Determination of the Dry Hiding of Pigmented Coatings PHILIP L. G O R D O N AND M I C H A E L A. GILDON', Capitol Paint and Varnish Works, Brooklyn, N. Y . such that the eye can no longer discern any differenee in opacity over black and white. Bruce (4) assigned a value of 0.98 as the contrast ratio representing ccmplcte hiding, hut Sawyer (84) quoted a reference to work by Kraemer and Schupp who showed that the critical contrast ratio is usually higher than 0.98 and may even be higher than 0.995 in some cases. The empirical equation presented by BNCCwaa reported as unsatisfactory by Gamble and F'fund (Sawyer, 84) and by the Baltimore Production Club (9). The Fell equation has been applied successfully to white paints hut no data were found on its application to colored painta. I n order to eliminate the disadvantages of the methods and equations previously used in determining hiding power, work was undertaken in the authors' la,hoboratary to find a means of measuring contrast ratio which could be dupliostcd with reasonable accuracy and to develop a general equation for the contrast ratio curve which would apply for white and odored paints and would hold in the higher range of oontmt ratios.
A n empirlul equation rxprnring the relationship between the conbast ratio of a dry pigmented coating and the weight of rnahrial applied is developed by rectification of the dry hiding SUNG, using (he method of averages (6). The equation is shown to apply to a swim 01 four unrelated paints, and furnisher the means for calculating the dry hiding et any desired contrast ratio. Tho method p n x n h d yields reproducible results, provides the opportunity lor neuhalizatlon of enon introduced by faulty tachniqur, and will also function where high dry hiding pigments are used. The equation is applied succenlvlly to data presented by other invertigaton where the range d conbast ratios covered reach- 0.999. A comparison between the derived equation and other empirical hiding power equations Indicates excellent agreement in tho case of white paints end a definite variation in the case of colored coatings. Theoreticd and practical interpretations of the data and comparisons are d i r curnd.
PREPARATION OF EXPERIMENTAL COATINGS
0.
NE of the controlling factors in the raw msterid cost of a pigmented coating is its opacity or hiding power, since in
I n order to determine whether any one equation would apply in all c~ses,a series of four unrelated paints was ground off. Pig-
most cmes the hiding pigment is the costliest portion of the paint when calculated on volume hasis. It is therefore necessary to hsve an accurate mertns of determining the hiding power of a paint both for formulating purposes and for estimation of the quantity of material required to coat a given area. The methods for measuring hiding power in use at present are the brushout (3,if, f8,17,80,df) anderyptometer (f0,88,83,87,83) for the wet hiding power and the dry fihn contrast ratio soecification for the dry hiding power ( 7 ) .
ments were selected which varied widely from each other in color and covering power. The extending pigments and vehicle were different in each case, as well as the gloss of the dry films. No attempt is made to correlate the results with any individual component of the paints, since the object of this investigation is to find an equation which can he applied regardless of the composition of the coating being tested. A formula breakdown of the test coatings is given in Table I.
The A.S.T.M. method (I) far relatlw U J u~wug-I UVY m wived wide acceptance and does not appear in government specifications. A number of system based on hotometric or photcgraphic measurement are described in the 8teraturc (&5,f4,16, 18, 4i5. 89). StutE and Hadhm (M)presented a general evaluation of the methods available at the time f?r hiding power determination, listing their individual shortcormngs. Msthematical dationships between paint 6lm constants were derived by Kubelks. and Munk (19) and Hsnstook (15). The equation of K u b e h and Munk correlating the e o n t m t ratio of a aint fihn over black and white against its reflectance at inlinite k m thiokness was tested h Jndd (18),who found that it did not hold closely in all CBRB. &anstock indicates B relrttionshp between the log of the quantity of light transmitted through a paint film and the thickness of the film. Sawyer (84) presents a comrehensive outline of the theoretical aspects of hiding power. 18 oltner evaluates the aDDlicahilitv of the Kubclka and Munk equa'tih, the e uation de&d hg Urwe, and an empirical q u a tion by l{. H. who ,bows & linear rclation betmrrii log 10 contrast ratio and rcriprorsl film tliirknesi over the N ~ F Cof from 0.7 upwards nearly to 1.0 contrast ratio. The Feu equation has been chwkrd hy a nurnhrr of workcri and found to hold very~lus~l~mtl~aca.;rufrhitcpil~nts.
%.
141.
PREPARATION OF TEST PANEL!
-I_-..-. Six brushouts were made of each paint on board paintout sheeta, which had an area of 1 square foot consisting of 17 white squares with B diffuse daylight reflectance of approximately 80% and 18 black squares with a reflectance of less than 5%. The brushouts were made as carefully as possible, the amount of paint being varied to obtain as wide a range of oontrast ratios as possible. I n the case of the strongly opaque green paint, it was neoessary ,bthin the coatings in order to get the lower contraat ratio readings, while in the ease of the more transparent Burgundy lake, two coats were used to get the higher readings, The test panels were permitted to dry far 24 hours. 111
CONTRAST RATIO DETERMINATIONS
The photovolt reflectometer, used to measure the contrast ratio is illtxtrated in Figure 1. This instrument consista of an indi&ting taut-wire suspension galvanometer, actuated hy e search unit which is constructed so that readings are obtained
The disadvantages of the hrurhout method for hiding power dpterniinstion are the diflirulty of reprodwing results by diflerent operators, the lnrgc error introduced by fsulry technique, and the fact that this method does not take into xcount loss of hiding of gloss paints on drying and the decided incrcsre iu hiding of flat paints formulnred w i t h high dry lliding p i p e n t s . The use of the cryptornerer docs ni,t owreonie these faults. The dry 6lrn contrast rJtio method specifies a contrast ratio (less than one) foro given volume of paint per square foot, painted over a stairda d brushout sheet. Tims, resulu of this rnathod nrtl relaiivc and vannot he used IO cslculak the quantity of paint w e d 4 to give complete covering.. The ideni hiding power ~ I U Pwould be that obtained over a standard burfare when the contrast ratio is 1
Figure 1.
A t p l ~ ~ inn tth e U n i t d S t 8 t e Navy.
442
Apparatus
for Measuring Contraat Ratio
ANALYTICAL EDITION
luly, 1944 Table
1.
Coating
3. 4.
White Chrome green Zinc chromate Burgundylake
1.
White
1.
2.
2. Chrome green
tion that additional constants were required and the foliowing general equation for hyperbolic curves waa investigated:
Composition of Experimental Coatings Finish Flat Semigloss Semiglosr
Gloss
TS-W
TS-V
TP-V TS-V
%
%
%
83.5 83.3 68 3 80.0
62 0 66.0
78.0 55.5 42.0 48.8
62 5 62.8
Hiding Pigment Extender Titanium dior- CaCOa ide Chromium ox- Magnesium ids silicate
3.
Zinc chromate
4.
Burgundy lake Burgundy lake
Zinc chromate
Diatomaceow silica CaCOa
H E TP-V Wt./Gal. % Pounds 30.2 15.1 7.5 13.3 68.5 11.0 14.8 12.1
Vehicle Bodied linseed oil 30
-
443
-
gallon dehydrated castor oil, maleic gum varnish Alkyd resin, phenol resin, tung oil vunish 45-pUOn ester gum linseed oil varnish'
TS-W total solids by weight TB-V,'total solids by volume TP-V total pigment volume HP-V', hiding pigment volume Wt./Gal., weight per gallon in pounds
equivalent to a 45" angle of incidence and 0" viewing angle. The light source was a 15-cp. lamp, and a filter was used which made the light spectrally equal to average daylight. Reflectance readings were taken on the 15 squares in the middle of the test sheets, omitting the outer row. The readings over black and white were averaged separately and the averages divided to give the contrast ratio. A variation of more than 2% between the highest and lowest reading over each set of squares of the same color was considered cause for rejection and a new paintout was made. The contrast ratio data are given in Tables I1 to IV. RECTIFICATION OF CONTRAST RATIO CURVE
The procedure used to set up empirical equations and test their applicability to the experimental contrast ratio data was the method outlined by Davis (6). Briefly, it involves rectification of the curve by plotting those functions of the variables that produce a straight line. The proper functions are ascertained by trial and error.
In view of the work by Hanstock (IS), the first relationship tried was that between the logarithmic functions of the weight and contrast ratio. However. dotting the data on lop and semilog paper resulted in curves, 6;hich inldicated that tgerere.was no proportional relationship between those functions. Since the contraat ratio curves are definitely hyperbolic, the next rectification attempted was the plotting of the red rocal of the wcighta against the reciprocal of the contrast ratio. khese functions would produce a straight line if the curve were an equilateral hyperbola. The resultant slight curve was an indica-
c=- W
where
C
W
C', WL a, b
- W'
+
'
a bW = contrast ratio = weight of paint to give contrast ratio of C =I any set of coordinates on contrast ratio curve = constants for contrast &io equation
For this equation to hold, a plot of W - w' against weight c - C' must yield a straight line. . The derivation of the e uation and proof of the rectification method are covered thorougay by Davis (6). The function c C' was calculated for each paint and the data are presented in Tables I1 to IV. Plotting these values against the corresponding W yielded a straight line for most of the coordinates for each paint. The equation for the straight line is ~
3 -
By setting up two series of simultaneous equations for each paint, constants a and b can be obtained. Substituting these values in the general equation permits the calculation of weight for any desired contrast ratio. A sample set of calculations for the white paint is given below.
TYPICAL CALCULATION
W' = 14.6 C' = 0.912 W - 14.6 -a+bW C - 0.912 Substituting data from Table I1 gives six equations 35.8 = a 3.7% 128.7 = a 16.4b 41.9 = a 4.16b 133.2 = a 18.2b 82.0 = a 10.50b 148.4 = a 19.8b 159.7 = 3a 18.44b 410.3 = 3a 54.4b
+ + + +
+++ +
The equations 159.7 = 3a 410.3 = 3a
(2) (3)
(4)
18.44b ++ 54.4b
(5) obtained by the addition of the two sets of equations above, are simultaneously to a = 10.2 b = 6.98
Substituting in the general equation,
W 10.2
- 14.6
+ 6.98W + 0"12
(6)
which is the contrast ratio curve equation for the white paint. A check in the cases where no straight line was obtained suggested that the deviation was due to the fact that the set of coordinates used was out of line in relation to the other coordinates. I n order to correct the deviations as much as possible, the rectification was carried out with the set of coordinates that were most dependablei.e., weight equal to 0-and the contrast ratio of the brushout sheet. Constants a and b were calculated for each paint using the coordinates W1 = 0, C1 0.043, as outlined in the typical calculation above. These constants were used in the hiding power equation .D
c = -a
Figure 2. Rectification of Contrast Ratio Curves 0 . O h w e d dab 0. Calculated data L e f t , white b.iinl1. R i g h t , chrome green paint P
+-bW + 0.043
and C was calculated for each W. These figures were tabulated in Tables I1 and 111. The observed and calculated contrast ratios were plotted against weight in Figures 2 and 3. In every case, calculated contrast ratios gave a smooth curve which seemed to align the observed data properly.
444
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
W 0 3.78 10.5 14.6 16.4 18.2 19.8
Table II. Contrast Ratio Data for White Paint 1 w-0 Calcd. C 0.043 0.610 0,882 0.912 0.926 0.939 0.947
C
- 0.043
0 b67 0.819 0.869 0.883 0.896 0.904
C
- 0.043
C
6:66
0:iis
12.82 16.80 18.57 20.31 21.90
0.855 0.910 0.926 0.939 0.949
I n order to test the equation in the higher range of contrast ratios, above 0.98, the rectification was applied to data from the Baltimore Club (9). Two paints were chosen, one in which the contrast ratio was determined up to 0.990, and the other where the highest contrast ratio was 0.999. The rectification data are tabulated in Table I V and the contrast ratio curves presented in Figure 4. The equation showed excellent adherence to the data, with negligible differences between observed and calculated contrast ratios. In brief review, the method for determining dry hiding power is 8s follows: The contrast ratio of the standard black and white hiding power sheet is measured by a suitable photoelectric instrument, using a daylight filter.
Table 111.
W
C
Contrast Ratio Data w-0
- 0.043
-
C 0.043 Chrome Green Paint 2 C
0 3.0 3.4 4.2 5.5 6.2 7.3 0 2.9 3.9 5.0 6.6 8.6 9.8
Vol. 16, No. 7
Calod. C
....
o:i02 0.490 0.629 0.749 0.787 0.841
7.46 6.94 6.67 7.35 7.87 8.68 Zinc Chromate Paint 3
....
0.043 0.572 0.681 0.770 0.789 0.841 0.914
5.49 6.12 6.88 8.73 10.06 11.25
Burgundy Lake Paint 4 0 5.5 8.6 13.6 24.5 25.4 30.2
....
0 205 0.336 0.457 0.642 0.651 0.696
26.81 25.60 29.73 38.15 39.00 40.80
Six brushouts are made on the hiding power sheets and allowed to dry for 24 hours. The reflectances of the center squares are measured and the results averaged. A variation of more than 2% indicates a poor brushout and a new brushout is made. The average reflectance over black is divided by that over white to give the contrast ratio. Values for
IV c- C'w1are
determined using the
coordinate I V l = 0, C1 being the contrast ratio of the unpainted brushout sheet. These values are substituted in the equation
wc -- C'IV' = a + bW
WEIGHT OF PAINT (GMS)
Figure 3.
Rectification of Contrast Ratio Curves
0 Observed deb 0. Glculatd data L e f t , Bu&undy lake paint 4.. Right, zinc chromate wlnl 3
to give six equations. The six equations are divided into two sets and added to give two equations which are solved simultaneously for constants a and b. The hiding power equation W C=-++c' a +bW where C1 is the contrast ratio of the unpainted hiding power sheet is solved for that value of c which is considered to indicate complete hiding. Thus value can be converted to square feet of coverage per gallon by using the equation 454 x W per gal. H.P. (sq. ft. Per gal.) mms of paint per sq. ft.
-
DISCUSSION
Figure 4. Rectification of Contrast Ratio Curves 0. Observed dab 0. Gleuletedd&
L e f t , ll*nated Illhopone wink ' R i g h t . high d n hldinr, lithopone mint
The measurement of hiding power has always been a controversial subject. In part, this can be attributed to the difficulty of applying uniform films of paint and misunderstanding as to what constitutes complete hiding. The theoretical conception of the contrast ratio curve is that it is asymptotic to unit contrast ratio, and that there must always be a difference b e tween the reflectance over white and &e reflectance over black, even though that difTerence be immeasurable. However, in making the reflectance measurements, we must use an instrument with definite sensitivity limitations. The data obtained must necessarily be bound by the limitations of the instrument used. Therefore, any empirical equation evolved from such data will represent a contrast ratio curve as developed by the reflectometer used. It Will be seen
ANALYTICAL EDITION
July, 1944 Table IV.
W 0
3.2 3.8 6.0
7.5 10.9 11.7 15.5 0 5.7 8.0 10.0 15.2 17.6 21.4
C
Rectification Data
-
0.0102 .... 0.750 0.7398 0.798 0.7878 0.868 0.8578 0.897 0.8868 0.036 0.9258 0.952 0.9418 0.990 0.9798 High Dry Hiding Lkhopone ( 9 , p. 0.0102 .... 0.938 0.9278 0.965 0,9548 0.976 0.9658 0.988 0,9778 0.993 0,9828 0.999 0,9888
C
-
....
4.325 4.825 6.995 8.475 11.77 12.42 15.82 29) 60% P / N V
....
6.13 8.38 10.36 15.54 17.90 21.62
Table
Calcd.
w-0 c 0.0102 c 0.0109 Titanated LithoDone (9. . . .D. 29)
445
V. Data lor Comparison with Fell Formula
Log of 10 x Log of 10 x Observed Calculated Calculated Contrast Contrast Contrast Weight Ratio Ratio Ratio White 1 3.78 0.264 0.610 0.7853 0.628 0.7980 10.5 0.0952 0.862 0,9355 0.855 0.9320 14.6 0.0685 0.912 0,9600 0.910 0,9590 16.4 0.0610 0.926 0,9666 0.926 0.9666 18.2 0.0550 0.939 0,9727 0.939 0.9727 19.8 0.0505 0.947 0.949 0.9773 0.9763 Chrome Green 2 3.0 0.333 0.445 0.6484 0.481° 0.6821 3.4 0.294 0.533 0.7267 0.531 0.7251 4.2 0.238 0.672 0.8274 0.623 0.7945 0,8802 5.5 0.182 0.792 0.8987 0.759 6.2 0.161 0.830 0,9191 0.825 0.9165 7.3 0.137 0.884 0.9465 0.923 0.9652 Zinc Chromate 3 0.345 2.9 0.572 0.7574 0.7731 0.593 3.9 0.681 0.8306 0.256 0.677 0.8331 5.0 0,200 0.770 0,8865 0,8727 0.746 6.6 0.152 0.789 0.8971 0.9128 0.818 8.6 0.842 0.116 0.9253 0.9450 0.881 0.102 9.8 0.914 0.9609 0.9586 0.909 High Dry Hiding Lith opone (0, p. 29) 60% P / N V 5.7 0.9745 0.175 0.938 0.9722 .. 0.943 0.125 8.0 0.965 0.9836 0.9845 0.963 10.0 0.100 0.976 0.9890 0.9894 0.975 15.2 0,988 0.0658 0.9048 0.990 0.9956 17.6 0.993 0.0568 0.9969 0.994 0.9974 21.4 0,0467 0.999 0.9996 0.9996 0.999 a From Table 111. Reciprocal of Weight
0,0102 0.747 0.784 0.868 0 900. 0 947 0.953 0.978 0.0102 0.943 0.963 0.975 0.990 0.994 0.e99
that in the author’s equation, a value can be obtained for contrast ratio of unity. From a practical standpoint, this value represents the point a t which the instrument used can no longer register a difference between reflectance over white and black, and may be considered complete hiding for that instrument. There has been little attempt made to apply hiding power equations to coatings other than white, perhaps because theoretically agreement is not to be expected except by spectrophotometric analysis and separate treatment for various wave lengths of the visible spectrum. After the present work was finished, the attention of the authors was drawn to the Fell equation ($4) which shows a relationship between the log of 10 X contrast ratio and the reciprocal of the film thickness. This equation has been shown to hold for several white paints, and it is of interest to see whether this relation applies as well as the hyperbolic equation proposed here to the same data including chromatic 0 as well aa white paints. Figures 5 and 6 and Table .9 V show this comparison; the agreement with the Fell equation is generally good. Indeed, for one of the chromatic paints (chrome green, Figure 5,
5
Observed Contrast Ratio
lower) the agreement is slightly better than with the hyperbolic form, since a straight line may be drawn which fits the observed data slightly more closely than the curve which corresponds to the hyperbola fitted by the authors’ method. It is probable, therefore, that an alternate method for finding conveniently the weight corresponding to complete hiding could be worked out for the Fell e q u a t i o n , though the authors have not tried to do this. CONCLUSION
The method for measuring dry hiding of pigmented coatings as outlined offers the means for eliminating many of the disadvantages now encountered. Its results are reproducible and it permits theoperator to detect and discard poor brushouts. The use of the hiding power equation gives its reading on the paint in the state in which the information is of the most value, the dried film. The equation offem the possibility of standardizing on a value for complete hiding which would be lese open to dispute than the values now used. LITERATURE CITED
(1) Am. 900. Testing Materials, $04
.06
Above, whih palnl 1.
Below, chrome irrcn pain1 P
.IO
.I2
-14
.I6
.I8
R E C I P R O C A L OF WEIGHT
RECIPROCAL OF WEIGHT
Figure 5. Comparison with Fell Equation 0 . Okmrd dab 0. Clculrted dah
-08
Figure 6.
Comparison with Fell Equation
Qlculahd Aboue,0. zincObenrd chmmihdab. paint 3. 0. Below, hiih data dw hldlnu llthoponr pain1
Standard MethodofTest, R e l a t i v e Dry Hiding Power of Paints, D344 (1939). (2) Ayera and Clewell, Am. Paint J . , 22,15-17,45-9, 50 (1938).
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
446
Ayers and Clewell, Paint, Oil,Chem. Rev.,100, N o . 3, 7-8, 30-2. 42 (1938).
Bruce, H. B., Bur. Standards Tech. Paper, 306 {1926). Davidsohn, A., Paint Manuf., 9, 340 (1939). Davis, D. S., “Empirical Equations and Nomography,” pp, 4-7, 24-6, New York, hfcGraw-Hill Co., 1943. Federal Standard Stock Catalog, Spcc. TT-P-23a, pp. 7-9 (March 22, 1940). Ibid., TT-P-Sla, p. 6 (Jan. 16, 193i). Federation of Paint and Varnish Production Clubs, Tech. Proc.,
(15) Haslam, G.S.,INU.E m . CHEAI., . A s ~ r . ED.. , 2, 69 (1930). (16) Ibid., 2, 319-22 (1930). (17) Jacobsen and Reynolds, I b i d . . 6, 303-5 (1034). (18) Judd, D. B., J . Resoarch .Vutl. Bur. Standards, 19, 287 (1937). (19) Kuhelkaand Munk, 2. tech. Physik, 12, 593 (1931). (20) Levy, S.A., Am. Pairit Varnish Mfrs. Assoc., Circ. 377, 135-0 (1931).
Morrison, R. A., Oficial Dioest Federation Paint & Varnish Pradwtion Clubs,112, 745 (1932). Pfund, A . H., Proc. A.S.T.M., Preprint 94, 0 (1930): Ibid., 30, Pt. 11,878 (1930). Sawyer, R. H., Am. Roc. Testing Materials, Symposiuni on
DP. 27-34 (1939).
Gardner, Sward, and Levy, Am. Paint Varnish hlfrs. Assoc.,
Color, pp. 23-37 (1941).
CP’TC. 362, 235-72 (1930).
Sohneerson. S. L., Byull. Obmena OpUt. Lakokrasoehn. Prom.,
Gardner, Sward, and Levy, Metal Cleanmg Finishing, 2 , 537-8 (1930).
Hallett, R. L., Proc. A m . SOC.Testing Materials, 30, Pt. 11, 895910 (1930).
Hanatock, R. F., J . Oil Coloztr Chem. Assoc., 20, 5-34 (1937). Hanstock, R. F., and Jordan, L. A,, Research Aasoo. of British Paint, Colour & Varnish Mfrs., British Patent 434,136 (Aug. 27, 1935).
Vol. 16, No, 7
10, 22-3 (1940). (26)
Stutz and Haalam, Proc. Am. SOC.Testing Materials, 30, P t .
(27)
Sward, G. G., Am. Paint Varnish Mfrs. Assoc., Circ. 433, 217-18
(28)
Sward, G. G., Natl. Paint, Varnish Lacquer Assoc., Sci. Sec.,
11, 884-90 (1930). (1943). --,\ - -
Circ. 433 (July, 1933). (29) Tichenow. E., Farben Ztg., 36, 1469-70 (1931).
Determination of Nitrate, Nitrite, and Ammonium Nitrogen Rapid Photometric Determination in Soil and Plant Extracts BENJAMIN WOLF, The G. L. F.-Seabrook Farms Raw Products Research Division, Bridgeton, N. J.
T
H E determination of soluble inorganic nitrogen fractions in plant and soil extracts is extremely useful in explaining crop growth. In a previous paper (4)a method was described for the determination of nitrate nitiogen. The present paper gives a more rapid method for determining nitrate nitrogen and in addition, proposes methods for determination of nitrite and ammonium nitrogen in the same extract.
CALIBRATION OF STANDARD CURVES. Aliquots of standard solutions are diluted with extracting solution to appropriate level and treated as in the determination of nutrients of soil and plant extracts. Photometer readings are taken using the a g propriate filter (Table I). Deflection concentration curves are drawn from the resultant data.
Table APPARATUS
Photoelectric colorimeter. The amounts of reagents and samples are based on the use of the Fisher electrophotometer. Vials (4),stirring rods, Waring Blendor, balance, oven, and 1- and 5-ml. pipets. REAGENTS AND SOLUTIONS
All reagents should be of C.P. grade. EXTRACTIONS. Morgan’s universal extracting solution ( 8 ) , acetic acid (0.5 N ) , buffered a t pH 4.8 with sodium acetate. DETERMINATION OF NITRATE iiITROGEN. Brucine (Merck), 1% in concentrated sulfuric acid, is prepared just prior to using. The sulfuric acid should be free of nitrates. Nitrate nitrogen standard. Sodium nitrate, 0,0910 gram in lo00 ml. of extracting solution to supply 15 p.p.m. of nitrogen. DETERMINATION OF KITRITE NITROGEN.Dimethylaniline (Eastman 97), 10 ml., in 1 to 6 hydrochloric acid. Concentrated hydrochloric acid. Nitrite nitrogen standard. Potassium nitrite, 0.1210 gram in lo00 ml. of extracting solution t o supply 20 p.p.m. of nitrogen. DETERMINATION OF AMMONIUM NITROGEN.To make Graves reagent ( 5 ) ,80 grams of sodium chloride are dissolved in 130 ml. of water and 100 ml. of a cold saturated solution of mercuric chloride (7%) are added with shaking. The salt is almost dissolved and 70 ml. of a saturated solution of lithium carbonate (lye) are added in small quantities and with continued shaking. Five grams of talc are added to the solution, whichisfiltered. Stored in a brown bottle and kept stoppered, it will keep for several weeks, but needs to be thoroughly shaken before use. Gum arabic, 0.25%; sodium hydroxide, 15%. Standard nitrogen. Ammonium chloride, 0.1521 gram in 1000 ml. of extracting solution to supply 40 p.p.m. of nitrogen. METHODS
PREPARATION OF SOILAND PLANT EXTRACTS. Detailed di-
rections for preparing extracts have been given (4).
I. Determination of Inorganic Nitrogen Fraction for Standard Curves and in Soil and Plant Extracts
Nitrogen Fraction Nitrate N
Material Standard soln. Soil extract Plant extract
Useful Range P.p.m. 0-2 0.6-12
Volume of Aliquots
M1.
Diluted to
M1. 15
0.6-12
0-2 2.5 2.6
Nitrite N
Standard soh. Soil extract Plant extract
0-5 2-10 2-10
0-5 10 10
20
Ammonium N
Standard soln. Soil extract Plant extract
0-5
0-25 5 5
20
4-80 4-80
DETERMINATION OF NUTRIENTS IN SOILAND PLANT EXTRACTS. Soil or plant
Nitrate Nitrogen i n the Absence of Nitrite Nitrogen.
extracts (2.5 ml.) are diluted to 15 ml. with extracting solution and 7.5 ml. of brucine reagent are added cautiously down the side of tube. The contents are immediately stirred with a flat-bottomed’ rod. Photometer readings are taken after 15 minutes, using a 425 blue filter and adjusting the blank to 100. Nitrate Nitrogen i n the Presence of Nitrile Nitrogen. Nitrite nitrogen will produce color changes similar to those produced by nitrate nitrogen with the brucine reagent. A concentration of nitrite nitrogen is approximately 3 times as effective as one of nitrate nitrogen, and so 3 p.p.m. of nitrate nitrogen can be subtracted for every p.p.m. of nitrite nitrogen present in the extract (for photometer readings 100 to 35). Nitrite nitrogen is determined by the method outlined blow. For more exact determination, standard amounts of nitrite nitrogen (0.1 to 1.0 p.p.m.) are treated with brucine as for the determination of nitrate nitrogen and a standard curve is drawn from the resultant data. The amount of nitrate nitrogen equivalent to the nitrite nitrogen present is calculated from the standard curves and deducted from the total amount. of nitrate nitrogen in the extract as shown by the brucine test. Nitn’te Nitrogen. Dimethylaniline solution (0.5 ml.) is added to each tube containing 10 ml. of soil or plant extract diluted to
