I S D r S T R I , I L An'D ESGIA-EERISG CHEMISTRY
April 15, 1930
contained approximately 20 per cent of the total nitrogen in the form of nitrate. The data (Table v) show that the two methods lead to practically identical results; they further demonst,rate that the can be safely employed for the determination of the total nitrogen in solutions of the complex mixture of substances found in leaf tissue.
193
Literature Cited (1) ASSOCU. Official Agr. Chem., Methods, 1925, p. 9. (2) Jones, IND.ENG.CHEM.,19, 269 (1927). (3) McCandless and Burton, J . Assocn. O f i c i a l A u . Chem., 10, 216 ( 1 0 2 : : . (4) Olsen, Comfit. rend. W a y . lab. Carlsbevg, 17 [3]. 1 (1927). 230 (192i) (5) Ranker, J , Assocn, 05ciai A g r , C h e m , , ( 6 ) Vickery and Pucher, IKD.ESG. CHEM.,Anal. Ed., 1, 121 (1929).
Titrometric Determination of Magnesium' J. Stanton Pierce and 11. B. Geiger GEORGETOWN C O L L E G E , GEORGETOWX, KY
M
AGSESIUlI has lieen determined titronietrically
in the presence of calcium by the use of the hydrogen electrode (2, d), Ti-ith trinitrobenzeiie as indicator (T), and iii 66 to 75 per cent alcohol solution Tvith thymolphthalein a3 indicator (.9). In the absence of calcium, magiiesiinn has !xen tleterniiiied 11sprecipitation of the hydroside with excess alkali! separation of the precipitate and solution, and titration of the e w e s alkali in a n aliquot of the solution i.3, S ) . Hou-euer, in tlie presence of a high concentration of calciuiii salts t,his method fails unless some meaiis is used to indicate the completeness of precipitation of mapnesiinii, for on the addition of more alkali calcium also is precipitated. Since calcium is associated so frequently with iiiagnesiuni in nature and in manufactured products, and since the determination of niagnesiuiii in such mixtures often is verp important, there is need for a practical, rapid, convenient, and fairly accurate laboratory method for the determination of magnesium in the presence of calcium. Direct titration of a neutral solution with alkali, using trinitrobenzene as indicator ( i ) ,gives results accurate enough for most control work, but requires a matching of colors. The method worked out in this laboratory for the determination of magnesium in cement' (6) gives fair results, but the filtrat'ion and washing of the gelatinous precipitate of magnesium hydroxide is tedious if this precipitat,e is large. Also, if the time required for this operation is long, there is danger of adsorption of carbon dioxide from the air and precipitation of calcium carbonate with the niagnesiuiii hydroxide. Since the time factor determines the value of analytical procedures in industrial laboratories, the nTork on niagnesiuni was continued so that tlie tedious filtration could be avoided and the method made more universal. In this n-ork magnesium is precipitated as the hydroside. Jt-itli standard carbonate-free alkali. When tlie end point for trinitrobenzene (a dark brick red) is obtained, the calcium remains in solution. The solution containing the precipitate is diluted t'o a definite volume, an aliquot filtered. and the escess alkali titrated. The accuracy of this method depends, not on the sharpness of the break of the neutralization curl-e in the titration of magnesium with alkali, but on the completeness of the precipitation of magnesium, with calcium remaining in solution, and the end point when the excess alkali is titrated with acid. This end point is around a pH of 4, and is quite definite. Iiolthoff (.$) states that calcium hydroxide does not precipitate in 0.01 -1-sodium hydroxide and in this work its concentration does not become half so great. Experimental Procedure
The following procedure for limestone, with slight modification, may be used for many substances containing magnesium. I
Received M a r c h 21, 1930.
Dissolye sample of 0.500 gram in shout 50 cc. of 0.25 S hydrochloric acid, boil out carbon dioxide, add 5 or 6 drups of 0.04 per cent alcohol solution of broniothymol blue, and add alkali until a blue color, iiidicatiiig neutrality, is obtained. Filter and wash tlie precipitate, cat'ching the ~vashiugs in the same beaker as the original filtrate. To the filtrate add 5 drops of 0.1 per cent alcohol solution of dimethylamiiioazoheiizene and 0.25 S hydrochloric acid to the appearance of a faint pink. Add 10 cc. of a saturated alcoholic solution of triiiit,robenzene and titrate wit'h 0.25 K carbonate-free sodium hydroxide (I?!) to the appearance of a deep red color. Transfer the solution t o a 100-cc. volumctric flask and continue titration, if necessary, t o hold the deep red color. Dilute to 100 cc.. mix, and filter through a dry filter paper into a dry beaker, keeping the funnel covered ivith a watch glass as much as possible. Pipet 50 cc. of the filtrate and add an indieator which a t a p H of about 4.0 has a color change easily recognizable in a reddish solution. Tit'rate with 0.25 S hydrochloric acid ( A , )to the color change of tlie indicator. X sufficiently close approximation of the per cent magnesium oside is as follows: B1 - 2 A 1
=
per cent MgO
Solutions of samples containing 110 cation whose hydroxide is insoluble a t neutrality are treated as is the neutral filtrate mentioned above. LIMESTOWS 3IgO Sample present 1
2 3
3IgO found
s
ci
1.0
1.0 4.1
L O
16.8
,n
l6.,
TSCIIXICAL S A L T S OF hlg Sample present
4 5
6
hIAGNBSIEY C I I L O R I D B
31g found
5%
c-
7 4
7 4 (contains CaCI:I
12 2 23.8
I
12.4 23.5
With the same samples the inagiiesium was determined as described in tlie article on the determination of magnesium in cement (6). satisfactory data were obtained, but the method is much longer than the one given above. Instead of filtering an aliquot of the solution above, the sample may be centrifuged after precipitation of the inagiiesiuiii hydroxide and a sample for analysis pipetted. If a large number of samples are t o be ruii ill which the iiiagiiesiuiii content is not very high, it may be advantageous to place each solution, after dilution to 100 cc., in a tall cylinder to allow the precipitate to settle, and to pipet a sample of the supernatant liquid for titration. Discussion of Method
Bromophenol blue was used as indicator for obtaining the data given above. This is a satisfactory indicator if a n aliquot of the alkaline filtrate is titrated right away. Trinitrobenzene in alkaline alcoholic solution readily changes to some compound which retains a distinct red color after
AL\7-4LYTICiALE D I T I O S
194
.Voie-Hepp ( I ) obtained a product with properties of a nitrated phenol b y nllowing an alcoholic solution of trinitrobenzene t o stand for a short time with 2 mols of potassium hydroxide. Lobry de Bruyn ( 7 1 obtained 3,j,3,,j,.tetranitroazohenzenzene and dinitrophenol by boiling trinitrobenzene with dilute sodium carbonate or b y allowing the mixture t o stand ;it room temperature for 3 days. Sodium- hydroxide reacted similarly sIo~vIy a t room temperature.
acidification. I n such a solutioii the color change of broinophenol blue cannot be detected easily. Congo red is a sat,isfactory indicator in such cases. and good results were obtained by using it, for cklaIlge froIn red t o blue-violet is detected Jvit,h in the presence of a high concentratioll of the Droduct from trinitrobenzene. Seither Congo red nor bromophenol blue is as satisfactory as dirnethylaininobenzene as an indicator for adjusting the acidity before the initial tit'ration. The red color of Congo red in alkaline solution interferes with the end point of trinitrobenzene. If the final titration is made with bromo-
blue in the solution increases the difficulty of rPc(ogniZing
\-01. 2 ,
so.2
this Doint. so this indicator is not atlcled ulitil the solutioli is about neutral. A slight error is introduced by using one indicator for tlio initial adjusting of the aciditv of t,he So~LltiOllalid anflthW for tlie final tiGation. The err(jr t1iLls introduced, honevcr. does not exceed 0.1 per cent of XgO. I
,
Acknowledgment
Ickiio\\-ledgmeiit is niade to the 1 1 0 ~C'lieinical C'IIIIIpany for the analyzed samples of niagi~esirunchloride -aIts used in the determinations. Literature Cited $ ~ ~ ; b ~r ~ n~ ~ ~ ~ a ~ (,on$ ~ . l p p~l C i lde r n , ,,1, t 3 ) Kolthoff, z , a n o y 6 . ailgem. C h t . m , 112, 11920). (4, m i t h o f f , Rec. I Y ~ L ' . citim., 41, 787 WXI.
'
, ~~~1~ ~ ~
(91 U'illitntter a n d \Valdschmidt-Leitz, B e y , 5 6 , 488 (1923)
Multiple Standard Colorimeter for pH Determinations' S. L. Leiboff BIOCHEMICAL LABORATORY, LEBAVON HOSPITAL. SEXUORK,S U
HIS coloriineter was especially designed for inatchiiig colors of uiikno~vnsolutions where a single standard
T
cannot be used, particularly in colorimetric hydrogenion determinations of wide range.
-
Figure 1-Box
f o r Standard Bottles
'C'sually such determinations are done in test t i i h i i i a block comparator where the tubes are placed side Iiy bide antl the color differences observed. I n the colorimeter here described all the standards are contained in sinall, rcctangular bottles of uniform size with two opposite parallel plane sides! t,he distance between ivhich is uniform in all the hottles. This allows t,he passage of light through equal depths of solution. T h e standard bottles are placed close together in a a black rectangular box. (Figure 1) This box contains circular fenestrz on two opposite sides parallel to each other. E a c h bottle fits snugly between a pair of fenestrzep,thus allowing the light to pass bhrougli a uniform depth of solutioii u-ith all the standards. Figure 2 s h o m the colorimeter \\-it11 the standards in place. It consists of compartment A ! which contains two mirrors, JI and JI', placed a t a n angle of 1 5 degrees aiid parallel to each other. Mirror -11' is half the size of mirror JI. The cylinder B carries a magnifying lens, L, aiid an eyepiece: E. Received December 13, 1929. Very suitable bottles are Kos. 31.190 and 31,192, Eimer 8r Amend Catalog, 1927, p. 706. These bottles are used for spectroscopic work and a r e inexpensive. 1 2
To facilit,ate the reading the reading tuhe itself may IIC sluped, with t,he mirrors tilted a t an appropriate angle. For the determination of pH the following arrangement is used: I n compartment C are placed the unknown) standards, and compensators. Bottle 1 contains the unkiiowi iolutioii with the appropriate indicator. Bottle 2 contains di,tilletl water. Bottle 3 contains the unknown solution, hut without indicator, to compensate for cloudines aiid color of the iinknown. The box containing the staiidards i: placed a t 1antl is moyed along the stage S horizontally by means of a rack aiid pinioii, R P , with the thumb screw T,5'. Thie Iiox is m o d along on the stage while the operator is looking t h r ( i u g 1 i the eyepiece until the E c l o s e s t match is obt a i n e d b e t w e e n t>he standard and the un-B known. T h e light p a s h i ~ i gthrough the feiiestrze F and F ' is reflected uplvard by the in i r r o r s-in i I' r o r M' c o y e r i n g one-half of mirror JT, antl p r o d u c e s , ~vlieiiv i e w e d through the eyepiece, the effect of a c i r c l e , o n e - half representing the itaiidard and the other half representing the unknown. Comparisons may be niade by artificial light by placing a few sheets of tissue paper between t h e s o u r c e of l i g h t TS and the colorimeter in RP Order to diffuse the Figure 2-Colorimeter w i t h S t a n d a r d s light. i n Place
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~
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