I X D VSTRIAL A S D ESGXNEERISG CHEMISTRY
130
sented in Figure 11. The wateiy blue particles of Alnhls have evidently reacted m-ith the liquid to form another bluegray enfringing constituent. Massive chunks of purple Fe-413 were found associated with M n d , but this type of Feilla was absent in the other alloys, in which the iron content was considerably lower. Heat treatment removed the cores nearly in entirety (compare Figures 12 and 13 with
VOl. 18, No. 2
Figures 9 and 10). Figure 14 is of hlnAls, the hlnSi(?) fringe, and the abundant intragranular precipitate of MnA13. Acknowledgment
Grateful acknowledgment is herewith made to A. C. Zimmerman, Clifford Mcllahon, and to John L. Hester for their assistance in this experimentation.
Factors Determining the Brightness and Opacity of White Paints’ By F. H. Rhodes and J. S. Fonda CORNELL UNIVERSITY, ITHACA, N. Y.
HE opacity of a paint may be defined as the ability absorbed, so that the brightness will be low. As the thickof the paint to obstruct the transmission of visible ness of the film of white paint is increased by increasing either light. This property is of very great importance the number of coats or the thickness of the individual coats, to the user, since upon the opacity depend the quantity and less and less light is transmitted and the brightness of the the number of coats of paint required to obscure the surface painted surface increases, until finally a point is reached a t over which the paint is applied. To obtain complete hiding which practically all of the incident light is either reflected with a paint of low opacity a large quantity of paint and a or absorbed by the paint film itself. When this point is large number of coats will be required, so that both the cost reached further increase in the thickness of the film produces of material and the cost of application will be high. By no measurable increase in brightness. The brightness of such an opaque film of any some authorities a distincp a r t i c u l a r paint may be tion has been drawn betermed the “ultimate brighttween opacity or degree of A formula is developed to express the relation between ness” of that paint. obstruction to visible light, the brightness of a film of white paint and the thickIt is obvious that the and hiding power or ability ness of the film, and experimental evidence in support thickness of the film of any to obscure the underlying of this formula is advanced. The well-known effect particular paint which is surface. This distinction is of the addition of a small amount of black pigment required to attain the ultinot a valid one; the two in increasing the hiding power of a white paint is exmate brightness for that terms are simply two plained. Attention is called to the possible effect of paint will decrease as the methods of expressing the the roughness of the surface of a paint film upon the opacity of the film increases. same property. brightness of the film. The actual brightness of All our so-called white any thin film of a paint is a paints absorb a t least a small functionof both theultimate amount of theincident light. so that the amount of lGht reflected by the painted surface is brightness and the opacity. The opacity of a film of paint is, always less than the amount of light incident to that surface. in turn, dependent on the thickness of the film, the ratio of I n other words, “white” paints are really gray instead of white. the refractive index of the pigment to the refractive index of The ratio between the amount of light reflected by a given film the vehicle, the volume percentage of pigment in the paint, the of paint and the amount of light incident to that film may average size (diameter) of the pigment particles, and the shape and structure of the individual particles of pigment. be taken as the measure of the brightness of the film. The substances that are used as paint pigments are, in The brightness of a white paint is of very great practical importance, particularly in the interior painting of resi- massive form, transparent. The whiteness of the white dences, office buildings, and factories. The intensity pigments in their usual finely divided condition is due to of illumination within a room depends not only upon the the multiple reflection and refraction of the incident light amount of light entering the room, but also upon the extent at the very large number of exposed surfaces. Since the to which this light is conserved by reflection from the walls amount of reflection that takes place in a light ray that passes and furnishings. Rooms painted with a white paint of a t a given angle from one transparent medium into another high brightness will show much greater average intensity increases with the difference between the refractive indices of illumination with the same input of light than will rooms of the two media, i t is apparent that the total amount of painted with a paint of low reflecting power. Moreover, reflection from a finely divided pigment suspended in a the use of a paint of high reflecting power facilitates uniform transparent vehicle will increase with the difference between illumination and minimizes the variation in the intensity the refractive index of the pigment and that of the vehicle. of light caused by the irregular distribution of light sources. Since the suspending medium in an ordinary paint is oxiIt is apparent that when a so-called white paint is applied dized linseed oil, the opacity of a paint should increase as over a black surface the brightness of the painted surface the difference between the refractive index of the pigment will depend to large extent upon the opacity and thickness and that of the oxidized oil. This fact has been recognized by most investigators in of the film. With very thin films or relatively transparent paints a considerable amount of the incident light will be this field. Several investigators have determined the brighttransmitted to the underlying dark surface and will there be ness and opacity of dry paint pigments or of suspensions of pigments in water and glycerol, and have proceeded to use I Received June 1. 1926.
T
.
February, 1926
INDUSTRIAL A S D ENGINEERI,VG CHEMISTRY
the data thus obtained as a basis for the comparison of the brightness and opacities of paints prepared from these pigments. In this previous work the tacit assumption has been made that the relation between the opacities of suspensions of any two pigments depends primarily upon the refractive indices of the two pigments alone and is substantially independent of the refractive indices of the suspending media. This assumption is not justified. Even the relative order of the opacities of a series of pigments may be different when the pigments are suspended in media of different refractive indices. Some work has been done upon the opacity of suspensions of pigments in unoxidized (liquid) linseed oil. Results thus obtained can probably be applied with a fair degree of accuracy to the comparison of the opacities of dried paint films from these pigments, although some slight error may be introduced because of the change in the refractive index of the vehicle during drying. I n determining the opacities of various substances when used as paint pigment the most logical procedure is, however, to measure directly the opacities of dry films of actual paints prepared from these pigments. I n comparing the hiding power and opacities of various pigments, many investigators have taken a unit weight of pigment as the basis of comparison. With paints made from pigments of the same refractive index and particle size the hiding power should vary with the percentage by volume of pigment in the paint. For this reason it is more logical to take a unit volume of pigment as the basis in comparing the opacities of different pigments. This fact has been generally, but not universally, recognized by investigators in this field. Since the brightness and opacity of a film of white paint are due to the multiple reflection and refraction of the light a t the interfaces between oil and pigment, it is obvious that opacity and brightness should increase as the number of such interfaces in the film increases-i. e., as the average diameter of the pigment particles decreases. The shape and the interior structure of the pigment particles themselves may also have some effect upon the hiding power of the paint. Pigments composed of particles that are irregular in shape and possess interior fracture should show higher opacity than pigments of which the particles are smooth and perfect. Previous M e t h o d s
Various methods have been used to determine the brightness and opacity of paints, A procedure2 in common use is to rub down a standard quantity of white pigment with a standard amount of linseed oil and ultramarine blue and to note the tint of the resulting paint. When two white pigments are thus tested, the one giving the lighter paint is said to have the higher hiding powere3 A modification of this method, using carbon black instead of ultramarine blue, has been used by Hallett.4 This method, however, does not necessarily give a quantitatively correct measure of the opacity or the brightness of the white paint pigment. Wolffb measured the opacity of a paint by measuring the exact thickness of a film of the liquid paint which was required to obscure completely a black surface covered by the paint. This method is subject to three possible criticisms: I-Measurements were made with liquid paints rather than with dry paint films. 2-Data thus obtained apply only t o thickness of film required t o give complete hiding and give no information as to the nature of the variation of opacity with film thickness.
3-Since the paints are used in the liquid form, any specific effect of the nature of the surface of the dry paint film upon the hiding power and brightness is neglected.
Pfund3has described an instrument which he terms a “cryptometer” and which is used to measure accurately the exact thickness of wet paint required to obscure completely a black surface. Measurements made by this cryptometer are subject to the same criticisms as those made by WOE. I n a subsequent article Pfundo gives some data as to the comparative brightness and opacity of a number of pigments, and discusses the effects of the addition of small amounts of black pigment upon these characteristics of a white paint. He found that, in general, the addition of black pigment sufficient to increase the hiding power 5 per cent decreased the brightness only 1 per cent. Other methods have been used to some extent. G . W. Thompson’ described an instrument called a n “opacimeter.” With this apparatus he measured the opacities of paints in the liquid form by measuring the light transmitted through various thicknesses of paint. Sheppards used a turbidimeter, which determined how well a liquid paint will hide a grating submerged in it. Hallettg points out that measurements of the hiding power of paints should be made on the dry paint film rather than on the liquid paint. By means of a special instrument devised by him and termed a “hidimeter,” he measured the comparative hiding powers of a series of white paint pigments. I n a subsequent article Hallett4 developed the following mathematical formula for correlating the hiding power of a film of paint, the hiding power of a unit thickness of film, and the thickness of the film expressed in terms of such unit thicknesses : ca = 1-1/14 where a = hiding power of unit thickness 1z = number of unit thicknesses in film S = hiding power of film expressed as fraction of complete hiding
He found that the hiding powers of the dry paint films as measured by his hidimeter agree closely with the hiding powers as predicted from cryptometer readings made with the wet paint, and showed that the hiding power and the tinting power of the pigments measured bore a direct linear relationship to each other. I n his work Hallett was concerned primarily with the hiding power of the paint and made no direct measurements of the ultimate brightnesses of the paints when applied in films thick enough to give complete hiding. Gardner’O has made some measurements of the whiteness of suspensions of pigments in nitrocellulose solution and in varnish, but the results are not, of course, quantitatively applicable to ordinary paints. Experimental
I n the present investigation an attempt was made t o determine the hiding powers and whitenesses of films of various white paints and to define the relationship of the hiding power to the thickness of film, refractive index of the pigment, particle size of the pigment, and volume percentage of pigment in the paint. Paints were prepared from a series of white pigments of known refractive index and measured particle size. From these paints films of known thickness and known volume percentage of pigment were applied and the whiteness and hiding power of each such film were measFranklin Inst., 196, 69 (1923). Paper presented before the American Institute of Chemical Engineers, December, 1912. 8 THISJOURNAL, 12, 167 (1920). 9 Proc. A m . Soc. Testing Materials, 20, 11, 426 (1920). 10 Paint Mfrs.’ Assoc. U. S., Circular 133 (September, 1921). a J.
7
2
Calbeck. Proc. A m . SOC.Tesling Materials, 22, 515 (1922).
4
Proc. A m . SOC.Tcsling Malrnials, 22, 11, 523 (1922). Farben-Ztg., 16, 2577 (1910); through Chcm. Zcnlr., 82, 11, 809 (1911).
* Pfund, J . Franklin Insl., 188, 675 (1919).
6
131
INDUSTRIAL A N D ENGINEERING CHEMISTRY
132
ured. A mathematical formula was developed to express the correlation of hiding power with its various determining factors and was found to apply with reasonable accuracy in all the paints which were studied. The paints used were prepared from pigment, pure refined linseed oil, and drier, and were thinned to the proper working consistency with pure gum turpentine.
The linseed oil used was pure refined oil from Xorth American seed, showing the following analysis: Refractive index Specific gravity Saponification number Iodine number Acid value
1.4790 0.933 192.5 181.1 0.5645
The drier used was lead-manganese linoleate dissolved in turpentine. This drier solution contained 1.16 per cent lead and 0.347 per cent manganese. I n preparing the paints the following standard formula was adopted, which gave paints of fairly satisfactory consistenclf with almost all of the pigments used: Pigment Oil Turpentine Drier solution
Vol. 18, No. 2
applied over the same area as the first. This procedure was found necessary in the case of two of the paints made from barytes and gave, therefore, a series of squares on the panel covered with 0 , 2 , 3 , 4 , 5 , and 6 coats, respectively. After the last coat of paint had thoroughly dried, measurements of the whiteness of each of the painted areas were made. The instrument used was an integrating photometer similar to that described by Taylor.ll This instrument has been shown to give true readings of the ratio of the amount of reflected light to the amount of incident light and therefore may be used for the direct determination of the whiteness of a painted surface. The average diameter of particle for each pigment was determined by the method described by Green.12 I n most cases the writers’ results agree fairly well with those obtained by Green for similar pigments, most of the variations being not greater than the differences in fineness which might exist among various samples of the same pigment. The pigment which the writers have termed simply “basic carbonate white lead,” and which was supplied as a representative sample of white lead made by the Dutch process, was, however, very much finer and more uniform in size than true Dutch process white lead, and was probably a precipitated basic carbonate. Since the discrepancy was not discovered until after the measurements of the whiteness and hiding power had been made, the data for this pigment are included, although they should be understood not to be typical of true Dutch process white lead. Calculations
Knowing the exact weight of paint applied, in each coat, per unit of surface, and the exact percentage by weight of pigment in each paint, it was possible to calculate the exact weight of pigment in each coat or series of coats on each unit area. From the density of the individul pigments as obtained from published tables,l3 the volume of pigment per unit area
Grams 67 33 8 1.8
The paints were applied to panels of white pine, 10 X 61 cm. (4 X 24 inches), which had been prepared as follows: The panels were first given a coat of flat black paint and allowed to dry. A coat of shellac was then applied in order to fill any pores of the wood and to prevent the flat black from staining any subsequent coats of white paint. The dry, shellaced surface was rubbed down lightly with very fine sandpaper to dull the gloss and to provide a slightly roughened surface to which the subsequent coat of paint would adhere. I n applying the white paint to the pan a beaker containing a portion of the paint, together with the brush with which it was to be applied, was weighed accurately. A coat of uniform and proper thickness was applied so as to cover a 51-cm. (20-inch) length the full width of the panel, leaving a 10-cm. (4inch) square a t one end unpainted. The beaker, brush, and unused paint were then weighed. After this first coat was thoroughly dry a similar coat was applied to a 41-cm. (16-inch) length of the panel thus leaving a 10-cm. (4-inch) square covered with but one coat of the white paint. Successive coats were similarly applied, thus leaving a series of squares covered with 0, 1, 2, 3, 4, and 5 coats of white paint, respectively. I n the case of some paints the hiding power of the first coat was so low as to be practically nil, so the second coat was
and the depth of pigment, considered as a solid layer, was calculated. From this result and the data as to the average particle diameter of the pigment, it is possible to calculate the number of superimposed unit layers of particles of pigment in any given coat, or series of coats of any particular paint could be determined. The formula for calculating the number of superimposed particles of pigment in any given film was: Bur. Standards, Sci. Paper 406. “A Photomicrographic Method for the Determination of Particle Size of Paint and Rubber Pigments,” Research Bulletin issued by New Jersey Zinc Company. 18 U. S. Geol. Survey, Bull. 679. 11
12
February, 1926
INDUSTRIAL A-VD ENGINEERING CHEMISTRY n = K
where n = G =
A =
P = d = t
=
K =
10,000 GP
Atd number of superimposed unit layers of particles weight (grams) of paint applied t o any known area area (sq. cm.) covered by G grams of paint fraction, by weight, of pigment in paint density of pigment average diameter of pigment particles, in microns, or 10,000 times the average diameter in cm. constant depending on the shape of the pigment particles and on their arrangement in the film
Since the value of K cannot be determined accurately for any individual paint, the value of one was arbitrarily assigned to this constant. The effect of the error thus introduced will be discussed later. When a ray of light falls on a film of white paint, a certain portion is reflected from the upper surface of the film while the remainder enters the body of the film. The amount of light reflected a t the upper surface of the pairit mill depend largely upon the roughness of the surface and upon the refractive index of 1,he vehicle, and will be practically independent of the nature of the pigment except as this may effect the roughness of the surface of the film. Thus, if from the same pigment two paints are prepared, one of which dries to a "flat" surface and the other to a glossy surface, thin films of the flat paint will show greater whiteness than will similar films of the glossy paint. I n the same way a plate of ground glass shows greater whiteness than a smooth glass plate, although the refractive index of the material is the same in both cases. Of the light which actually enters the film a portion is reflected a t the interfaces between pigment and vehicle and is thus returned as reflected light, a portion is transmitted to the under surface, and a portion is absorbed in the film itself. Of that light which is transmitted through the film a portion may be reflected at the under surface and reappear a5 reflected light. I n the writers' work, however, the amount of light obtained by reflection from the black under surface was so small that it was neglected. It should be possible to develop a mathem:ttical formula which will correlate the hiding power and whiteness of a film of any particular paint and the thickness of that film expressed in terms of the average diameter of the pigment particles. Such a formula may be developed as follows: the fraction of the incident light which actually enters the film 1--a = the fraction of the incident light which is reflected from the upper surface of the film n = the number of unit layers of pigment particles in the film R = portion of light incident t o any one unit layer of pigment particles which is directly reflected upward t o next upper layer A = portion of light incident t o any one unit layer of pigment particles which is directly absorbed within t h a t layer T = portion of light incident to any one unit layer of pigment particles which is directly transmitted through that layer U = amount of light reflected by any one unit layer and passing upward out of film B = brightness B , = brightness due t o reflected light passing upward from withithin the film B , = brightness of any film of n unit layers of pigment particles B, = ultimate brightness-i. e., brightness of a film of paint so thick as to be opaque By definition T = 1-A-Rand (1) B, = (Ui V, - Un) (2) Let
a
=
+
-+
The amount of light reflected by the upper layer of pigment particles, Ui, is equal to R . The amount of light transmitted by the upper layer of particles is T.
133
Of the light reaching the second layer of particles, T , a portion, RT, is reflected by this second layer. Of this reflected light, one part, A R T , is absorbed by the upper layer, a second part, R R T , is re-reflected downward by the first layer, and a third part, RT-ART-RRT, or RT2, emerges from the paint film. Of the light re-reflected downward by the first layer of particles, R2T, a portion, R3T,is again reflected upward by the second layer and R3T2emerges from the film. Thus, the total light reflected by the second layer of particles and reappearing as emergent light is RT2
+ R3T2 + R 6 T 2- - - + R U T 2 + + - - - R") + R2 + R4 - - + R")
= RT2 (1 R2 R4 = RT2S, where S = (1
I n a similar manner the total light emerging from the film after reflection from the third unit layer of pigment particles is and
Us = RT'S' U. = RT2S (T2Ss2)(n-2) or = R TZSM(n-2)
where
M = T2S2
(3)
. Thus far it has been tacitly assumed that all the incident light actually enters the film. As a matter of fact, some of the light, 1-a, is reflected a t the interface between the paint and the air, and only a portion, a, really enters the film. Thus we have
+ aR + -aRT2S+ -aRT2SA4- - - +uRT*SM(n-*) = (1--a) + aR + aRT2S (1 + iM - - M("-2))
B,, = (1-u)
(4)
From this we derive the equation
or From (3), where
KI-KZB, = M(n-1) M = T2S2 S = (1+R2+R4- - - R " ) =
1 1-R2
The maximum value of R is (1- T ) ,so that for the maximum value of R , S=
1 l-(l-T)2
=-
1
2T--2
(
The corresponding value of M is 2&T)2* The minimum value of R is 0, in which case M becomes equal to T2. Since T is always less than 1, M("-') approaches 0 as n is increased.LThus we have KI-KZB.
= 0
(7)
INDUSTRIAL AND ENGINEERING CHEMISTRY
134
Similarly, when n = 1, M("-') = 1 and K1-K2B1 = 1 (8) From these equations we can evaluate the constants K1 and K1 in terms of B, (the ultimate brightness) and B1 (the brightness of a film consisting of a single layer of pigment particles). We thus obtain the values:
K1= Bu-& ,and K2
=
1 BU-BI
-
+ +
to the tendency of the particles of zinc oxide to agglomerate. In plotting the results obtained with the zinc oxide pigments the greatest weight is given to the data obtained with the thinner films, since the probable error in determining B, B. increases rapidly as Bn approaches B,. As has been stated above, the values of n are calculated from the formula n = K 10,000 GP
-
Atd
Substituting these in Equation 6 we have
or B,-Bn = (B,-BJ M(n-1) or log (B,-B,) = log (Bu-B1) ( n - I ) log M log (By-Bn) K (n-1) log M
Vol. 18, No. 2
(9)
in which the constant K has been assigned the arbitrary value of 1. The effect of using this admittedly incorrect arbitrary value instead of the unknown true value is simply to multiply n by a constant factor and thus to change the slope of the graph but not its essential form.
(10)
Obviously, therefore, when the logarithms of B, - B n are plotted against the corresponding values of (n- 1) a straight line should result. The values of Bn are obtained by direct measurement, while values of B, are gotten by extrapolation from graphs of Bn plotted against n. The data concerning the various pigments and paints are presented in Table I. Table I-Data
Concernin$ Pi$ments a n d Paints Used Refractive Av. diam. Ultimate index Sp. gr. of pigment whiteness Paint PIGVENT of pigment of pigment Microns of paint 0.341 0.825 1 L/thopone 2.125 4.35 0.381 0.935 2 Lithopone 2.125 4.35 0.375 0.873 3 Lithopone 2.125 4.35 0.520 0.860 4 Lithopone 2.125 4.35 0.371 0.875 5 Lithopone 2.125 4.35 6 Sublimed white lead 0.435 0.883 (basic sulfate) 1.88 6.4 7 Carter process white 6.64 0.620 0.860 lead (basic carbonate) 2.01 8 Basic carbonate white 0.385 0.860 lead 2.01 6.64 1.99 0.470 9 Barytes 1.64 4.50 2.77 0.470 10 Barytes 1.64 4.50 11 Blancfixe 1.64 4.50 0.948 0.800 0.460 0.800 12 Blancfixe 1.64 4.50 2.51 0.600 13 American whiting 1.572 2.715 14 Foreign whiting 1.572 2.715 1.96 0.600 16 French process zinc oxide 2.02 5.23 0.350 0.902 0.500 0.892 16 "U.S. P."zincoxide 2.02 6.23 17 "Lehigh" American process zinc oxide 2.02 5.45 0.500 0.873
,-Bn are plotted For each of these paints the values o against the corresponding values of n- 1, using arithmeticallogarithmic paper and plotting the values of Bu-Bn on the logarithmic scale. It will be observed that in almost every case the points fall on or close to a straight line, thus
From the graphs of B,-Bn plotted against n-1 the comparative hiding powers of the various white paint pigments can be computed. According to Konig and Brodhun, the least difference in brightness that is perceptible to the human eye is from 1 to 2 per cent. It follows, therefore, that complete hiding is secured when Bu-Bn becomes approximately equal to 0.02. If the number of unit layers (of any given pigment) required to give a value of 0.02 for B,-Bn be multiplied by the average diameter of the particles of that pigment, the results should be proportional to the volume of that pigment required to give apparently complete hiding. The reciprocals of these values will be proportional to the hiding powers of the various pigments, expresssd in terms of volume. I n Table I1 the hiding power of lithopone is assigned the arbitrary value of 80 and the values for the other pigments are calculated from their ratios to this arbitrary standard. Corresponding values obtained by Hallett are given for comparison. Table 11-Relative Hiding Power per Unit Volume of P i g m e n t PIGMBNT Rhodes and Fonda Hallett 80 Lithopone 72 Basic lead sulfate 100 Carbonate white lead 16.4 Blanc fixe Whiting 15 t o 20 90 96 Zinc oxide
demonstrating the correctness of the formula given above. The least consistent results were obtained in the case of the zinc oxide Diements. The authors are inclined to attribute the someihgt erratic results obtained with this pigment
Since the two sets of measurements were made by different methods, with samples of pigment which probably differed somewhat in particle size and other properties, the results are reasonably consistent. It should be noted that in calculating back to thickness in terms of unit volume of pigment any error due to arbitrary assumptions as to the arrangement of the particles is eliminated. The value of B1 for each paint represents the portion of the total incident light which wo,uld be reflected by a film of paint containing a single layer of pigment particles when illuminated by parallel light striking the surface a t the
February, 1926
INDUSTRIAL A N D ENGINEERING CHEMISTRY
particular angle of incidence adopted in the photometer used. Obviously, B1 is equal to the sum of the light reflected a t the interface between the air and the film of oil above the first layer of pigment particles and the light reflected a t the interface between the pigment particles and the film of oil above them. The writers’ data do not afford sufficient basis for the separate evaluation of these factors. It is possible that the light reflected a t the interface between the air and the paint film is a rery considerable portion of the total light reflected by the film. If so, the brightness of a film of white paint should depend, in part, upon the viscosity and the surface tension of the vehicle, extent to which the vehicle shrinks during drying, and other factors which affect the smoothness of the surface of the film. The study of the effects of these factors on brightness offers an interesting field for further study. A number of investigators have found that when a small amount of dark pigment, such as carbon black or Prussian
135
blue, is added to a white paint the increase in the hiding power of the paint is very much more pronounced than is the decrease in brightness. The authors have not seen any adequate explanation of this phenomenon. It may be explained as follows: From Equation 9 we have B,-B, where or B,-B,
= (B,-BJ M(n-11 M = TZS2 = (B,-&) (T*S*)(n-I)
If a dark pigment is added to the paint, T will be decreased will be decreased very greatly. For any and (T2S2)fn--l) given thickness of film the value of B,--B, will be very much less than before, and the point a t which the differences between B, and B , ceases to be visible will be reached with a much thinner film than before the dark pigment was added. I n other words, the addition of the dark pigment should very greatly increase the hiding power of the paint.
A Quick Method for the Determination of Ozone’ By H. B. McDonnell ~4ORICULTURAL EXPERIMENT STATION, U N I V E R S I T Y OF h f A R Y L A N D , COLLEGE P A R K ,
HIS method is used in checking the operation of an ozonizer a t the University of Maryland Agricultural Experiment Station. As ordinarily geared this machine delivers approximately 2800 liters (100 cubic feet) of ozonized air per minute at 9 cm. of water pressure. The method is a modification of one used by Todd,2 which is, briefly, as follows: From a nozzle with a n outlet 1.5 mm. (1/16 inch) in diameter the ozonized air is allowed to flow for 15 seconds through a glass tube dipping 2.5 cm. (1 inch) into 30 cc. of a 1 per cent potassium iodide solution containing starch. The color obtained is compared with that from an equal volume of solution containing 5 cc. of a 0.01 per cent iodine solution to which is added starch solution. Instead of comparing the blue solutions the writer found i t more convenient to add to the testing solution of potassium iodide and starch a n amount of sodium thiosulfate solution equivalent to 5 cc. of the 0.01 per cent iodine solution and pass the ozonized air through until the blue color appeared, noting the time with a stopwatch. The amount of ozone varies inversely with the time. Under the conditions of the test it was found that approximately 790 cc. of ozonized air passed through the test solution in 15 seconds, which would indicate approximately 90 parts of ozone per million by weight, which was the concentration desired.
T
Solutions
(1) Starch. Mix 2 grams of soluble starch with about 5 cc. of cold water and add 100 cc. boiling water. This should be kept cool and renewed after 2 to 4 days. (2) Iodine, 0.01 per cent. Dissolve 0.1 gram iodine in about 10 cc. water, with the aid of about 2 grams potassium iodide, and make up to 1 liter. This solution is fairly stable for 1 or 2 months if kept in the dark. (3) Sodium thiosulfate, approximately equivalent to solution (2). Test by taking 5 cc. of (2) in a flask, add the same amount of water and 3 or 4 drops of (1). From a buret add 1 Presented before the Division of Water Sewage and Sanitation a t the 69th Meeting of the American Chemical Society, Baltimore, Md., April 6 to 10, 1925. 2 “Experiments with Oxygen on Disease.”
MD.
solution (3) till color is discharged. Use this amount in each test; it should be approximately 5 cc. The solution is not stable and should be standardized daily. (4)Potassium iodide, 1 per cent. Apparatus
(1) A4brass nozzle to screw into the main supply pipe. The tip of the nozzle for 6 mm. (l/d inch) is drilled to 1.5 mm. (l/le inch) in diameter. (2) A glass tube about 15 cm. long and 4 mm. bore, bent about 45 degrees 3 cm. from one end, the other end ground off at an angle so that the flow of gas will not be obstructed when the end of the tube rests against the bottom of the test bottle. This tube is attached to the nozzle by a short piece of rubber tubing. (3) Test bottle. A 60-cc., wide-mouth packing bottle, which 30 cc. should fill to a height of 2.5 cm., where i t should be marked with a file. Test To the test bottle add the proper amount of the standard thiosulfate solution; fill to the mark with potassium iodide solution and add 4 or 5 drops of starch solution. Attach the glass tube to the brass nozzle, bring the test bottle over t h e end of the glass tube till the end of the tube touches the bottom of bottle, and note time for the appearance of blue color by use of a stopwatch. The concentration of ozone i s inversely proportional to the time. The method is applicable to low concentrations where the amount of ozone used in the test is not over 1mg. For higher concentrations the potassium hydroxide liberated causes low results. (The reviewer suggests the addition of a little boric acid to the thiosulfate and potassium iodide solution to adapt the method to higher concentrations.) The pressure and standards are arbitrary and may be varied to meet individual conditions. This method can also be used for dilute mixtures of chlorins and air. Nolc-Riesenfeld and Benrker [Z. anorg. Chcm., 08, 167 (1916) 1 have also reported work on the action of ozone on inorganic iodine compounds.
