,FejC!\per
a
Llier
7.6
tquilibrium Concentr
TABLE Adsorption of K,Fe(C1;)6 by Cd3(FeC6K6)~ Solutions mixed (Total volume 2 j o cc)
CdCli
K&(CNh
Equilibrium concentration of K3FefCN)B millimols, '1.
K3Fe(CS)aadsorbed gram per gram .o
IO0
50
0.000
0
IO0 IO0
55
0.0340
60
IO0
io
IO0
80
IO0 100
90 IO0
0.638 I . 3j r 3.266 5.333 7.373 9.416
100
140
14.800
mol per mol 0 0
O.Oi8jO
o 0618
0,1428
0.0773
0,1686
0.0802
0.18j4 0.1972
0.0853 0,0902
0,2085
0.0892
0.2062
ADSORPTIOS APiD PERMEABILITY O F J I E M B R A S E S
I833
Adsorption of K3Fe(C S ) s by CdaiFeC,S6)? gel. The adsorption of &FC?(CS)~ by Cds(FeCJ8)? was determined by the same procedure outlined for determining the adsorption of K 3 F e t C x ) ~by the copper salt. The results are given in Table T and shown graphically in Fig. 5 . It will be noted that the curve does not approach the y-axis so closely at lower equilibrium concentrations as does the corresponding curve for copper ferricyanide
FIG. j Adsorption of K3Fe(CS)8by Cd,(FeC,S,),
Diffusion Experiments Since sufficiently marked positive adsorption of a solute by a membrane renders the latter impermeable to the solute, one would expect that, other things being equal, the more strongly a solute is adsorbed by a membrane the more slowly it would diffuse through it. I t is, of course, impossible to realize ideal experimental conditions, since the size, extent of hydration, and the mobility of ions and molecules are specific. Xevertheless, it seemed desirable to determine the relative rates of diffusion through copper ferricyanide membranes of the three salts: K3Fe(CS)e,&SO4 and KCl whose adsorption by copper ferricyanide gel is recorded above. This was especially true since Hartung reports that li2SOi is adsorbed more strongly than IK1 by copper ferrocyanide and the latter diffuses much more readily than the former through the ferrocyanide membrane, in line with the prediction made in the first sentence of this paragraph. To make the diffusion experiments, copper ferricyanide membranes were prepared by impregnating Carl Schleicher and Schiill parchment thimbles with the gel. This was accomplished by placing S / j &Fe(CS)G on the CuClz soluinside of the thimble and suspending it by a wire handle in l j ! ' ~ tion. After acting for a day or two the solutions were interchanged, the copper solution being placed inside the parchment and the ferricyanide around the outside. By repeating this process four or five times in the course of a week, membranes were formed with which quite reproducible diffusion data
I834
HARRY B. WEISER
were obtained. Before using, the membranes were soaked for several days in repeated changes of water. The membrane thimbles were suspended to the bottom of a stopper which fitted a 2 5 0 cc bottle. After placing 15 cc of electrolyte in the thimble i t was hung in 150cc of sugar solution isotonic with the electrolyte, and allowed to remain 24 hours. Duplicate experiments were carried out first with KC1, then with K$04 and finally with K3Fe(CN),, using the same thimbles for each series of observations. The amount of diffusion was determined using the methods of analysis previously described under the adsorption experiments. The results are given in Table VI. It will be seen that the very TABLE
VI
Diffusion of Salts through cUs(FeC~N6)2Membranes Membrane NO.
KCI in cup a t start cc M/25
I
I5
2
15
grams
cc M/?s
grams
I118 0.1118
12.75
0.0950
12 20
0.0910
0
K2SOa in cup a t start cc I
K2SOaoutside cup after 24 hours
KC1 diffused through membrane
85 .o 81.3
c2 KzSOI diffused
M ' 2 j BaSOl equiv. grams
I5
0
I400
I5
0
I400
KsFe(CN in cup a t start cc
KCI outside cup after 24 hours
W 2 j
grams
I
15
0
2
15
0
I975 I975
K3Fe C S 8 outside cup after 2 4 hours
K3Fe/CN)6diffused through membrane
c>
ci' hl 2 j
grams
o 68 o 64
o 0089
4 5
o 0083
4 2
strongly adsorbed I13Fe(CS)G diffuses quite slowly, and the weakly adsorbed KC1 quite rapidly while the KZSOI occupies an intermediate position. I t appears therefore that the presence of an adsorbed solute in the membrane retards the rate of diffusion of that solute into and through the pores. As has been shown,' if the adsorption is sufficiently strong, there is no diffusion of the solute, the membrane becoming completely semipermeable. Potassium ferricyanide was found to diffuse through a cadmium ferricyanide membrane much more readily than through a copper ferricyanide membrane, There are two reasons for this; first, the adsorption is more readily reversible at low concentrations; and second, the salt is less gelatinous and hence gives a membrane with larger pores. Weiser: Colloid Symposium Annual, 7,275 t1930); J. Phys. Chem., 34,335 (1930).
ADSORPTION AND PERMEABILITY OF MEMBRANES
I835
Discussion The above experimental results support the theory that a membrane will be impermeable to a dissolved solute ( I ) provided it exhibits sufficiently strong negative adsorption that the adsorbed film of pure solvent fills the pores full or ( 2 ) provided it exhibits sufficiently strong positive adsorption that the pores are filled with a net-work of oriented chains of adsorbed solute molecules to the point where no more can enter, within the range that the adsorption is practically irreversible. The adsorption theory of the action of the semipermeable membrane is opposed to the view that the membrane merely acts as a sieve or ultra-filter with pores sufficiently small that dissolved molecules above a certain size are held back while smaller ones can pass through. Nevertheless, pore size is quite as important for the true semipermeable membrane which functions by an adsorption mechanism, as it is for the true ultrafilter which functions as a sieve without the intervention of adsorption phenomena. For example, when a solute fails to pass a membrane because of negative adsorption, the pores must be sufficiently small that the adsorbed film of solvent fills the pores full, otherwise the solute may pass through the center of the pores. I n other words, a membrane of a given composition may be impermeable to a solute because of negative adsorption if the pores are small enough and may be permeable to the same solute in spite of negative adsorption if the pores are too large. This does not mean that the porous membrane becomes a molecular sieve or filter when the pores become small enough that the adsorbed film completely fills them. On the contrary, it means that a porous membrane which does not exhibit marked adsorption for a solvent, ordinarily acts as an ultrafilter or sieve allowing molecules in solution to pass but holding back particles of colloidal dimensions; but if there is some adsorption of the solvent, and the pores of the membrane are made sufficiently fine, the membrane may become impermeable to certain molecules in solution because of negative adsorption. Collodion membranes may act in this way. It is well known that collodion makes a very satisfactory ultrafiltration membrane for separating colloidal particles from solutions of electrolytes. But a collodion membrane may be prepared that is said to hold back dissolved molecules. Thus Michaelis' makes what he calls molecular sieve membranes by evaporating practically all of the organic solvent in which the collodion is dispersed instead of evaporating a part of the liquid and then immersing in water. I t is claimed that KCl molecules in solution do not pass through such membranes into pure water even after days although non-electrolytes of comparahlc molecular size will diffuse. If sodium nitrate solution is placed on the opposite side of the membrane from the potassium chloride there is an exchange between the potassium and sodium ions but the diffusion of the chloride ion is extremely slow even in this case. Such observations do not support' the view that collodion membranes with very fine pores are molecular
' Colloid Symposium Monograph, 5 . 135 (1927); J. Gen. Physiol., 8, 33 (192 j);Michaelis and Prrlziveig: Ibid., 10, 515 ( 1 9 2 7 1 ; Mirhaelis and Fujita: Biochem. Z., 141, 47 (1925); hlichaelis, Ellsworth, and IIeech: J . Gen. Physiol., 10, 671 ( 1 9 2 7 , .
1836
HARRY B. WEISER
sieves. For if the dried collodion is merely a molecular sieve it is difficult to account for its permeability to non-electrolytes similar in molecular size to potassium chloride. Moreover, a sieve which allows potassium ion to pass would hardly prevent the passage of chloride ion which is almost identical in size. I t is more likely that collodion exhibits some negative adsorption for chloride just as sugar is adsorbed negatively by copper ferricyanide. I n the case of the chloride and collodion, the negative adsorption is insufficient to prevent free diffusion through ordinary collodion but is enough to inhibit or prevent diffusion through the very minute pores of dried collodion. In other words, a collodion membrane is usually an ultrafilter but it may become a semipermeable membrane for certain solutes if the pores are sufficiently fine. Collander' considers the dried collodion membrane to be a molecular sieve through which solutes diffuse at a rate inversely proportional to the molecular size. But he finds that the velocity of diffusion is not determined exclusively by molecular size and suggests casually that the discrepancies. may be due to solution and adsorption processes in the membrane. Grollman2 finds that the sieve-like action of collodion membrane is influenced by a layer of adsorbed liquid on the pore walls. I t is hoped to consider this behavior further in a later communication.
Summary The following is a brief summary of the results of this investigation. ( I ) A study has been made of (a) the adsorption of cane sugar, K3Fe(CN)6,KzS04, and KCl by copper ferricyanide gel and (b) the permeability of the copper ferricyanide membrane to the several compounds in aqueous solution. Copper ferricyanide exhibits strong negative adsorption for sugar (2) from aqueous solution and the membrane is impermeable to sugar because of thisnegative adsorption. (3) Copper ferricyanide gel adsorbs salts in the following order: K3Fe(CN)6>K9SO4>KC1. Thus, a t an equilibrium concentration of 2 5 millimols per liter of the several salts the adsorption in mols of adsorbate per mol of copper ferricyanide gel is: K,Fe(CS), = 0 . 2 4 ; K2S04 = 0.05;and KCl = 0.003. (4) Although the adsorption of K3Fe(CS)6by copper ferricyanide gel is qui,* strong, the adsorption is not completely irreversible at any concentration; hence the salt will diffuse very slowly through the copper ferricyanide membrane. ( 5 ) The rate of diffusion of salts through a copper ferricyanide membrane is in the order KC1>K2S04>K3Fe(CiY)6.This is the reverse of the order of adsorption of salts by copper ferricyanide. (6) The presence of an adsorbed solute in a membrane retards the rate of diffusion of that solute into and through the pores. If the adsorption is sufSOC. Scient. Finn., Comm. Biol., I1 6 (1926). J. Gen. Physiology, 9, 813 (1926).
ADSORPTION AND PERMEABILITY O F MEMBRANES
I837
ficiently great, there is no diffusion of the solute, the membrane becoming completely semipermeable. by cadmium ferricyanide gel is of the ( 7 ) The adsorption of KBFe(Cr\’)6 same order of magnitude as the adsorption of this salt by copper ferricyanide gel. This permeability of the cadmium ferricyanide membrane to K3Fe(CS)6 is greater than that of the copper ferricyanide membrane, since the adsorption by the former is more readily reversible a t low concentrations and since the cadmium salt is less gelatinous and hence gives a membrane withlargerpores. (8) The observations support the view that a distinctly porous membrane will be semipermeable to a dissolved solute (a) provided it exhibits sufficiently strong negative adsorption that the adsorbed film of pure solvent fills the pores full or (b) provided it exhibits sufficiently strong positive adsorption that the pores are filled with a network of oriented chains of adsorbed solute molecules to the point where no more can enter, within the range that the adsorption is practically irreversible. ( 9 ) The ideal semipermeable membrane differs fundamentally from the ideal ultrafilter in that the former functions through an adsorption mechanism whereas the latter functions as a sieve with fine pores, without the intervention of adsorption phenomena. A membrane which ordinarily acts as an ultrafilter allowing dissolved molecules to pass but not colloidal particles may become a semipermeable membrane for certain solutes if the pores are sufficiently fine that negative adsorption, which is negligible with larger pores, becomes a predominating factor. I t is suggested that the alleged impermeability of extremely fine-pored, dried collodion membranes to certain dissolved solutes is due to negative adsorption rather than to a true “molecular sieve” action. The Rice Instctute, Houston, Tezas.
