Process for the preparation of silver dispersions for filter layers and antihalation layers

The process for the preparation of silver dispersions for antihalation and filter layers in photographic material by reducing silver salts in the presence of a heterocyclic 5-membered or 6-membered compound which has a ##STR1## group in its 1,2,3-position and a 2-aminoalkyl or 2-hydroxyalkyl group or an alkylene chain which is linked through the 1- and 2-position. The process provides neutral grey silver dispersions without using cadmium salts, for the reduction of the silver salts.

This invention relates to a process for the preparation of silver 
dispersions for antihalation and filter layers. 
Silver dispersions for filter layers and antihalation layers have hitherto 
been prepared mainly by the reduction of silver nitrate in the presence of 
a binder such as gelatine with phenols such as hydroquinone or tannin. 
With these processes it is in most cases only possible to obtain blue 
silver dispersions which do not have a uniform absorption over the whole 
spectrum. Another serious disadvantage of these processes is that the 
resulting oxidation products of the reducing agent have a hardening effect 
on gelatine, even when only small quantities of oxidation products are 
left in the emulsion when the silver dispersion is being prepared. 
When reduction is carried out with hydrazine, black silver dispersions are 
obtained only if silver nuclei are added to the gelatine solution. 
Moreover, vigorous foaming occurs due to the evolution of nitrogen. This 
foaming causes serious manufacturing problems when large quantities of 
silver dispersions are being prepared, and these problems are difficult if 
not impossible to overcome since antifoaming agents have a harmful effect 
when used in the preparation of photographic layers. The problems of 
gelatine hardening and of foam formation could be solved by the method 
described in German Offenlegungsschrift No. 1,917,745. According to this 
method, silver dispersions are prepared by the reduction of silver salts 
with ascorbic acid or its isomers, and either pure yellow or blue black 
silver dispersions can be obtained as desired, depending on the pH, the 
nature of the protective colloid used and other additives as well as the 
reducing agents, blue toners, etc.. 
Neutral grey silver dispersions which have a very uniform absorption over 
the whole spectrum and can be used for antihalation layers can be obtained 
by the reduction of silver salts with ascorbic acid in the presence of 
cadmium salts. 
About 1 to 10% by weight of cadmium salts, based on the quantity of silver 
nitrate, are used for preparing the silver dispersions and, when preparing 
the dispersions, it is necessary to ensure that the dissolved cadmium 
salts are removed from the waste water. On the other hand, there is also a 
risk of the cadmium salts left in the dispersion being dissolved out of 
the photographic material when the exposed film is processed and thereby 
contaminating the waste water. 
As part of the intensified efforts on the part of industry to restrict the 
use of potential environmental toxins, the possibility has been 
investigated of restricting the use of cadmium salts for the preparation 
of silver dispersions of antihalation layers without reducing the quality 
of the photographic materials produced. 
The present invention is therefore based on the problem of developing a 
process for the preparation of neutral grey silver dispersions, which 
dispenses with the use of cadmium salts for the reduction of aqueous 
silver salts as reducing agents but still yields silver which is as far as 
possible neutral grey in colour and does not have any of the disadvantages 
of contaminated effluent mentioned above. 
The invention thus relates to a process for the preparation of dispersions 
of metallic silver by reduction of an aqueous silver salt in the presence 
of a protective colloid and of a reducing agent of the kind commonly used 
for precipitating silver, such as phenols, hydrazine, compounds of the 
pyrazolidone-3 series, hydroxylamine or reducing sugar compounds such as 
dextrine or compounds of the oxytetronic acid series. 
The process according to the invention is characterised in that reduction 
of the silver salt compound is carried out in the presence of a 
heterocyclic 5-membered or 6-membered compound which has a 
##STR2## 
group in its 1,2,3-position and a 2-aminoalkyl or 2-hydroxyalkyl group or 
an alkylene chain which is linked through the 1- and 2-position. 
The reducing agent used is preferably an oxytetronic acid compound such as 
ascorbic acid or its isomers or homologues which may be partly replaced by 
other conventional reducing agents but preferably not to a greater extent 
than 50%. The following are suitable examples: Hydrazine hydrate, 
hydroxylamine, compounds of the pyrazolidone-3 series and adduct compounds 
of amines with boranes or dextrin. 
The process according to the invention gives rise to silver dispersions 
which have an excellent neutral grey colour with high covering power 
without having any deleterious effect on the effluent. Furthermore, the 
above mentioned heterocyclic compound used according to the invention is 
photographically completely inert so that it has no deleterious effect on 
photographic materials if the silver dispersions prepared according to the 
invention are used as antihalation layers in photographic materials. 
The process according to the invention is preferably carried out in the 
presence of a protective colloid which contains polyvinyl pyrrolidone as 
additive. 
The quantity of polyvinyl pyrrolidone to be used may vary within wide 
limits and depends mainly on the nature of the reducing agent used and of 
the heterocyclic compound. It has generally been found suitable to add 
from 0.5 to 50 g of polyvinyl pyrrolidone per mol of silver nitrate, 
preferably from 2 to 10 g/mol of silver nitrate. 
The protective colloids used for the process according to the invention may 
be hydrophilic, water-soluble film formers, e.g. natural polymers such as 
starch or degradation products of starch such as dextrans or dextrins, or 
proteins, preferably gelatine. Suitable synthetic film formers include 
polyvinyl alcohol, partially saponified polyvinyl acetate and polyvinyl 
pyrrolidone, which has already been mentioned above. 
The reducing agents used for the process according to the invention are 
preferably oxytetronic acid compounds. According to "Chemie der Zucker- 
und Polysaccharide," by F. Micheel, 2nd Edition, published by akademische 
Verlagsanstalt Geest und Portig KG, 1956, pages 35 to 39, oxytetronic acid 
compounds are sugar compounds with reducing properties. 
L-Ascorbic acid, which is the most frequently occurring among the reducing 
sugar compounds which may be used according to the invention should 
therefore be regarded as L-threo-oxytetronic acid. Other suitable reducing 
agents include isoascorbic acid, (D-erythro-oxytetronic acid); 
6-desoxi-L-ascorbic acid, (6-desoxi-L-threo-oxytetronic acids) and 
4-methyl-D, L-oxytetronic acid. 
The heterocyclic compounds used for the process according to the invention 
may be 5-membered or 6-membered compounds and may contain condensed 
aromatic rings, in particular a condensed benzene ring which may be 
substituted. The compounds used according to the invention may also be 
used in the form of their quaternary salts after reaction with a suitable 
alkylating agent. 
Particularly suitable among the heterocyclic compounds which may be used 
according to the invention are those represented by the following general 
formula 
##STR3## 
in which R.sup.1 represents a straight or branched chain alkyl group which 
may be substituted with a hydroxyl or amino group and preferably has from 
1 to 11 carbon atoms, such as a straight or branched chain methyl, ethyl, 
propyl, butyl, pentyl or undecyl group. The amino group may be substituted 
with lower alkyl groups, preferably with alkyl groups having up to 4 
carbon atoms such as methyl or propyl; aryl, in particular phenyl; 
aralkyl, in particular phenethyl or benzyl; or with an acyl group derived 
from an aromatic or aliphatic short chain carboxylic acid having up to 5 
carbon atoms such as acetic acid, propionic acid, butyric acid or 
isobutyric acid or from aromatic carboxylic acids such as benzoic acid or 
toluic acid. Alkyl groups which are substituted by the hydroxyl or amino 
group selectively on the 3rd to 8th carbon atom from the point of 
attachment are preferred. 
R.sup.2 represents hydrogen; an alkyl group preferably having from 1 to 6 
carbon atoms such as methyl, isopropyl or pentyl; a cycloalkyl group such 
as cyclopentyl or cyclohexyl; an aralkyl group such as phenethyl or 
benzyl; or an aryl group, in particular a phenyl group or 
R.sup.1 and R.sup.2 together represent an alkylene chain which may be 
substituted by shorter alkyl groups having up to 6 carbon atoms such as 
methyl, isopropyl or pentyl or cycloalkyl such as cyclopentyl or 
cyclohexyl or aryl such as phenyl and which is capable of forming a 
5-membered to 7-membered heterocyclic ring together with the nitrogen atom 
in the 1-position and the carbon atom in the 2-position of the 
heterocyclic ring; 
R.sup.3 represents a free electron pair; a hydrogen atom or a substituent 
suitable for forming a quaternary salt, such as an alkyl group, in 
particular an alkyl having from 1 to 6 carbon atoms such as methyl, 
isopropyl or butyl; aryl, in particular phenyl, or aralkyl, in particular 
benzyl; 
X represents an anion required to complete a quaternary salt or an ammonium 
salt, which anion is photographically inert and normally used for 
completing quaternary salts and ammonium salts in photographic materials, 
for example anions of inorganic acids such as halides, in particular 
chlorides, or sulphates, or anions of organic acids, such as tosylate or 
mesylate; X is absent when R.sup.3 represents an electron pair; and 
A represents the carbon atoms required to complete a 5-membered or 
6-membered ring, such as the carbon atoms required to complete an 
imidazole, imidazoline, dihydropyrimidine or tetrahydropyrimidine ring, 
and the heterocyclic ring may also carry condensed aromatic rings, e.g. a 
benzo or naphtho ring, for example benzimidazole, perimidine or 
dihydroquinazoline which condensed rings may in turn be substituted, for 
example by an alkyl group having up to 6 carbon atoms such as a methyl, 
isopropyl or tert-butyl group, a cycloalkyl group such as a cyclopentyl or 
cyclohexyl group, an aralkyl group such as a benzyl group, an aryl group 
such as a phenyl group or halogen such as chlorine. 
The following are examples of suitable heterocyclic compounds which may be 
used according to the invention: 
Table 1 
__________________________________________________________________________ 
##STR4## 
No. 
##STR5## R.sup.1 R.sup.2 
X.sup.- 
.degree. C 
__________________________________________________________________________ 
1 Benzimidazole [CH.sub.2 ].sub.3NH.sub.2 
H -- mp: 
119-120.degree. 
2 Benzimidazole [CH.sub.2 ].sub.4 NH.sub.2 
H -- mp: 70.degree. 
3 Benzimidazole [CH.sub.2 ].sub.5NH.sub.2 
H -- mp: 99.degree. 
4 Benzimidazole [CH.sub.2 ].sub.11NH.sub.2 
H -- mp: 
73,5-74,5.degree. 
5 Benzimidazole [CH.sub.2 ].sub.3NHCH.sub.3 
H -- mp: 137,5.degree. 
6 5-Methylbenzimidazole 
[CH.sub.2 ].sub.3NHCH.sub.3 
H -- mp: 126.degree. 
7 Benzimidazole [CH.sub.2 ].sub.5NHCH.sub.3 
H -- bp.sub.0,3 : 
195-205.degree. 
8 Benzimidazole [CH.sub.2 ].sub.5NH.sub.2 
C.sub.6 H.sub.5 
-- bp.sub.0,3 : 
185.degree. 
9 Benzimidazole [CH.sub.2 ].sub.5NHCH.sub.2C.sub.6 H.sub.6 
H -- bp.sub.0,1 : 
239-240.degree. 
10 Benzimidazole [CH.sub.2 ].sub.5NHCOC.sub.6 H.sub.5 
H -- mp: 
201-202.degree. 
11 Benzimidazole CH.sub.2CH.sub.2CH.sub.2CH.sub.2CH.sub.2 
-- mp: 126.degree. 
12 Benzimidazolinium 
CH.sub.2CH.sub.2CH.sub.2CH.sub.2CH.sub.2 
Cl.sup.- 
mp: 
226-228.degree. 
13 Benzimidazole CH.sub.2CH.sub.2CH.sub.2CH.sub.2 
-- mp: 106.degree. 
14 Benzimidazole CH.sub.2CH.sub.2CH.sub.2 -- mp: 114.degree. 
15 3-Methylbenzimidazolinium 
CH.sub.2CH.sub.2CH.sub. 2CH.sub.2CH.sub.2 
Tosylate 
mp: 171.degree. 
16 Benzimidazole [CH.sub.2 ].sub.3OH H -- mp: 163.degree. 
17 Benzimidazole [CH.sub.2 ].sub.5OH H -- mp: 143.degree. 
18 5-Methylbenzimidazol 
[CH.sub.2 ].sub.5OH H -- mp: 134.degree. 
19 Benzimidazole 
##STR6## H -- mp: 123.degree. 
20 4,6-Dimethylbenzimidazole 
[CH.sub.2 ].sub.3OH H -- mp: 169.degree. 
21 3,4,5,6-Tetrahydropyrimi- 
[CH.sub.2 ].sub.4NH.sub.2 
H -- bp.sub.10 : 
158.degree. 
dine 
22 3,4,5,6-Tetrahydro- 
[CH.sub.2 ].sub.5NH.sub.2 
H -- bp.sub.0.05 : 
137.degree. 
pyrimidine 
23 3,4,5,6-Tetrahydro- 
CH.sub.2 ].sub.5NH.sub.2 
[CH.sub.2 ].sub.3NH.sub.2 
-- bp.sub.0,2 : 
155-60.degree. 
pyrimidine 
24 Perimidine [CH.sub.2 ].sub.5NH.sub.2 
H -- mp: 193.degree. 
25 7-Chlorobenzimidazole 
CH.sub.2CH.sub.2 CH.sub.2CH.sub.2CH.sub.2 
-- mp: 115.degree. 
26 4-Chlorbenzimidazole 
CH.sub.2CH.sub.2CH.sub.2CH.sub.2CH.sub.2 
-- mp: 136.degree. 
27 2-Imidazoline [C.sub.2 ].sub.5NH.sub.2 
H -- mp: 78-81.degree. 
28 5-Methyl-2-imidazoline 
[CH.sub.2 ].sub.5NH.sub.2 
H -- bp.sub.0,05 : 
116-120.degree. 
29 2-Imidazoline [CH.sub.2 ].sub.11NH.sub.2 
H -- mp: 96-97.degree. 
30 3,4,5,6-Tetrahydro- 
CH.sub.2CH.sub.2CH.sub.2 -- bp.sub.11 : 
97-99.degree. 
pyrimidine 
31 3,4,5,6-Tetrahydro- 
CH.sub.2CH.sub.2CH.sub.2CH.sub.2CH.sub.2 
-- bp.sub.13 : 
130.degree. 
pyrimidine 
32 Benzimidazole CH.sub.2C(CH.sub.3).sub.2NH.sub.2 
H -- mp: 210.degree. 
33 Benzimidazole 
##STR7## H -- bp.sub.0,05 : 
215-18.degree. 
34 Benzimidazole [CH.sub.2 ].sub.5N(CH.sub.3).sub.2 
H -- bp.sub.0,05 : 
197.degree. 
35 Benzimidazole 
##STR8## -- mp: 143.degree. 
36 3,4,5,6-Tetrahydro- 
[CH.sub.2 ].sub.5NH.sub.2 
CH.sub.3 
-- Bp.sub.15 : 
168-170.degree. 
pyrimidine 
37 3,4,5,6-Tetrahydro- 
[CH.sub.2 ].sub.6NH.sub.2 
H -- Bp.sub.10 : 
188.degree. 
pyrimidine 
38 Imidazoline [CH.sub.2 ].sub.5NH.sub.2 
[CH.sub.2 ].sub.2NH.sub.2 
-- Bp.sub.0,1 : 
128-30.degree. 
39 Imidazoline [CH.sub.2 ].sub.4NH.sub.2 
H -- Bp.sub.12 : 
155.degree. 
40 Imidazoline [CH.sub.2 ].sub.6NH.sub.2 
H -- Bp.sub.10 : 
175.degree. 
41 Imidazoline CH.sub.2CH.sub.2CH.sub.2CH.sub.2 
-- Bp.sub.0,03 : 
42-44.degree. 
42 Benzimidazole Dihydrochloride of compound No. 3 mp: 
240.degree.-250.degr 
ee. 
43 Benzimidazole Dinitrate of compound No. 3 mp: 
145.degree.-146.degr 
ee. 
__________________________________________________________________________ 
*In the compounds mentioned in the above Table, X.sup.- and + are absent 
when R.sup.3 is a free electron pair. 
The following are further examples of suitable benzimidazole derivatives of 
the process according to the invention: 
2-(.gamma.-Isobutylaminobutyl)-; 2-(.epsilon.-acetylaminopentyl)-; 
2-(.epsilon.-dimethylaminopentyl)-; 2-(aminohexyl)-; 
2-(.omega.-amino-heptyl); 2-(.delta.-hydroxybutyl)-; 
2-[2'-(2"-hydroxycyclohexyl)-ethyl]-; 2-(.delta.-hydroxypentyl)-; 
2-(.omega.-hydroxyheptyl)-; 2-(.epsilon.-hydroxypropyl)- 1-methyl; 
2-(.omega.-hydroxybutyl-1-ethyl)-; 2-(.epsilon.-hydroxypentyl)-1-phenyl; 
2-(.gamma.-aminobutyl)-1-ethyl-; 1,2-(1'-methyltrimethylene)-; 
1,2-(2'ethyltrimethylene)-; 1,2-(3'-methyltrimethylene)-; 
1,2(1'-benzyltrimethylene)-; 1,2-tetramethylene-; 
1,2-(3-phenyltetramethylene)-; 1,2-(3'-isopropyl-4'-methyltetramethylene)-; 
1,2-pentamethylene; 1,2-(3'-tert.-butylpentamethylene)-; 
1,2-(5'-cyclohexylpentamethylene)-benzimidazole, its 4-, 5-, 6- or 
7-methyl compound, its 5-trifluoromethyl, 5-tert.-butyl or 5-phenyl 
compound, its 4,6- or 5,7-dimethyl compound; its 4-, 5-, 6- or 7-chloro or 
bromo compound or its 5,6-dichloro compound. 
As already mentioned above, the compounds used according to the invention 
may be put into the process in the form of their bases or their salts and 
it is left open which nitrogen atom of the given compound is used for salt 
formation. 
The quantity of heterocyclic compound to be used may vary within wide 
limits and depends on the nature of the reducing agent and the quantity of 
polyvinyl pyrrolidone added. It can easily be determined by a few 
laboratory tests. It has generally been found sufficient to add from 5 to 
500 mg and preferable to add from 20 to 100 mg per mol of silver salt. 
The heterocyclic compounds used according to the invention may be prepared 
by known methods. 
2-(.omega.-Aminoalkyl)-benzimidazoles, for example, may be prepared by 
reacting phenylene diamines with amino acids or amino acid derivatives as 
described in Chem. Reviews 74, 279 to 283 (1974); German Pat. No. 
1,131,688 and British Pat. No. 1,023,792 or with reactive lactime 
derivatives as described in German Offenlegungsschrift No. 2,110,227 or 
with lactams as described in German Offenlegungsschrift No. 2,321,054. 
2-(.omega.-Hydroxyalkyl)-benzimidazoles, for example, may be prepared by 
reacting phenylene diamines with lactones as described in Ann. 596, 208 
(1955); Khim. Geterotsikl, Soedin. (1972), pages 641 to 644; J. Org. Chem. 
24, 419 to 421 (1959) and Z. Naturforsch. 25B, 928 to 931 (1970) or with 
N,N',N" -tris-(.omega.-hydroxyalkyl)-triazines as described in Bull. Chem. 
Soc. Japan 38, 897 to 901 (1965). 
Finally, 1,2-alkylene-benzimidazoles can be prepared, for example, by acid 
catalysed thermal splitting of 2-(.omega.-aminoalkyl)-benzimidazoles as 
described in German Patent Application P 24 35 406.1, by ring opening 
condensation of phenylene diamines with lactones as described in Ann. 596, 
209 (1955); J. Org. Chem. 24, 419 to 421 (1959) and Z. Naturforsch. 25B, 
928 to 931 (1970) or by ring closing condensation of o-phenylene diamines 
with .omega.-halogencarboxylic acid iminoether hydrochlorides as described 
in J. Org. Chem. 27, 2165 (1962). The preparation of 1,2-alkylene 
imidazolines has been described in J. prakt. Ch. [2] 140, 59 [1934] and 
the preparation of 1,2-alkylene-3,4,5,6-tetrahydropyrimidines has been 
described in Synthesis 11, 591 [1972]. 
The salts of the basic compounds according to the invention with 
photographically inert acids are prepared by the usual methods. Quaternary 
salts of 1,2-alkylene benzimidazoles are prepared by quaternisation with 
suitable alkylation agents such as alkyl or aralkyl halides, tosylates, 
sulphates or mesylates. 
The preparation of compound 11 is described below by way of example. 
5 g of methanesulphonic acid are added to 200 g (1 mol) of 
2-(5'-aminopentyl)-benzimidazole and the mixture is heated to 300.degree. 
C for one hour with stirring and kept at 300.degree. to 320.degree. C for 
about 10 to 15 hours until evolution of ammonia ceases. Subsequent 
fractional distillation of the reaction mixture yields 159 g (85.4% of the 
theory) of 1,2-pentamethylene-benzimidazole, b.p..sub.0.05 mm 145.degree. 
to 148.degree. C, m.p. 126.degree. C, colourless crystals after 
recrystallisation from 3 parts of ethyl acetate. 
As already mentioned above, part of the reducing agent used in the process 
according to the invention, preferably up to 50 mol percent, may consist 
e.g. of hydrazine hydrate, hydroxylamine, compounds of the pyrazolidone-3 
series such as 1-phenylpyrazolidone-3, addition products of amines and 
boranes, or dextrin. When reducing agents which liberate gases are also 
used, they should be added in such small quantities that unwanted foaming 
will not occur and any reducing agents which have a hardening effect 
should also be used in such small quantities that they have no deleterious 
effect. 
After reduction, the silver dispersions are adjusted to a pH of between 5.5 
and 6.5 and when they have solidified they are shredded and rinsed. If 
desired, rinsing may be replaced by a process of flocculating, for example 
using ammonium sulphate or any of the usual flocculating agents.

When the soluble salts have been washed out, the shreds are melted and if 
necessary a further portion of protective colloid is added, e.g. gelatine. 
EXAMPLE 1 
(a) comparison Example I 
Preparation of a silver dispersion using an ascorbic acid solution 
50 ml of a 7 percent by weight aqueous gelatine solution (salt free bone 
gelatine) were mixed with 64 ml of a 30% aqueous ascorbic acid solution. 
The pH of the solution was about 1.2. 34 ml of a 50% silver nitrate 
solution were added to this mixture at 30.degree. C with vigorous 
stirring. The silver nitrate was thereby reduced and a dark brown silver 
dispersion was obtained. 
When solid, this silver dispersion was shredded and rinsed. After rinsing, 
the dispersion was melted and 5 g of gelatine were added to the melt. 
After the gelatine had been dissolved at 40.degree. C, 4 ml of a 2.5% 
methanolic solution of phenol were added to the melt as bactericide and 2 
ml of a 7.5% aqueous solution of saponin were added as wetting agent and 
the melt was applied to a transparent cellulose acetate substrate. The 
dark brown material obtained in this way was examined behind various 
filters of a Zeiss filter photometer and the colour densities determined 
at the wavelengths indicated below. To calculate the covering power, the 
colour densities were divided by the quantity of silver applied, 
calculated as silver nitrate. The values obtained for the covering power 
at different wavelengths are given below: 
Table 2 
______________________________________ 
Wavelength [nm] 
420 490 530 590 750 
______________________________________ 
Covering power 
1.8 1.6 1.5 1.35 1.15 
______________________________________ 
(B) Comparison Example II 
Preparation of a silver dispersion according to Comparison Example I using 
cadmium chloride additionally. 
A silver dispersion was prepared in the same way as described in Comparison 
Example I except that 20 ml of a 10% cadmium chloride solution were added 
to the gelatine solution before the silver nitrate solution was run in. 
Subsequent treatment and determination of the covering powers were carried 
out as described in Comparison Example I. 
Table 3 
______________________________________ 
Wavelength [nm] 
420 490 530 590 750 
______________________________________ 
Covering power 
2.3 2.2 2.1 1.95 1.8 
______________________________________ 
Comparison Example II shows that when cadmium salts are used, neutral grey 
silver dispersions are obtained which have a practically uniform covering 
power over the whole wavelength range of the spectrum. 
(C) Example according to the invention 
This is a repetition of comparison Example 1 except that, before the 
addition of silver nitrate solution, 500 mg of a polyvinyl pyrrolidone 
(e.g. polyvinyl pyrrolidone K 90 having a molecular weight of 90,000 
manufactured by BASF, Ludwigschafen) and 7 ml of a 0.1% solution of 
1,2-pentamethylene benzimidazole hydrochloride were added. Subsequent 
treatment of the material and determination of the covering powers were 
carried out as described in Comparison Example I. 
Table 4 
______________________________________ 
Wavelength [nm] 
420 490 530 590 750 
______________________________________ 
Covering power 
2.2 2.1 2.1 2.0 1.95 
______________________________________ 
When the values for covering power obtained according to the invention are 
compared with the corresponding values in the comparison examples, it is 
found that by adding the substances used according to the invention it is 
possible to prepare a neutral gray silver dispersion with high covering 
power which compares favourably with the neutral grey silver dispersion 
obtained according to Comparison Example II in being even more uniform 
over the whole spectral range. 
This is shown graphically in the accompanying FIGURE where the relationship 
between covering power and wavelength in Comparison Example I is 
represented as the full line curve A, that of Comparison Example II as 
broken line curve B and that of the Example according to the invention as 
dash-dot curve C. In the graph, the values for covering power are plotted 
along the ordinate and the wavelengths in nm along the abscissa. 
EXAMPLE 2 
Example 1C was repeated, except that instead of 
1,2-pentamethylene-benzimidazole hydrochloride used in Example 1C, the 
compounds shown in Table 5 below were used in the quantities indicated. 
Table 5 
______________________________________ 
Quantity Covering power at 
Compound mg/mol wavelengths [nm] 
No. silver nitrate 
420 490 530 590 750 
______________________________________ 
1 90 2.2 2.2 2.1 2.1 1.95 
3 100 2.3 2.0 1.95 1.9 1.8 
9 80 2.0 2.0 2.0 1.95 1.9 
14 100 2.3 2.1 1.95 1.85 1.8 
13 80 2.1 2.1 2.1 2.0 2.0 
16 90 2.2 2.0 1.85 1.90 1.85 
19 90 2.2 2.0 1.85 1.90 1.85 
______________________________________ 
The covering power can be determined within the limits of error of .+-.5 %. 
EXAMPLE 3 
Example 1C was repeated except that instead of using the quantity of 
reducing agent indicated in Example 1C, 48 ml of a 30% ascorbic acid 
solution and 5 ml of a 10% hydrazine hydrate solution were added and the 
quantity of 1,2-pentamethylene benzimidazole hydrochloride was increased 
to 9 ml of the 0.1% solution. 
The following results were obtained: 
Table 6 
______________________________________ 
Wavelength [nm] 
420 490 530 590 750 
______________________________________ 
Covering power 
2.3 2.2 2.0 2.0 1.85 
______________________________________