Quantitative reflectometry—II

Tc~lur~w Vol

21 pp 475-410

Perpmon

Precs. 1974 Prmted m Great Bntam

QUANTITATIVE PRECISION

REFLECTOMETRY-II* AND INTERFERENCES DAVID KEALEY

School of Chemical and Physical Sciences, Kmgston Polytechmc. Kingston-upon-Thames, England. (Rewired

19 Ocrober 1973. Accepted IO December

Surrey.

1973)

detailed assessment of the precision of direct reflectance measurements has been made, usmg commercially available test-strtps for nickel. The overall relative precision is 3.2-4.6 O0 for the concentration range 10-100 ppm. The effect of interfering ions has been studied and the mechanism of Interference discussed in terms of dtffusion. Non-commercial test-strips have been prepared and compared with the commercial product.

Summary--A

The feasibility of using direct reflectance measurements for quantitative analysis was examined in the first paper of this series’ and it was shown that the relative precision is 2-496 for IO-100 ppm of nickel. A more detailed assessment of the precision has now been made by an analysis of variance,’ and interferences have been investigated. During the course of the current work, nickel test-strips were prepared with a larger surface area than those available commercially. The principal advantage of such “homemade” strips is the ability to make reflectance measurements with an attachment fitted to a spectrophotometer. Although spectrophotometric measurements using the commercial strips had been made during the earlier work, a rather tedious routine of mounting six strips on a card was necessary to provide adequate instrumental response. The properties of the larger test-strips have been compared with those of the commercial product. EXPERIMENTAL The reagents and apparatus were those described earlier,’ with the addition of plastic TLC plates coated with sihca gel (with binder) or cellulose (no binder). Groups of up to five test-strips were mounted m a large cork which was then clamped in position for the strtps to be immersed simultaneously (no stirring) in the appropriate sample solution (for 2.0 min i 1 set). The strips were then rinsed briefly with dtstilled water and dried for 3 min with a hot-air blower. For the stattstical evaluation of preciston. sets of five strips were developed for nickel concentrations of 10, 25, 50, 75 and 100 ppm. The procedures and condittons for reflectance measurements with the “Chromoscan” and SP 800 instruments were as previously described.’ The SP 800 spectrophotometer was used only for the non-commercial strips. which were large enough to be mounted directly m the SP 890 reflectance attachment

The effects of K. Ca. La. Fe(IIIl. Cu(I1). Co(H) and Hg(II) on a 100 ppm mckel standard were mvestigated at levels up to 20.000 ppm. Each metal was added as the nitrate and the pH of the sample solutton adjusted to between 5 and 7 with sodium acetate if necessary. In the case of Fe(W), tartrate was also added to prevent prectpitation of the hydrated oxide. Interference from Cu(II) at htgh levels was masked with sodium thiosulphate. from Fe(III) with potassium fluoride and from Hg(I1) with sodtum chlortde. * Part I: 7alnnrrr. 1972. 19.1563. 475

DAVID KEALEY

476 Preparation

of non-commercial

test-strips

Plastic silica-gel and cellulose TLC plates were each cut mto strips approximately 1 x 0.75 in. and the adsorbent layer removed frpm one end so as to leave a test area of about 1 x 1.75 m. Each strip was immersed briefly in a 1 : 1 mixture of saturated solutions of .dimethylglyoxime and anhydrous sodium acetate in methanol. After rinsing in methanol to remove excess of reagent, the strips were allowed to dry in air. RESULTS

AND

DISCUSS,ION

Assessment of precision

In the development of an analytical procedure, it is of interest to know the precision that can be expected from a “batch” of measurements made with a given set of reagents and within a given working period, and the overall precision of measurements made in a number of such periods. The overall precision may be affected by the use of different sets of reagents and by gross changes in laboratory conditions. In the present work a “batch” is a set of measurements made with test-strips taken from one particular box of 100. during one working session. Good estimates of”within-box”, “between-box”and “overall” precision can be made by statistical analysis of variance on data from sets of five replicates from each of four boxes. This has been done for solutions containing 10. 25, 50, 75 and 100 ppm of nickel, the optimum range of concentration being indicated by linearity in the Kubelka-Munk and Ringbom plots.’ The precision of repeated measurements on a set of prepared standards over a long period (10 weeks) has been assessed similarly. Table 1. Analysis of variance formulae Source. of variability Between boxes Within boxes Total

Sums of squares, Si

Degrees of freedom N,=m-1

S, = 1 B2/n - (c B)‘/mn S,, = z X2 - c B2/n S, = 1 X2 - (1 B)‘/mn

No = m(n N, = mn -

1) 1

Mean squares M, = S,IN, MO = SolNo

X = reflectance measurement: B = sum of a set of n replicates: m = number of boxes or number of sets of measurements made on a set of standards over a period of time: M, = \,$ and M, = II.\,, + sw where \H and gw are csttmates of the “withm-box” and “between-box” standard deviattons.

The formulae used in the analysis of variance are shown in Table 1. If “between-box” variations are significantly greater than “within-box” variations, then the following relationships are valid: SW- 4% sg =

s, =

Ml-MO \ ii

n

Js$ + &)

where s, , sBand s, are estimates of “within-box ” , “between-box’* and “overall” standard deviations respectively, the other symbols being defined in Table 1. This will hold only if M, is significantly greater than M,, as indicated by application of the statistical F-test, and was found to be the case at all the concentrations studied except the 10 ppm level, which was anomalous in that M, was significantly less than MO

Quantitative reflectometry Table 2. Relative precision of reflectance measurements nickel

[Nil, ppm

471 in the determination

Within boxes

Relative precision. y, Between boxes

Overall

3.2 2.0 1.4 1.4 1.5

3.1 2.9 2.9 4.3

3.7 3.3 3.2 4.6

10

25 50 75 loo Srngle 100

duy

1.3

OfTI.

10

Wrc~L!

1.3

of

01 cwrll

1.8

so that only “within-box” precision could be calculated at that level. The source of the anomaly, which may have been an undetected but determinate error, has not been investigated further. The calculated precisions are given in Table 2 and are in general agreement with the results obtained in the preliminary study.’ Interferences

in rejlectometry

The colour of test-strips immersed for 2 min in a nickel solution intensifies appreciably during this period even though the solution is unstirred. The process clearly involves diffusion since nickel ions at the surface of the test-strip are removed from the solution by the chromogenic reaction, thereby creating a concentration gradient, nickel ions then diffusing from the bulk solution to the surface until all the reagent is consumed. If the chromogenic reaction is fast, diffusion will be the rate-controlling process. The lower reflectance (increased reading) for test-strips immersed in stirred solutions, where mechanical convection augments diffusion, can be adduced as evidence to support this assumption.’ It is also of interest to note from the earlier results that the effect of stirring is greater at the 10 ppm level, where a relatively small concentration gradient is established, than at the 50 ppm level where the gradient is much larger. The higher rate of diffusion expected in the 50 ppm solution is proportionately less affected than that prevailing at the lower concentration. The rate of diffusion of an ionic species can also be altered by the formation of an electrical double-layer at the surface of the test-strip. In general, electrical double-layers form at any interface between a solid and an electrolyte solution. The double-layer, consisting of a monolayer of adsorbed ions, or an ionized surface, and a more diffuse layer of counterions extending into the bulk solution, behaves like a condenser in that a potential gradient is established normal to the surface. Other electrolytes in the sample solution can have a profound effect on the magnitude of the potential gradient, and if at high enough concentrations, may reduce the bulk diffusion rate of the ions of interest (in this case. nickel). Non-electrolytes may have a similar effect solely by increasing solution viscosity. High concentrations of ionic species’ which do not react with dimethylglyoxime but which result in a negatively-charged surface with an outer layer of positive counter-ions will therefore inhibit the diffusion of nickel ions to the surface of the test-strip. For a given immersion time, an increased reflectance (lower reading) will be observed, although the colour of the strip will not be visibly affected. These conclusions are supported by the results given in Table 3 which show the effects of

478

DAVID KEALEY Table 3. Effect of K, Ca and La on the reflectance of a lOO-ppm nickel standard Interfering ion none

’ K+

Ca2+ La-‘+

Concentration, ppm zoo0

20.000 WQO 20,000 2000 20,000

Reflectance (mean of 5 replicates)

Relative precision, I’”

9.80 7.16 6.93 7.20 5.50 560 5.60

1.6 1.5 1.1 1.7 1.4 1.3 1.6

K+ Ca2+ and La3+, all added as nitrates and none of which reacts with the reagent. The precision of these measurements, also given, does not differ significantly from-that of those made with pure nickel solutions. There is some indication from the results shown in Table 3 that the degree of interference is related to the formal charge on the cation, tervalent lanthanum causing very serious interference even at the 2000 ppm level. At the 20,000 ppm level. La3+ and Ca2+ have an effect comparable to but more pronounced than that of K+, but at such a high concentration of electrolyte the inhibition of bulk difIusion may be the dominant factor. It is possible that interference of this type could be correlated with other parameters such as charge density, but further work would be necessary. It can be assumed that any electrolyte present in addition to nickel salts constitutes an interference, the threshold concentration at which reflectance is measurably affected being determined by the nature of the electrolyte. For analytical purposes, the situation may be regarded as a matrix effect which can be obviated by use of standards with a composition similar to that of the sample. If the interfering ion also reacts with the reagent to form a coloured complex, additional changes in reflectance will occur. This is because of the competition for the reagent between nickel and the interfering ion at the surface of the test-strip. If there is a considerable excess of the interfering ion present, it is likely that there will be a significant reduction in the amount of nickel complex formed in addition to the effect caused by a slower rate of diffusion. Likely interfering ions of this type, viz. Cu(II), Fe(III), Co(I1) and Hg(II), have been investigated at concentration levels between 50 and 5000 ppm. These concentration levels and suitable masking reactions were based on data provided with the commercial test-strips. The results are summarized in Table 4 and show that Cu(I1) can be tolerated up to at least 1000 ppm but at 5000 ppm the test-strip is discoloured by a green deposit and the reflectance increases. Although the addition of sodium thiosulphate prevents formation of the green discolouration and prevents the copper from competing for the reagent, the diffusion of nickel is still significantly inhibited. Interference by Fe(II1) at both the 1000 and 5000 ppm levels can be minimized by complexing the iron with tartrate or fluoride but the diffusion of nickel is nevertheless impaired. At 5000 ppm of Fe(III), a slight brown discolouration of the test-strip is noticeable. Interference by Co(I1) is serious and is observed even at 50 ppm. the reflectance decreasing owing to formation of a brown cobalt complex. Bathing the strips in aqueous ammonia (1 + 20) after immersion in the sample solution dissolves some cobalt complex but leaves only a very pale pink colour due to nickel and results in a higher reflectance. Up to at least 5000 ppm of Hg(I1) can be masked successfully

Quantitatwe

reflectometry

479

Table 4 Effect of Cu(II). Fe(III). Co(J1) and Hg(J1) on the reflectance of a 100 ppm nickel standard

Interfermg ion

Concentration, iw

None clJ*+

1.000 5.000 5.000 1,000 5.000 50 1.000 1.000 1,000

Fe3+ CO*+

Hg*+

Reflectance (mean of 5 replicates) lOGO 10.30 7.10 8.41 9.48 I 8.46 I 1060 not measured 8.30 not measured 10.03 9.34

Masking agent

Relative precision, “/i

-

aqueous ammonia -

1.4 1.7 47 3.1 1.4 1.7 2.6 6.6 -

NaCl NaCl

2.0 3.1

Na&A tartrate or fluoride -

-

with sodium chloride. the test-strips showing a deep red-brown colour in the absence of the masking agent. It should be noted that in many cases, especially at high concentrations of the interfering ion, the relative precision is significantly reduced. Assessment of non-commercial test-strips Strips prepared from plastic TLC plates coated with silica gel or cellulose were impregnated with dimethylglyoxime, and sodium acetate as a buffer to enable them to be used down to pH 2-3. Comparison with the commercial strips showed that the colour intensity produced at a given nickel concentration was greater for both types of “homemade” strips. However, the silica gel coating tends to flake off during immersion in the sample solution, and the precision at the 25- and 100 ppm levels (5 replicates) is not as good as that of the commercial product (Table 5). The cellulose layers are mechanically stronger. give a very even colour and a precision somewhat better than that of the commercial product (Table 5). The reflectance spectrum of the developed cellulose-coated strips is similar to that of the commercial strips except that the maximum is slightly less broad and is shifted from 547 f 1 nm to 538 & 1 nm. This hypsochromic shift is to be expected from the finer and more even particle size of the powdered cellulose used in the manufacture of the plates as compared to the coarse, fibrous structure of paper. As in the case of the commercial strips, the Kubelka-Munk law is obeyed up to about 100 ppm of nickel and the optimum range for quantitative measurements indicated by a Ringbom plot is lo-100 ppm. Table 5. Comparison of commerical and non-commercial Strip Commercial product Cellulose TLC plates SiO, TLC plates

test-strips

[Nil,mm

Reflectance (mean of 5 replicates)

Relative precision, %

25 100 25 100 25 100

4.80 10.05 5.32 11.14 5.03 1095

2.2 1.6 1.5 0.7 7.4 3.1

480

DAVID KEALEY CONCLUSIONS

An analysis of variance at several concentrations between 10 and 100 ppm of nickel has confirmed that the overall relative precision of direct reflectance measurements on commercial test-strips lies in the range 3*2-4.6%. Within a single working session and with one particular box of test-strips, a relative precision of 1+3.2% can be expected. Larger strips, prepared from cellulose-coated TLC plates, give a more even colour and marginally better precision, but similarly-prepared silica gel coated strips are not satisfactory. Interfering ions which do not react with the chromogenic reagent do not appear to affect the precision significantly, but the colour produced is reduced in intensity (the reflectance is increased). The degree of such interference may be related to formal charge or charge density and is considered to be caused by changes in the rate of dihusion ot mckel ions to the surface of the strip during the immersion period. Cobalt at the 50 ppm level interferes by reacting with the reagent, but iron, copper and mercury interference can be minimized by using suitable masking agents. In such cases, the precision may be adversely affected. For quantitative analysis, samples and standards should have as similar a composition as possible. Acknowledgement-I should like to express my gratitude to Professor E. J. Shellard of Chelsea College of Science and Technology for the use of the Joyce-Loebl “Chromoscan”. REFERENCES 1. D. Kealey, Talonta, 1972, 19, 1563. 2. A. L. Wilson, ibid., 1970,17,31. Zwammenfasamg-Mit Hilfe handelsiiblicher Testreifen Rir Nickel wurde die Genauigkeit direkter Reflexionsmessungen im enxelnen abgeschlttxt. Im Konxentrationsbereich 10-100 ppm ist die relative Genauigkeit insgesamt 3,2-4,6x. Der Einflul3 stijrender Ionen wurde untersucht und der Mechanlsmus der Stiirung im Sinne der Diffusion diskutiert. Es wurden selbst Test* streifen hergestellt und mit den k&it&hen verglichen. Resmne-On a fait une appreciation detail& de la precision de mesures de pouvoir rtflecteur direct, en utilisant des lames-essai commercialement accessibles pour le nickel. La precision globale relative est 3,2-4.6 % pour le domaine de concentration 10-100 ppm. On a etudit l’influence d’ions genants et discute du mecanisme d’interference en fonction de la diffusion. On a prepare des lames-essai non commerciales et les a comparees au produit commercial.