Separation and determination of fluoride by diffusion with hexamethyldisiloxane and use of a fluoride-sensitive electrode

R SARA and E WANNINEN

1034 A

Fig 2 Agtatmg apparatus A-tray B--axIs, C-fork, D-eccentnc

A

for dGuszon Ashes, cam, E-motor

A Metrohm fluoride-sensltlve electrode was used with a Metrohm Ag/AgCl electrode with fibre Junction as reference electrode The em f’s were measured with the expanded scale of an Orton 701 pH mV-meter Materrals The fluonde standard solutions were prepared from Merck “Suprapur” sodium fluoride and were made 0033M m both sodium nitrate and hexamethylenetetramme (Merck pa ), the pH was adjusted to 5.4 with concentrated mtnc acid Perchlorrc acid (2 7&f, prepared from the Merck pa 60% acid) saturated with he~methy~dIslloxane (HMDS, Fluka AG) was used for the dlffuslon In some experiments 10% v/v ethanol was added to the perchlorlc acid before Its saturation with HMDS The perchlorlc acid solution was stored m a separatory-funnel under a layer of HMDS As trapping solutron, 0 1M sodmm hydroxide was used Procedure

The d&Tusion dishes are placed m an mchned posihon according to Fig 3, three separated compartments are thus formed m each The samples (maximally 10ml) are plpetted mto the left-hand compartments, 5 ml of trappmg solution are plpetted mto the middle compartments, and 5 ml of HMDS-saturated perchlorlc acid mto the right-hand cornp~~ent~ after winch the hd IS ~m~ately replaced and sealed w&h Vaseline Thzs procedure ensures mm~mal loss of fluoride smce the compartments for the sample and the HMDS-saturated perchlorlc acid are several centlmetres apart The lmmedlate replacement and sealing of the hd further reduce the risk of losmg fluoride When all the dlffuslon dishes have been prepared, the sample and the saturated perchlorz acid can be mIxed by rocking the dish When analysmg tooth-paste, ca 4g of a 1 5 w/w mixture of tooth-paste and water 1s weighed mto the dlffuslon dish For determmatlon of soluble fluoride the mixture IS centrifuged at 4OOOrpm for 10 mm and the supernatant hqmd IS analysed for fluoride The dlffuslon time necessary varies with the cornpositIon of the sample but 3 hr IS suffiaent for most samples When the drffuslon IS complete the dish IS opened and the trap pmg solution IS neutrahzed and buffered with 5 ml of 0 l&4 mtrlc acid and 5 ml of hexamethylenetetramme buffer The solution IS then analysed for fluoride with a fluonde-sensltlve electrode by the method of standard addition The fluoride concentration CF m the buffered trapping solution (volume V,) IS calculated from

where C, 1s the concentration and V, the volume of the added standard, AE the difference m the e m f before and after the addition (mV) and k the Nernstlan slope for the electrode

Fig 3 Preparation of the d@uslon dish

RESULTS AND DISCUSSION

The first expernnents, with 0 1M hydrochloric acid used to neutralize the trapping solutton, showed that the response tnne was rather long and dependent on the wncentratton of the preceding solution measured If the difference m wn~ntration was more than one order of magnitude the response was notably retarded The use of nitric acid for neutrahzmg shortened the response time by about 50% A typical response tune for 10e4kf fluoride with the mtric acid system was 15mm, but the tune could vary between qmte wide hnnts In ~ticul~, the first two or three measurements every morning were very slow A recorder connected to the pH-meter was found to be of great help m determmmg when the potential was stable The reproducibility of the emf measurement was found to be reduced because of heatmg of the solution by the magnettc stnrer motor Attempts to insulate the solutton with foam plastic dtd not give sattsfactory results for very dilute fluoride solutions because of the longer response time Much better reproductblhty was obtamed by using a magnetic stirrer driven by the~os~~~~y-controll~ water The em f of the m~suring cell seemed to change shghtly every ttMe the electrodes were hfted out of the solution It was to avoid thrs that the method of standard addition was used A cahbratron curve was constructed by adding known amounts of fluorrde standard to a buffered sodmm fluoride solution of known con~ntratlon It was linear over the pF range 24. The Nernstlan slope for the curve was 57 8 mV at 20” Calculations showed that the em f’s measured did not differ from the calibration curve values by more than 0 1 mV Another advantage of the method of standard add&on IS that the response time after the adddton IS much shorter than for a separate solution with the same concentration Also, several additions can be made to give nnproved accuracy In the method of Taves, small amounts of 6M hy drochlorz aad saturated with HMDS are mjected mto the dlf%szon dish through a small hole m the hd 8 Smce hydrochloric acid often contams small amounts of hydrogen fluoride’ It has to be pur6k.i before use The Taves8 and Hall9 methods were compared experunentally and smce HMDS-saturated perchlonc acid gave much faster d&&on it was used m all later expernnents No loss of iluonde was observed (relatlve to the mjectlon method) when the dlffusIon procedure described m the experimental part was used If larger amounts of fluoride are to be separated it is advantageous to increase the amount of HMDS present, either by usmg larger volumes of the acid reagent or addmg laO/, of ethanol to the perchlorlc aad before saturating It with HMDS and using the normal volume To determine the effect of agitation on the diffusion time, two series of dfluslon dishes were prepared wntammg the same amount of sodium fluoride One

Determination

of fluoride

1035

Table 1 Recovery of sodmm fluonde

Time, hr

Agltatlon

No of samples

Recovery, %

+ 5 ml 2 7M HC104 + 5 ml 2 7M HCIO, + 5 ml 2 7M HCIO.,

3 1 1

None Rocking Rocking

5 5 7

985 + 06t 987kO6 997+09

+ 5 ml 2 7M HC104 + 5ml 27M HQO.,

3 1

Rockmg Rockmg

7 7

98-O+ 10 978kll

Sample* 12 18pmole NaF 12 18pmole NaF 12 18pmole NaF + alcoholj 37 38 pmole NaF 37 38mole NaF + alcohol$

* The perchlorlc acid solution was saturated with HMDS t Relative standard devlatlon $ The perchlorlc acid solution contained 10% v/v ethanol

.

12

9 Y P 4

0

0

30

60 Time,

90

120

min

Fig 4 Diffusion time us amount of fluoride found Dlffuslon of 12 18 wale of NaF l No agltatlon, dlffuslon usmg 27M HMDS-saturated HClO, o Rockmg agltatlon, dlffuslon usmg 27M HMDS-saturated HClO, A Rockmg agitation, dlffuslon using 2 7M HMDS saturated HClO, contammg 10% ethanol

series was rocked m the agitator and the other was kept static The boxes were opened successively after different tnnes and the trapping solutions were analysed for fluoride The effect of agltatlon IS very clear (Fig 4) The dlffuslon tnne for 12 mole of fluoride was shortened from 2 hr to 30mm The effect of alcohol m the perchlorlc acid solution was small for this amount of fluoride (Fig 4), but for larger amounts of fluoride the addition of alcohol considerably shortened the diffusion time, e g , from over 2 hr to 30 mm for 37 pmole of fluoride The reproduclblhty for samples contammg sodium fluoride can be seen m Table 1 Fluoridated teeth have been found to be less susceptable to canes, which IS why most tooth-pastes contam fluoride There are reports of methods for determma-

tlon of fluoride m tooth-pastes by usmg a fluondesensitive electrode”* I1 but these methods cannot be used for all tooth-pastes because of the dBerences in composition Many tooth-pastes contam for mstance monofluorophosphate (MFP) wlch IS also active agamst caries Smce MFP cannot be determined directly with the fluoride-sensltlve electrode the compound must be hydrolysed before analysis These dlfficultles can be overcome by separating the fluoride from the tooth-paste by dlffuslon Even samples contammg MFP can be analysed, because MFP IS hydrolysed by the perchlorlc acid used m the dlffuslon Experlments show that the rate of hydrolysis IS not a hmltmg factor for the speed of diffusion Since fluoride even 111Insoluble salts, such as calcium fluoride, can be separated by diffusion, the method can be used to determme both total and soluble fluoride m the tooth-paste Tooth-pastes are generally quite thick, which IS why they must be dduted with water before analysis A suspension of one part of tooth-paste m five parts of water (w/w) was found to be suitable Such a suspension can easily be centrifuged for the determmation of soluble fluoride A sample of the suspension 1s weighed directly mto the dlffuslon dish for the determmatlon of total fluoride The dlffuslon times for tooth-paste (contammg solid CaHP04 2H20 and 0 lW/, fluoride as MFP) were determined as for sodium fluoride solutions, lW/, recovery bemg achieved m ca 33 hr with agltatlon, but no ethanol present, ca 2 hr with agitation and with ethanol present, but over 7 hr without agltatlon or ethanol

Table 2 Recovery of fluoride from tooth-paste contammg solid CaHPO, Sample*

Time,

4 0 g suspenslon$ + 5 ml 2 7M HC104 40g suspension + 5ml 2 7M HC104 4 0 g suspension + 5 ml 2 7M HC104 + alcohol * The perchlorlc acid solution was saturated with HMDS t Relative standard devlatlon ‘j One part of tooth paste and five parts of water (w/w)

22 4 3

hr

2H,O and 0 100% fluoride

Agltatlon

No of samples

Recovery, %

None Rockmg Rocking

5 5 5

992 f 08t 993+09 993+09

1036

R SARA and E WANNINEN

The recovery of fluoride when analysmg a toothpaste containing 0100% fluoride can be seen m Table 2 Tooth-pastes contammg the followmg solid abrasives have been analysed CaHPO, 2H20, hO1, A1203. The tune for complete dlfFusion was ca 2hr for tooth-pastes contammg phosphates or slhcon dioxide and ca 5 hr for tooth-pastes containing alumma Full advantage of the rapidity of dfluslon can be taken only d the dlffuslon time 1s determined separately for each type of sample Analysis of tooth-pastes contammg carbonate does not give good results, owing to the formation of carbon dloxlde If the absorbing capacity of the alkaline trapping solution 1s exceeded, there will be overpressure m the diffislon dish, causmg leakage of carbon &oxide and trtmethylfluorosllane. The method 1s also useful for many other types of saqple. It has been applied to determme fluoride, often as calcium salts, in such samples as water, foodstuffs and rocks. Fluoride concentrations have been determmed m waters with alummmm concentrations up to 1M The dfluslon tnne for such samples rs, however, considerably longer, around 10 hr

Acknowledgement-The authors wish to thank Mr M Plckermg for prehmmary revlslon of the English text of this paper

REFERENCES

M Frant and J W Ross Jr, Scrence, 1966, 154, 1553 2 H H Willard and 0 B Winter, lnd Eng Chem, Anal Ed, 1933, 5, 784 3 W Horw~tz, O&la1 Methods of Analyst of the Assoc~ atton of Analytical Chetmsts, A 0 A C, Washington DC, 1970 4 R H Powell and 0 Mems, Anal Chem, 1958, 30, 1546 5 M J Nardozzl and L L Lewis, ~bld, 1961, 33, 1261 6 L Singer and W D Armstrong, hrd, 1954, 26, 904 7 D R Tavcs, lbrd, 1968, 40, 204 8 Idem, Talanta, 1968, 15, 969 9 R J Hall, hd, 1969, 16, 129 10 N Shane and D Miele, J Phurm Sci., 1968, 57, 1260 11 E Vuori and J 0 Kdpio, Suomen Hammaslaakardehtl -FInlands Tandlakartldntng, 1974, 21, 129 12 K D Moffett, J R Slmmler and H A Potratz, Anal Chem, 1956, 28, 1356 13 C Eder and A Yarden, lb&, 1956, 28, 512